haircare

Abstract

A barrel (102, fig.1) for a haircare appliance (100, fig.1) comprises a heater 432, an absorbent layer 434 disposed over the heater, and an exterior surface. The heater is operable to heat the absorbe

Description

BACKGROUND Haircare appliances are generally used to treat or style hair, and some haircare appliances may treat or style hair by heating liquid. SUMMARY According to a first aspect of the present disclosure, there is provided a barrel for a haircare appliance, the barrel comprising: a heater; an absorbent layer for retaining liquid, the absorbent layer disposed over the heater; and an exterior surface, wherein the heater is operable to heat the absorbent layer to vaporise the liquid retained thereby to generate a vapour for transmission out of the barrel, through the exterior surface, and the heater, the absorbent layer and the exterior surface are configured to permit air within the barrel to flow out of the barrel, through the heater, the absorbent layer and the exterior surface. Conventional curling wands use heated plates or barrels and can scorch hair when the hair is placed directly on the heated barrels or surfaces at high temperatures. This can cause damage to the hair. The structure of the barrel according to the first aspect can reduce damage to the hair compared to such devices, while delivering a desired styling effect. In particular, the structure of the barrel enables vapour and air to be delivered through the exterior surface to hair wrapped around the barrel. The absorbent layer being disposed over the heater, for example so that the absorbent layer overlaps the heater or is on the heater (with or without at least one intervening component between the absorbent layer and the heater), enables the heat generated by the heater to be transferred readily to the absorbent layer, to vaporise the liquid retained by the absorbent layer. The vapour can then pass out of the barrel, through the exterior surface, and onto the hair. The air can similarly be transferred through the heater, absorbent layer and exterior surface, and towards the hair. With this approach, the vapour applied to the hair may be at a lower temperature than a heated barrel that is placed in contact with the hair in existing appliances. The structure of a hair fibre includes two protein structures within a cortex of the hair: a crystalline region of keratin (comprising ordered, structured protein fibres), and an amorphous region of keratin (comprising randomly oriented protein fibres). Both of these structures have different bonds that give hair its strength and shape. Typically, when drying wet hair, the hair remains cool as the water is evaporated and then, once the water has evaporated, the temperature increases. Styling in this manner without extreme heat (such as temperatures of 150 degrees Celsius and above) may only enable access to salt-hydrogen bonds within the amorphous region of the keratin. Without wishing to be bound by theory, the application of the vapour to the hair (which may be referring to as flash steaming) may enable a warm, moist styling environment in which the vapour is also able to penetrate the crystalline regions at temperatures of around 100 degrees Celsius. The vapour may thus be able to access more hydrogen bonds, increasing the probability of bond disruption to achieve a particular styling effect. Applying the vapour to the hair can improve style retention as it affects bonds in both the crystalline and amorphous regions, whereas ambient humidity typically cannot access the crystalline region as it is at room temperature. To create a well-defined style, excess moisture and / or surface wetness of the hair may be removed by applying the air to the hair. Removing moisture can allow new hydrogen bonds to form in the new position the hair is being held in (e g. with the hair wrapped around the barrel) so as to create the style, such as a curl or a wave. Removal of moisture from the hair by applying air to the hair in this manner can enable the style to be created more quickly, as opposed to leaving the hair to dry naturally after applying vapour. The absorbent layer being disposed over the heater may allow the vapour to be generated close to the exterior surface of the barrel (which for example faces away from the absorbent layer). This may avoid having pressurised vapour moving through the barrel, which can result in condensation of the vapour and inefficiencies. Instead, the vapour may be generated locally to the surface of the barrel so that the vapour is inhibited from escaping from the barrel without passing through the hair wrapped round the barrel. In contrasting approaches, a vapour is generated at a distance from the surface of the barrel and ejected under pressure from holes in the barrel. This means that a higher proportion of the vapour is likely to exit the barrel from holes that are not covered by the hair, which provide less restriction to the flow of the vapour. In addition, in these contrasting approaches, the vapour may exit preferentially from holes facing upwards. The structure of the barrel of the first aspect herein may be more effective at delivering the vapour to the hair than these contrasting approaches by generating the vapour adjacent to the surface of the barrel. In addition, the structure of the barrel of the first aspect herein may enable a style to be created with a substantially lower amount of vapour than the aforementioned contrasting approaches, thereby also providing improved efficiency. The heater may be configured to be heated from an off state to a temperature of at least 100 degrees Celsius in less than 5 seconds. Such a temperature is for example sufficient to vaporise the liquid retained by the absorbent layer to generate the vapour. Hence, such a heater may enable the vapour to be generated within 5 seconds of initiating styling. This may enable hair to be styled more efficiently than otherwise. For example, the heater may be considered to be a so-called “low thermal mass” heater, which can be heated and cooled relatively quickly. The heater may be configured to be heated from a temperature of 60 degrees Celsius to a temperature of at least 100 degrees Celsius in less than 2 seconds. The heater may be maintained at an idle temperature of around 60 degrees Celsius in between styling hair segments. For example, the user may wrap a tress of hair around the barrel, then utilise the barrel to style the tress of hair (which may involve heating the heater to a temperature of at least 100 degrees Celsius). The user may then unwrap the tress of hair and locate a different tress of hair to be wrapped around the barrel. In between styling the tress of hair and commencing styling of the different tress of hair, the heater may stay at the idle temperature. Once styling of the different tress of hair is initiated, the heater may then be heated to at least 100 degrees Celsius in less than 2 seconds, so as to generate the vapour in less than 2 seconds. This may reduce the time taken to style the hair. The heater may be configured to be cooled from a temperature of at least 100 degrees Celsius (such as a temperature of 120 degrees Celsius) to a temperature of less than 70 degrees Celsius (such as a temperature of around 60 degrees Celsius) in less than 5 seconds, for example by applying an airflow through the heater of at least 4 litres per second. This may allow the heater to reach a temperature that is less likely to cause burns more rapidly. The heater may have a thickness of less than 1 millimetre (mm) in a plane perpendicular to a longitudinal axis of the barrel. Preferably, the heater may have a thickness of less than 0.6 millimetres (mm) in a plane perpendicular to a longitudinal axis of the barrel. The heater may be thinner than heaters of existing appliances, which may enable the heater to be heated more rapidly to a sufficiently high temperature to vaporise the liquid. The heater may be formed of or comprise a metal with a relatively high thermal conductivity, such as aluminium. Other metals than aluminium may be used, though, such as stainless steel (for greater strength), copper (for improved thermal conductivity) or an alloy. The weight of the heater typically depends on the size of the barrel, and the barrel may comprise a plurality of heaters. For a barrel with a length of 140mm, the total weight of the heater(s) may be around 11 grams (g) for a barrel with a diameter of 20mm, around 15g for a barrel with a diameter of 25mm or around 22g for a barrel with a diameter of 38mm. The heat capacity of the heater is typically proportional to the size and weight of the heater. For a barrel with a diameter of 25mm and a length of 140mm, the heat capacity may be around 14.7 Joules per Celsius (J / C). The barrel may comprise a cover layer disposed over the absorbent layer and the exterior surface may comprise an exterior surface of the cover layer. The cover layer may protect the underlying layers of the barrel during use of the haircare appliance. The exterior surface may comprise an exterior surface of the absorbent layer. In such cases, the barrel may comprise at least one rib disposed over the absorbent layer and extending along a longitudinal axis of the barrel. The exterior surface may further comprise at least a portion of an exterior surface of each of the at least one rib. The at least one rib may provide a smoother exterior surface in order to aid removal of hair from around the barrel, after styling is complete. For example, snagging of the hair on the absorbent layer may be reduced by providing the at least one rib. Each respective rib of the at least one rib may be retractable relative to the absorbent layer to adjust an extent to which a cross-section of the respective rib in a plane parallel to the longitudinal axis of the barrel protrudes from the absorbent layer. The extent to which each rib protrudes may be adjusted in this manner for example to increase the extent of protrusion to provide a smoother tress withdrawal surface for withdrawing hair from the surface and / or to decrease the extent of protrusion to improve transfer of heat and / or vapour to the hair during styling. The heater may comprise perforations for the air to flow through. The perforations may enable a more even distribution of the air through the heater, so as to more evenly apply the airflow to the hair. The air flowing through the perforations may be heated by the heater, enabling a heated airflow to be generated by the same heater as that used to heat the absorbent layer to generate the vapour. This may allow a dedicated heater for heating the air to be omitted, while still allowing a heated airflow to be generated, for example to heat the air to dry the hair after the vapour has been applied. The perforations may be disposed circumferentially about a longitudinal axis of the barrel. For example, the perforations may entirely encircle the longitudinal axis of the barrel. The circumferential positioning of the perforations may allow air to flow radially out of the heater, through the perforations, for example so that air flows generally omnidirectionally out of the heater. This can aid distribution of the air to the hair compared to an airflow that flows through the hair in a single direction or a more limited range of directions. If the air is applied through the heater after the vapour is applied to the hair, this may allow the vapour to be removed from the hair more efficiently than otherwise. The heater may comprise a plurality of first longitudinal rows of heater element portions, each of the first longitudinal rows extending along a longitudinal axis of the barrel. The first longitudinal rows may allow the heat to be applied to the hair along the longitudinal axis, so as to more evenly heat hair that is wrapped around the barrel. The perforations may be arranged in a plurality of second longitudinal rows, each of the second longitudinal rows extending along the longitudinal axis of the barrel. The first longitudinal rows may allow air to flow through the heater at various positions along the longitudinal axis. This may allow the air to be distributed to hair wrapped along the length of the barrel so that the air is more evenly applied to the hair. Each of the first longitudinal rows may be interleaved between a respective pair of adjacent second longitudinal rows of the plurality of second longitudinal rows. Interleaving the first and second longitudinal rows may increase heater coverage without unduly sacrificing distribution of the air, which may improve the overall performance achievable with the barrel. Each of the heater element portions may have a generally sinusoidal profile in a plane of the heater parallel to a surface of the heater facing the absorbent layer. A generally sinusoidal profile for example refers to a shape that is a smoothly varying curve with generally constant frequency and amplitude in the plane of the heater. This shape may allow for the heater element portions to be more closely packed relative to the perforations, so as to increase the heating effect without unduly reducing the density of the perforations. For example, there may be a substantially constant distance between the heater element portions and a closest perforation, taken in a direction at a tangent to the heater element portions, along a length of the heater element portions. The perforations may be first perforations and at least one of: the absorbent layer may comprise second perforations for the air to flow through; and (in examples in which the barrel comprises a cover layer and the exterior surface is an exterior surface of the cover layer) the cover layer may comprise third perforations for the air to flow through. The first, second and / or third perforations may comprise slits, holes, apertures or other openings, and may enable the air to be more evenly transferred out of the barrel. In other examples, though, at least one of the heater, absorbent layer and / or cover layer may be configured to permit air to flow therethrough in a different manner. For example, at least one of the heater, absorbent layer and / or cover layer may comprise a plurality of separate elements, with a gap between adjacent elements to permit the airflow. The elements may themselves be perforated. The second perforations may be aligned with at least one of: the first perforations and the third perforations. Aligning the first, second and / or third perforations may aid transmission of the air out of the barrel. Perforations may be considered to be aligned if a perforation of given set of perforations (such as the first perforations) is overlapped by an overlying perforation of an overlying set of perforations (such as the second perforations), e.g. so that a transverse axis, perpendicular to the longitudinal axis of the barrel, passes through both the perforation and the overlying perforation. The second perforations may be considered to be aligned with the first and / or the third perforations if at least some of the second perforations are aligned with perforations of the first and / or the third perforations. The cover layer may comprise the third perforations, and each perforation of the third perforations may have a diameter of less than 70 micrometres. A cover layer with this structure may protect inner layers of the barrel, such as the absorbent layer and the heater, and reduce ingress of debris into the barrel, through the cover layer, without unduly compromising flow of air out of the cover layer or surface temperature. Hair fibres on average have a sub-millimetre diameter, with relatively fine hair tresses typically having a diameter of around 70 micrometres or more. Hence, having third perforations with the diameter of less than 70 micrometres may inhibit the hair from entering the third perforations, which may reduce snagging of the hair during use. The volume of the cover layer not occupied by the third perforations may be selected to allow the ready release of vapour and air to the hair and to limit the thermal mass of the barrel, to enable effective styling. The cover layer may be a mesh. A mesh for example offers a suitable structure for effective operation of a haircare appliance comprising the barrel, with a suitable balance of the third perforations and closed portions (not occupied by the third perforations). The geometry provided by a mesh may be more suitable than that provided by other materials such as those that have undergone chemical etching, weaving or punching to create the third perforations. The mesh may be a relatively fine mesh, such as a mesh of stainless steel 316, which has a relatively high thermal conductivity and is relatively resistant to damage by common chemicals. In other examples, a different material may be used for a mesh for the cover layer, such as a polymer, e g. polyester or polyamide. Polymers such as these may contribute to a greater build-up of static electricity on the hair, but they may be less susceptible to mechanical deformation and may be easier to customise than stainless steel. The barrel may comprise a conduit for transporting the liquid to the absorbent layer. The use of a conduit may provide greater control in transporting the liquid than other approaches, such as the use of wicking alone to transport the liquid. For example, the provision of a conduit may allow a predefined amount of the liquid to be transported to the absorbent layer, via the conduit, which may allow the vapour generation to be more effectively controlled so as to generate a desired quantity of the vapour. The conduit may be elongate along a longitudinal axis of the barrel and may comprise outlets arranged along a length of the conduit for the liquid to flow through to reach the absorbent layer. This may enable the liquid to be more evenly distributed along a length of the absorbent layer than if the liquid is wicked into the absorbent layer from one end. Distributing the liquid to the absorbent layer in this manner may result in a more even distribution of vapour along the length of the absorbent layer, which can enable the vapour to be applied more evenly to hair wrapped around the length of the barrel. The outlets may be substantially evenly distributed along the length of the conduit, such as evenly distributed, evenly distributed within manufacturing and / or measurement tolerances, or sufficiently evenly distributed to achieve an acceptably even distribution of the liquid across the absorbent portion. This may aid in even distribution of the liquid to the absorbent layer. The barrel may comprise a plurality of conduits for transporting the liquid to the absorbent layer, which may further facilitate the distribution of the liquid to the absorbent layer. The heater may comprise a plurality of heating areas, and a respective conduit of the plurality of conduits may be disposed between each pair of adjacent heating areas of the plurality of heating areas. This arrangement may enable the heat generated by the heater to be concentrated on the regions in which the liquid is retained by the absorbent layer, which may be a more efficient use of the heat than heating the conduits. The plurality of conduits may be substantially evenly distributed about a circumference of the barrel. For example, there may be n conduits distributed every (360°¼) about the circumference. This may allow the liquid to be evenly distributed circumferentially about the longitudinal axis, to enable vapour generation that is more even about the circumference of the barrel. Distributing the plurality of conduits in this manner may also reduce the time to wet the absorbent layer sufficiently evenly to commence styling, so that styling can be performed more rapidly. The absorbent layer may be removeable from the barrel. This may allow the absorbent layer to be washed (or otherwise cleaned) or replaced, which can extend the overall operational lifetime of the barrel. The absorbent layer may comprise a wicking material configured to spread the liquid provided to the absorbent layer, for example via the conduit, across a surface of the absorbent layer using capillary action, also known as wicking. This may enable the liquid to be spread more evenly throughout the absorbent layer A texture and / or weave of the absorbent layer may be selected to favour wicking in certain direction(s) to improve coverage of the absorbent layer by the liquid. According to a second aspect of the present disclosure, there is provided a haircare appliance comprising the barrel of the first aspect, and a body. The body may comprise an airflow generator for generating an airflow out of the barrel, through the heater, the absorbent layer and the exterior surface. This may provide a compact arrangement for generating the airflow, which may allow the haircare appliance to be handled more easily during use. The heater may be operable to heat the airflow generated by the airflow generator. The heater may therefore provide a dual functionality of heating the absorbent layer to generate the vapour and heating the airflow, for example to generate a heated airflow for setting a style. Using a heater to provide dual functionality in this way may mean that a dedicated air heater can be omitted, meaning that the haircare appliance may be more compact and / or lower in weight than otherwise. This may make the haircare appliance easier to handle for use in hair styling. The body may comprise a reservoir for storing the liquid. The reservoir may be fluidly connected to the absorbent layer to provide the liquid from the reservoir to the absorbent layer. This may allow the liquid to be supplied to the absorbent layer more straightforwardly and with less mess than other approaches such as using a manual or automated spray to apply liquid to a surface of the barrel or by passing a wet brush over the surface of the barrel. The haircare appliance may comprise a liquid delivery arrangement for delivering a predefined amount of the liquid from the reservoir to the absorbent layer. This may provide greater control over the amount of vapour generated, which can in turn provide increased control over the styling effect achieved by the haircare appliance. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic side cross-sectional view of a haircare appliance, according to an example; Figure 2 is a schematic cross-sectional view of a barrel of a haircare appliance in a plane perpendicular to a longitudinal axis of the barrel, according to an example; Figure 3 is a schematic cross-sectional view of a portion of a barrel of a haircare appliance in a plane perpendicular to a longitudinal axis of the barrel, according to a further example; Figure 4 is a schematic perspective view of the portion of the barrel of Figure 3; Figure 5 is a schematic view of a portion of a barrel comprising a thin-film structure according to an example; Figure 6 is a schematic view of a portion of a barrel comprising a thin-film structure according to a further example; Figure 7 is a schematic top view of the portion of the barrel of Figure 6; Figure 8 is a schematic side cross-sectional view through the plane P in Figure 7; Figure 9 is a schematic view of a portion of a barrel comprising a thick-film structure according to an example; Figure 10 is a schematic view of a portion of a barrel comprising a thick-film structure according to a further example; Figure 11 is a schematic top view of the heater trace similar to that shown in Figure 7; Figure 12 is a schematic top view of a heater, according to an example; Figure 13 is a schematic cross-sectional view through a barrel comprising a heater, according to a further example; Figure 14 is a schematic cross-sectional view through a barrel comprising a heater, according to a further example; Figure 15 is a schematic perspective view of a reservoir for storing a liquid, according to a first example; Figure 16 is a schematic perspective view of a reservoir for storing a liquid, according to a second example; Figure 17 is a schematic perspective view of a reservoir for storing a liquid, according to a third example; Figure 18 is a schematic perspective view of a reservoir for storing a liquid, according to a fourth example; Figure 19 is a schematic perspective view of a reservoir for storing a liquid, according to a fifth example; Figure 20 is a schematic unwrapped view of a conduit, according to an example; Figure 21 is a schematic unwrapped view of a conduit, according to a further example; Figure 22 is a schematic unwrapped view of outlets of a conduit, according to an example; Figure 23 is a flow diagram of a predefined styling procedure, according to an example; Figure 24 is a plot of surface temperature over time, during execution of a predefined styling procedure according to an example; Figure 25 is a plot of surface temperature and heater power consumption over time, during execution of a predefined styling procedure according to an example; Figure 26 is a schematic perspective view of a barrel of a haircare appliance, according to an example; and Figures 27 and 28 are simplified cross-sectional views of the barrel of Figure 26 along a line A-A in Figure 26. DETAILED DESCRIPTION Figure 1 shows an example haircare appliance 100. The haircare appliance 100 comprises a barrel 102, a body 104 and an electrical cable 106 extending from the body 104, for supplying electrical power to electrical components of the haircare appliance 100. The electrical cable 106 comprises a plug (not shown in Figure 1) at a distal end of the cable 106 with respect to the body 104, which can be plugged into a socket in order to supply mains power to the haircare appliance 100. The electrical cable 106 may be referred to as an electrical cord. The body 104 includes airflow inlets 108, a reservoir 110, an airflow generator 112, a start button 113, a network interface 115, a gas removal button 117, a control system 114, a pump 116 and liquid outlets 118, 120. The body 104 has a generally hollow tubular housing formed of plastic. The reservoir 110, the airflow generator 112, the control system 114, the pump 116 and the liquid outlets 118, 120 are within the tubular housing of the body 104. A central longitudinal axis of the body 104 coincides with a central longitudinal axis of the haircare appliance 100. The airflow inlets 108 are each generally circular perforations in a side wall of the tubular housing of the body 104. The airflow inlets 108 are arranged in an irregular pattern in the side wall. However, in other examples, the airflow inlets may be arranged in evenly distributed longitudinal rows along a part of a length of the side wall and in evenly distributed circumferential rows around a circumference of the side wall. The reservoir 110 is for storing liquid (in this case, water), which is delivered to the barrel 102 by the pump 116. The reservoir 110 is shown schematically in Figure 1. However, various example structures that may be used for the reservoir 110 are shown in Figures 15 to 19. The reservoir 110 in this case is an integrated water tank, which may be removable from the haircare appliance 100, for example to top up the reservoir 110 with additional liquid or to clean the reservoir 110. The haircare appliance 100 comprises a reservoir sensor 111, which in Figure 1 is arranged in the body 104 and is coupled to the control system 114. The reservoir sensor 111 is configured to sense whether the reservoir 110 is connected to the haircare appliance 100. The reservoir sensor 111 may be a mechanical switch, a Hall effect sensor paired to a magnet on the reservoir 110 or a radio frequency identification (RFID) sensor to sense the presence of the reservoir 110. The haircare appliance 100 of Figure 1 is operable to deliver a predefined amount of the liquid from the reservoir 110 to the barrel 102 in dependence on the reservoir sensor 111 sensing that the reservoir 110 is connected to the haircare appliance 100. For example, the predefined amount of the liquid may be delivered in response to input from a user (e g. in response to a user initiating a predefined styling procedure or wetting of the absorbent layer), provided that the reservoir sensor 111 senses that the reservoir 110 is connected to the haircare appliance 100. The reservoir 110 is fluidly connected to the pump 116 by pipes 122 extending longitudinally within the tubular housing of the body 104. The pump 116 delivers water from the reservoir 110 to the barrel 102 via the liquid outlets 118, 120 of the body 104. The airflow generator 112 includes a motor 124 and an impeller 126 and may be considered to be an air blower for blowing air into the barrel 102. The motor 124 is attached to the impeller 126 and is operable to rotate the impeller 126 when switched to an on state, e g. by the control system 114. The impeller 126 is operable to generate an airflow from the airflow inlets 108 and into the barrel 102. In Figure 1, the airflow enters the barrel 102 through an airflow outlet 128 of the body 104, which is fluidly connected to a channel 130 within the barrel 102. The airflow outlet 128 is defined by circular perforations at an end of the tubular housing of the body 104. In other cases, though, the airflow generated by the airflow generator 112 may enter a different region of the barrel 102 and / or via a different outlet or outlets than the airflow outlet 128 of the body 104. The barrel 102 comprises a heater 132, an absorbent layer 134 for retaining a liquid (in this case, the liquid delivered from the reservoir 110), and a cover layer 136. The heater 132 has a generally hollow tubular shape. An interior hollow chamber of the heater 132 defines the channel 130 within the barrel 102. The absorbent layer 134 is disposed over the heater 132 and the cover layer 136 is disposed over the absorbent layer 134, so that the cover layer 136 overlaps the absorbent layer 134 and the heater 132, and the absorbent layer 134 overlaps the heater 132. The absorbent layer 134 is disposed between the cover layer 136 and the heater 132. The absorbent layer 134 and the cover layer 136 each have a generally hollow tubular shape. A cross-sectional area of the absorbent layer 134 is larger than that of the heater 132 but smaller than that of the cover layer 136. The heater 132, the absorbent layer 134 and the cover layer 136 are concentric, and each extend along the central longitudinal axis of the haircare appliance 100 (and of the barrel 102). In Figure 1, an exterior surface 135 of the barrel 102 corresponds to an exterior surface of the cover layer 136. In use, a tress of hair is wrapped around the exterior surface of the barrel 102 to style the tress. The heater 132 is operable to heat the absorbent layer 134 to vaporise the liquid retained by the absorbent layer 134 to generate a vapour. The vapour generated by vaporising the liquid retained by the absorbent layer 134 is transmitted through the exterior surface 135 of the barrel (in this case, through the cover layer 136) and out of the haircare appliance 100, in use. The exterior surface 135 of the barrel 102 is thus permeable by the airflow and the vapour. The heater 132 may also be used for heating an airflow generated by the airflow generator 112, in addition to heating the absorbent layer 134 to generate the vapour. The heater 132 is a so-called low thermal mass heater, which can be heated and cooled relatively rapidly. In this case, the heater 132 is configured to be heated from an off state to a temperature of at least 100 degrees Celsius in less than 5 seconds. The temperature of at least 100 degrees Celsius is generally high enough to begin vaporising the liquid retained by the absorbent layer 134. In the off state in this example, the control system 114 does not supply electrical power to the heater 132, so the heater 132 remains at room temperature. The heater 132 is also configured to be heated from a temperature of 60 degrees Celsius to a temperature of at least 100 degrees Celsius in less than 2 seconds. The temperature of 60 degrees Celsius for example corresponds to an idle temperature, which may be low enough that a user can handle the haircare appliance 100 to some extent without burning themselves but which is high enough to reduce the time to begin vaporisation of the liquid. The heater 132 has a thickness of less than 0.6 millimetres in a plane perpendicular to a longitudinal axis of the barrel 102, allowing it to be heated and cooled relatively rapidly. In Figure 1, the pump 116 is configured to deliver a predefined amount of the liquid (in this case, a dose of water) from the reservoir 110 to the absorbent layer 134. For example, the predefined amount of the liquid may be selected to generate a desired amount of vapour to achieve a particular style without overly wetting the hair and increasing a time taken to dry the hair and set the style. For an average tress of hair, the amount of liquid to achieve effective styling results tends to be between around 0.5g and 1g. To deliver the predefined amount of the liquid, the pump 116 may be configured to pump the liquid at a predefined rate for a predefined amount of time to deliver the predefined amount of the liquid from the reservoir 110 to the absorbent layer 134. The predefined amount of the liquid may be delivered as part of a predefined styling procedure, for example during a wetting phase which is performed before a vaporisation phase, as discussed in more detail with reference to Figures 23 to 25. The pump 116, the pipes 122 to deliver liquid to the pump 116 from the reservoir 110, and at least one conduit (not shown in Figure 1) to deliver the predefined amount of liquid from the pump 116 to the absorbent layer 134 may be considered to form a liquid delivery arrangement. In some cases, though, the pump 116 may be directly connected to the reservoir 110 rather than being indirectly connected to the reservoir 110 via the pipes 122 or another conduit. The reservoir 110, the liquid delivery arrangement, the heater 132 and the absorbent layer 134 may together be considered to be a vapour generation system. The absorbent layer 134 absorbs the liquid (in this case, water) delivered by the liquid delivery arrangement. The absorbent layer 134 of Figure 1 comprises a wicking material, which distributes the water throughout the absorbent layer 134 due to a capillary effect. The water absorbent material of the absorbent layer 134 is layered on top of the heater 132. The heater 132 can be heated relatively rapidly, which, in turn, can rapidly heat the wet absorbent layer 134 on the heater 132 to generate steam. This allows steam to be generated close to a surface of the barrel 102 (which corresponds to an exterior, e g. outer, surface of the cover layer 136, which is opposite to an interior, e g. inner, surface of the cover layer 136 facing the absorbent layer 134). In Figure 1, there is a gap between the absorbent layer 134 and the heater 132. However, in other examples, the absorbent layer 134 may be in contact with the heater 132 to allow steam to be generated more efficiently upon heating the heater 132. A material for the absorbent layer 134 may be selected for the absorbent layer 134 to receive the liquid and distribute the liquid across the absorbent layer 134 relatively evenly, to withstand the temperatures generated by the heater 132 in generating the vapour, and to be sufficiently thin to permit thermal energy generated by the heater 132 to be transferred to hair of a user (which is wrapped around an outer surface of the barrel 102, in use). In Figure 1, the absorbent layer 134 is formed of a microfibre material, such as a mix of polyester with polyamide. For a single-ply woven layer of polyester with polyamide, a suitable thickness of the absorbent layer 134 is around 500 micrometres, with a fibre diameter of around 10 micrometres. The absorbent layer 134 of Figure 1 is removeable from the haircare appliance 100 by removing the cover layer 136 overlying the absorbent layer 134 and then removing the absorbent layer 134 (or by removing the cover layer 136 and the absorbent layer 134 together). For example, a clip or other locking mechanism may be disengaged to allow the cover layer 136 and / or the absorbent layer 134 to be moved relative to other components of the haircare appliance 100. The cover layer 136 and / or the absorbent layer 134 may then be slid in a direction parallel to the central longitudinal axis of the haircare appliance 100, away from the body 104, so as to disconnect the cover layer 136 and the absorbent layer 134 from the barrel 102. In another example, the absorbent layer 134 and the cover layer 136 may be wrapped around the heater 132 and fixed to a particular location along the barrel 102 by a fixture to enable appropriate tensioning of the absorbent layer 134 and improve thermal contact between the absorbent layer 134 and the heater 132. In this example, the fixture can be disengaged to allow the absorbent layer 134 and the cover layer 136 to be unwrapped from the barrel 102 and removed from the haircare appliance 100. The cover layer 136 is a metallic mesh structure layered on top of the absorbent layer 134 in Figure 1. In Figure 1, there is a gap between the cover layer 136 and the absorbent layer 134. However, in other examples, the cover layer 136 may be in contact with the absorbent layer 134. The cover layer 136 of Figure 1 is formed of stainless steel 316, with a plain weave structure. For a single-ply cover layer 136 with this structure, a suitable thickness is around 50 micrometres, with a wire diameter of around 25 micrometres, an open area between adjacent wires of around 40 micrometres and a total open area of around 40%. In general, a diameter of a perforation of the cover layer 136 (e.g. corresponding an open area between adjacent fibres for a mesh structure) may be less than around 70 micrometres to limit snagging of hair on the cover layer 136, as the diameter of a relatively fine hair is typically around 70 micrometres. The heater 132, the absorbent layer 134 and the cover layer 136 are configured to permit air within the barrel 102 (such as the air within the channel 130, generated by the airflow generator 112) to flow out of the barrel 102, through the heater 132, the absorbent layer 134 and the cover layer 136. In the example of Figure 1, the heater 132, the absorbent layer 134 and the cover layer 136 each comprise perforations for the air to flow through (which may be referred to as first, second and third perforations, respectively). In the example of Figure 1, the perforations in the heater 132 correspond to through-holes in the material of the heater 132. For example, the heater 132 may be a perforated shim. The perforations in an absorbent layer such as the absorbent layer 134 of Figure 1 may also be through-holes, like those of the heater 132, which may align with at least one of the perforations in the heater 132 and / or the cover layer 136. However, in Figure 1, the perforations in the absorbent layer 134 correspond to pores in a porous microfibre material. The mesh structure of the cover layer 136 of Figure 1 has gaps between adjacent strands that are woven or otherwise connected together to form the mesh. These gaps form the perforations in the cover layer 136, which allow air to flow through. In other cases, though, the cover layer 136 may also or instead have through-holes, which may align with at least one of the perforations in the heater 132 and / or the absorbent layer 134. In addition to being permeable to air, the cover layer 136 is also permeable to the vapour generated by vaporising the liquid retained by the absorbent layer 134. The vapour can pass out of the barrel 102 through the cover layer 136 via the perforations in the cover layer 136, such as via the open areas in a mesh structure of the cover layer 136. The vapour generated by the haircare appliance 100 is delivered to the hair wrapped around the barrel 102, in use, to generate a desired style (such as a curl or wave in the hair). After the liquid has been vaporised from the absorbent layer 134, the haircare appliance 100 can be operated to deliver airflow generated by the airflow generator 112 to the hair, through the heater 132, the absorbent layer 134 and the cover layer 136. If the heater 132 is an on state during airflow generation (e.g. so that electrical power is being supplied to the heater 132 so as to heat the heater 132 above room temperature), the heater 132 can heat the airflow passing through the heater 132, so that the airflow delivered to the hair, through the cover layer 136, is a heated airflow for drying the hair. Subsequently, the heater 132 can be turned off. Styling may cease at this point, in which case the airflow may also be turned off. However, in other examples, the airflow may remain on so as to deliver a cooling airflow to the hair, through the cover layer 136, to set the style. The haircare appliance 100 of Figure 1 includes a liquid detection system 138 to generate a liquid detection signal indicative of whether liquid is present within the reservoir 110, the liquid delivery arrangement and / or the absorbent layer 134, a power sensor 139 for obtaining a power signal indicative of a power consumed by the heater 132, and a barrel temperature sensor 140 to sense a temperature at a surface of the barrel 102 (in this case, corresponding to a temperature at a surface of the cover layer 136). A respective output of the liquid detection system 138, the power sensor 139 and / or the barrel temperature sensor 140 may be used by the control system 114 to aid control of the haircare appliance 100, for example to initiate gas removal as described further with reference to Figures 15 to 22 and / or to perform a predefined styling procedure as described further with reference to Figures 23 to 25. The haircare appliance 100 may also or instead comprise a further sensor, such as a further temperature sensor to sense a temperature at a surface of other layers of the haircare appliance 100, such as the absorbent layer 134 or the heater 132. The haircare appliance 100 of Figure 1 may be configured to provide a predefined amount of liquid of between 0.5 grams (g) and 1.5g, depending on a size of the barrel 102, and to spread the liquid relatively evenly to the absorbent layer 132 within a few seconds, such as in less than 4 seconds. For example, the haircare appliance 100 may be operated to evaporate liquid at a rate of around 3 grams per minute. This means that a typical liquid dose of 0.5g will be evaporated by the heat applied to the absorbent layer 134 by the heater 132 in around 10 seconds. A heated airflow, at a temperature of around 95 degrees Celsius or more, may be achieved for up to 10 seconds, and a temperature at a surface of the barrel 102 may be lowered to less than around 75 degrees Celsius in 5 seconds or less. Operation of the airflow generator 112 and the pump 116 is controlled by the control system 114, which is electrically connected to the motor 124 of the airflow generator 112 and to the pump 116. The control system 114 is configured to control the haircare appliance 100 to perform a predefined styling procedure, as discussed in more detail with reference to Figures 21 to 23. The predefined styling procedure may be used to perform surface steam curling of hair of a user of the haircare appliance 100. The control system 114 in Figure 1 is an electronic controller comprising circuitry configured to control the haircare appliance 100 to perform the predefined styling procedure. The circuitry may implement internal components of the control system 114 such as a processor, storage, at least one data interface for transferring data to another component and / or receiving data from another component, such as a sensor of the haircare appliance 100, e.g. the liquid detection system 138, the power sensor 139, the barrel temperature sensor 140 and / or another sensor of the haircare appliance 100. The circuitry may also implement a bus to allow communication between the various internal components of the control system 114. The processor of the control system 114 may receive data from a sensor, which may be referred to as sensor data and may be in the form of at least one signal from the sensor, via the at least one data interface. The sensor data may be stored in the storage and processed using the processor for example based on instructions stored within the storage defining processing to be performed by the control system 114, such as instructions to implement the predefined styling procedure. The processor may generate at least one configuration signal for configuring component(s) of the haircare appliance 100, based on the instructions. For example, the processor may generate a configuration signal to send to the airflow generator 112 to control a configuration of the airflow generator 112, e.g. to switch the airflow generator 112 from an off state to an on state or vice versa and / or to adjust a parameter, such as a flow rate, of the airflow generated by the airflow generator 112. The processor may also or instead generate a configuration signal to send to a heater 132 of the barrel 102 to control a configuration of the heater 132, eg. to switch the heater 132 from an off state to an on state or vice versa and / or to adjust a parameter, such as a set temperature or power, of the heater 132. In this way, the control system 114 can adjust a configuration of components of the haircare appliance 100 over time, in order to achieve the predefined styling procedure. In Figure 1, the predefined styling procedure may be initiated by a user pressing the start button 113. Pressing the start button 113 generates a control signal (which in this case is an initiation signal to indicate that the predefined styling procedure is to be initiated). The start button 113 is an example of a mechanical user interface for receiving a control signal (in this, a mechanical control signal representing whether the start button 113 is depressed or not). In other examples, though, a haircare appliance otherwise similar to the haircare appliance 100 of Figure 1 may also or instead include a user interface of different type than the start button 113 (which may be mechanical in nature, such as a slider, or may be an electronic user interface, such as a touchscreen). The haircare appliance 100 of Figure 1 also comprises the network interface 115 for controlling the haircare appliance 100 remotely, for example without touching the haircare appliance 100. The network interface 115 may comprise an input / output interface, such as a Bluetooth connector or a network connector, for communicating with a remote system via a suitable telecommunications network. For example, a user of the haircare appliance 100 may download a suitable software application (which may be referred to as an “app”) to an electronic user device, such as a smartphone or tablet computer. The app may provide a graphical user interface (GUI) for the user to interact with to control the haircare appliance 100. The GUI may provide an option for the user to initiate the predefined styling procedure, e.g. by selecting a particular menu item within the app. Selection of this item may generate a control signal, e g. an initiation signal, which is sent to the haircare appliance 100 via the network interface 115. The haircare appliance 100 of Figure 1 includes both the start button 113 and the network interface 115 so the haircare appliance 100 can be controlled remotely and by physical interaction with the haircare appliance 100, to provide flexibility. In other cases, though, a haircare appliance otherwise the same as the haircare appliance 100 need not comprise both a user interface and a network interface. The user can also use an interface of the haircare appliance 100 to control various parameters of the predefined styling procedure, such as a duration of at least one phase of the predefined styling procedure, a quantity of the vapour, and a temperature of the barrel 102 for at least one phase of the predefined styling procedure. This allows for customisation of the predefined styling procedure for different hair types and desired styles. For example, the user may use a user interface of the haircare appliance 100 (e.g. a touchscreen, a button or other switch, or a slider) or an app on an electronic user device to select a desired value for at least one parameter of the predefined styling procedure. For example, a user may adjust the predefined styling procedure using a mode button of the haircare appliance 100 (not shown in Figure 1) to adjust the at least one parameter so as to change a style achieved (such as a curl tightness) and / or a thermal comfort of the user. The desired value(s) of the at least one parameter may be represented by customisation data instructing customisation of at least one parameter to be used for the predefined styling procedure. In these cases, the control system 114 receives the customisation data, e.g. from the user interface and / or the network interface 115, and controls operation of the haircare appliance 100 to perform the predefined styling procedure based on the customisation data. The control signal obtained via an interface of the haircare appliance 100 may also or instead comprise a gas removal signal to initiate removal of gas from a vapour generation system of the haircare appliance 100. In the haircare appliance 100 of Figure 1, a user can press the gas removal button 117 to provide a gas removal signal. In response to the gas removal button 117 being pressed, the control system 114 configures the haircare appliance 100 to remove gas from the vapour generation system. The user can also or instead initiate gas removal via an app on an electronic user device, to cause the gas removal signal to be sent to the control system 114 via the network interface 115. In other examples, though, the gas removal signal may be received via a different interface than the gas removal button 117 or the network interface 115, such as a different user interface, e.g. an electronic touchscreen, which may be operable to obtain the initiation signal in addition to the gas removal signal in dependence on which procedure a user opts to select via the user interface. In response to receiving the gas removal signal, the control system 114 of Figure 1 configures the pump 116 in an on state for a predefined amount of time, to pump liquid from the reservoir 110 through the liquid delivery arrangement to the absorbent layer 134, so that the liquid traversing the system displaces gas within the reservoir 110 and / or the liquid delivery arrangement. In other examples, though, the control system 114 may configure the haircare appliance 100 in a different manner to remove gas from the reservoir 110 and / or the liquid delivery arrangement, in response to the gas removal signal. In examples, gas may be removed from the reservoir 110 by a user while refilling the reservoir 110 with liquid. For example, the reservoir 110 may have a cap that, when pressed or screwed onto the reservoir 110, would pressurise the reservoir 110 to an extent, so as to cause gas to be removed from within the reservoir 110, e.g. if held in a particular orientation such as upright. In some examples, a cap (or other closure) such as this may be operated automatically by the control system 114 rather than by a user, so as to remove gas from the reservoir 110. Figure 2 shows a side cross-sectional view of a barrel 202 for a haircare appliance, such as the haircare appliance 100 of Figure 1. The barrel 202 of Figure 2 is similar to the barrel 102 of Figure 1, and comprises a series of concentric, generally cylindrical layers. The layers of the barrel 202 are, in order from innermost to outermost: a polyimide layer 242 comprising a heater trace layer (e g. formed of copper), an adhesive layer 244, an aluminium layer 246, an absorbent layer 234 (formed of a microfibre material), and a cover layer 236 (formed of a steel mesh). The polyimide layer 242 comprising the heater trace layer, the adhesive layer 244 and the aluminium layer 246 together function as a heater, which defines a channel 230 for air to flow through the barrel 202, e g. from an airflow generator in a body of a haircare appliance. Although not shown in Figure 2, it is to be appreciated that the polyimide layer 242 may further include a sensor trace layer, as discussed in more detail with reference to Figures 5 to 11. The barrel 202 also includes four conduits (each labelled with the reference numeral 248) for transporting the liquid to the absorbent layer 234. The conduits 248 may be coupled to a pump such as the pump 116 of Figure 1, for delivering a predefined amount of the liquid from a reservoir of the body to the absorbent layer 234. The conduits 248 are elongate along the longitudinal axis of the barrel 202 and are evenly distributed about a circumference of the barrel 202 so as to evenly distribute the liquid to the absorbent layer 234 circumferentially. The distribution of the conduits 248 may be selected to reduce reliance on wicking for delivering liquid evenly to the absorbent layer 234 and to enable sufficient coverage longitudinally and circumferentially by the heater 323 to achieve a desired evaporation capability for vaporising the liquid. In Figure 2, the conduits 248 are pipes, which each comprise outlets arranged along the longitudinal axis of the barrel 202 to distribute the liquid more evenly along in a longitudinal direction. A liquid delivery arrangement for delivering the liquid to the absorbent layer 234 need not comprise pipes, though. For example, the liquid may be delivered by at least one conduit embedded in a structure of the barrel 202 (which may or may not be a pipe and may instead be a channel, for example). A portion of a barrel 302 for a haircare appliance is shown in cross-section in Figure 3 and in perspective view in Figure 4. The barrel 302 of Figures 3 and 4 is similar to the barrel 202 of Figure 2, but certain layers of the barrel 202 of Figure 2 are omitted in Figures 3 and 4 for clarity such that only a portion of the barrel 302 is shown in Figures 3 and 4. The barrel 302 of Figures 3 and 4 includes a heater (formed of four heating areas 332a-332d, and collectively referred to with the reference numeral 332), an absorbent layer 334 disposed over the heater and four conduits (one of which is labelled with the reference numeral 348 in Figures 3 and 4) to distribute the liquid to the absorbent layer 334. The barrel 302 also includes a cover layer disposed over the absorbent layer 334 (not shown in Figures 3 and 4). The heating areas 332a-332d are separated by the conduits 348, so that a respective conduit 348 is disposed between each pair of adjacent heating areas 332a-332d. The heating areas 332a-332d are each perforated heating areas 332a-332d, which comprise perforations (one of which is labelled with the reference numeral 350 in Figure 4) to allow air to flow through. As can be seen in Figure 4, the perforations 350 in the heating areas 332a-332d are arranged in evenly distributed longitudinal rows along a length of the barrel 302. The perforations 350 are also arranged in evenly distributed circumferential rows around a circumference of the barrel 302. The perforations 350 are circular in this example, but may be other shapes in other examples. In other examples, there may be a heater with open slots to allow conduits, such as water pipes, to reach a surface of the barrel. Each of the conduits 348 is a pipe which is elongate along a longitudinal axis of the barrel 302. The conduits 348 each comprise outlets (one of which is labelled with the reference numeral 352 in Figure 4) arranged along a length of the respective conduit 348 for the liquid to flow through to reach the absorbent layer 334. The outlets 352 are slits in Figure 4, but in other examples may have other forms such as circular holes or other openings. For example, an outlet of a conduit (such as at least one of the outlets 352) may comprise a one-way valve to permit the liquid to flow in a first direction out of the outlet and towards the absorbent layer 334, and inhibit fluid flow in a second direction opposite to the first direction. A oneway valve such as this may be pressure-activated, such as a duckbill valve. The outlets 352 are evenly distributed along the length of each of the conduits 348 so as to evenly distribute the liquid to the absorbent layer 334 in a longitudinal direction. The barrel 302 includes a mechanical support structure 354 (shown in Figure 3), which is formed by struts, with each strut supporting two opposing conduits 348. The mechanical support structure 354 aids the mechanical resilience of the barrel 302. The mechanical support structure 354 may be formed of a plurality of struts with the cross-sectional form shown in Figure 3, e g. disposed at regular intervals along the longitudinal axis of the barrel 302. Alternatively, a given strut may extend longitudinally so as to support opposing conduits longitudinally along at least part of the longitudinal axis of the barrel 302. Heaters such as the heaters 132, 232, 332 of Figures 1 to 4 may take the form of a thin-film or a thick-film structure. Figure 5 shows a portion of a barrel 402 comprising a thin-film structure, which in this example comprises an etched flexible printed circuit (FPC). The barrel 402 comprises a heater 432, an absorbent layer 434 disposed over the heater 432 and a cover layer 436 disposed over the absorbent layer 434. The heater 432 comprises a polyimide layer 442, an adhesive layer 444 and an aluminium layer 446. A heater trace 456 is disposed within the polyimide layer 442. Two holes 458, 460 are shown in Figure 5 as extending through both the heater 432 and the absorbent layer 434 to permit air to pass through the heater 432 and the absorbent layer 434. The cover layer 436 is a metal mesh, which is permeable to both air and vapour generated by vaporising liquid retained by the absorbent layer 434. Figures 6 to 8 show a portion of a barrel 502 comprising a thin-film structure according to another example. The portion of the barrel 502 of Figure 6 is shown in top view in Figure 7, and in cross-section through a plane P (labelled in Figure 7) in Figure 8. The barrel 502 comprises a heater 532, an absorbent layer 534 disposed over the heater 532 and a cover layer 536 disposed over the absorbent layer 534. The heater 532 comprises a polyimide layer 542, an adhesive layer 544 and an aluminium layer 546. A heater trace 556 and a sensor trace 562 are disposed within the polyimide layer 542. The sensor trace 562 is for example for distributing electrical signals to and / or from a sensor, such as the liquid detection system 138, the power sensor 139, and the barrel temperature sensor 140 of Figure 1. In a plane of the heater 532 parallel to a surface of the heater 532 facing the absorbent layer 534, the heater trace 556 has a generally sinusoidal profile, which is for example an oscillating, wave-shaped profile. The sensor trace 562 may have a similar profile to the heater trace 556 or a different profile. Two holes 558, 560 are shown in Figures 7 and 8 as extending through the heater 532 and the absorbent layer 534 to permit air to pass through the heater 532 and the absorbent layer 534. The cover layer 536 is a metal mesh, which is permeable to both air and vapour generated by vaporising liquid retained by the absorbent layer 534. Figure 9 shows a portion of a barrel 602 comprising a thick-film structure, which for example comprises printed traces on a metal substrate. The barrel 602 comprises a heater 632, an absorbent layer 634 disposed over the heater 632 and a cover layer 636 disposed over the absorbent layer 634. The heater 632 comprises a printed insulator layer 664 and an aluminium layer 646. A printed conductor 656 is disposed within the printed insulator layer 664, to form a heater trace. The heater 632 and absorbent layer 634 are permeable to air (e.g. by virtue of comprising perforations). The cover layer 636 is a metal mesh, which is permeable to both air and vapour generated by vaporising liquid retained by the absorbent layer 634. Figure 10 shows a portion of a barrel 702 comprising a thick-film structure according to a further example. The barrel 702 comprises a heater 732, an absorbent layer 734 disposed over the heater 732 and a cover layer 736 disposed over the absorbent layer 734. The heater 732 comprises a printed insulator layer 764 and an aluminium layer 746. A printed conductor 756 is disposed within the printed insulator layer 764, to form a heater trace. A printed sensor trace 762 is also disposed within the printed insulator layer 764. The heater 732 and absorbent layer 734 are permeable to air (e g. by virtue of comprising perforations). The cover layer 736 is a metal mesh, which is permeable to both air and vapour generated by vaporising liquid retained by the absorbent layer 734. Figure 11 shows a schematic top view of a heater trace 556 similar to the heater trace 556 partly shown in Figure 7; Figures 7 and 11 use the same reference numerals for corresponding features due to the similarities between the heater traces 556 in these Figures. The heater trace 556 is comprised by the barrel 502 of Figure 7, which comprises holes (two of which are labelled with the reference numerals 558, 560 in Figure 7) through the heater 532 and absorbent layer 534. Respective areas 566, 568 including the holes 558, 560 and adjacent portions of the heater trace 556 are indicated in Figure 11. Heating of a heater such as the heater 532 of Figure 11 can be achieved more rapidly by increasing coverage of the heater trace 556. A width and length of the heater trace 556 can be selected to achieve a desired resistance (which for example depends on a desired power dissipation and an available voltage). To enable adhesion of insulation surrounding the heater trace 556, it may be desirable to leave a particular clearance between the heater trace 556 and the perforations (one of which is labelled with the reference numeral 570 in Figure H). The perforations 570 of Figure 11 are relatively small to increase heat exchange with airflow passing through the perforations. In Figure 11, the perforations 570 (which in this case are holes) are evenly distributed across the barrel 502 and with a generally constant distance between the heater trace 556 and neighbouring perforations 570. This can allow more evenly distributed heat transfer to the airflow passing through the heater 532. Although not shown in Figure 11, it is to be appreciated that the barrel 502 may include at least one additional sensor, for example a surface mounted temperature sensor such as a PT 1000 or NTC thermistor. An additional copper trace layer between the heater trace 556 and the aluminium layer 546 (which for example acts as a heat spreader) may be used for temperature sensing. A width of a temperature sensing trace may be reduced to increase resistance (and hence sensitivity). Areas that are not covered by the heater trace 556 (such as an area in which an electrical connector is disposed) may absorb heat and cause a locally cooler area in an adjacent heater area. Areas that are not covered by the heater trace 556 may be reduced to reduce these effects. In the example of Figure 11, the heater 532 comprises a plurality of first longitudinal rows (two of which are labelled with the reference numerals 572a, 572b) of heater element portions (eg. corresponding to longitudinal sections of the heater trace 556 that extend along a length of the barrel 502). The first longitudinal rows 572a, 572b each extend along a longitudinal axis of the barrel 502. The perforations 570 of the heater 532 are also arranged in a plurality of second longitudinal rows (two of which are labelled with the reference numerals 574a, 574b), which each extend along a longitudinal axis of the barrel 502. Each of the first longitudinal rows 572a, 572b is interleaved between a respective pair of second longitudinal rows 574a, 574b, for example so that first and second longitudinal rows alternate across the barrel 502. The heater element portions (which in Figure 11 correspond with respective rows of the first longitudinal rows 574a, 574b) have a generally sinusoidal profile in a plane of the heater 532 parallel to a surface of the heater 532 facing the absorbent layer 534. Due to the generally sinusoidal profile of the heater element portions, and the even spacing of the perforations 570, there is an approximately constant spacing between the heater trace 556 and each of the perforations 570, along a length of the heater element portions. Figure 12 shows a further example of a heater 832, which may be used as the heater 132 in the haircare appliance 100 of Figure 1. In Figure 12, the heater 832 comprises a heater trace comprising two heater trace portions 831a, 831b, which are each elongate portions that extend along a longitudinal axis of the heater 832. The heater trace portions 831a, 831b are connected by a connecting portion 881 of the heater trace, which is disposed perpendicularly to the longitudinal axis of the heater 832. The heater 832 comprises heater terminals 875a, 875b for supplying an electrical current to the heater trace to heat the heater trace. The heater 832 comprises a perforated metal shim 878, which is arranged in contact with the heater trace and acts as a heat spreading element to spread heat generated by the heater trace across the barrel. In this example, the heater trace portions 83 la, 83 lb are unperforated and may be impermeable to an airflow. However, the metal shim 878 is perforated and comprises a plurality of airflow holes to permit airflow to flow through the heater 832 for application to hair wrapped around the barrel. Figure 13 illustrates a heater 1832 according to a further example. The heater 1832 of Figure 13 comprise similar elements to the heater 832 of Figure 12 and comprises four heater areas 1876a-1876d. The heater 1832 of Figure 13 comprises a structural element 1873, upon which other elements of the heater 1832 are arranged. The structural element 1873 is a generally cylindrical element to provide a structural core or backing for the other elements of the heater 832. In Figure 13, the structural element 1873 also comprises two struts (one of which is labelled with the reference numeral 1871 in Figure 13) extending outwardly, towards an exterior surface of the barrel. Each of the heater areas 1876a-1876d is formed by a stainless steel heater shim portion 1831 disposed on the structural element 1873, an electrical insulator portion 1879 disposed on the heater shim portion 1831 and a metal shim portion 1878 disposed on the electrical insulator portion 1879. The heater shim portions 1831 are each formed by a respective heater trace portion similar to the heater trace portions 831a, 831b of Figure 12. In Figure 13, the heater shim portions 1831 of different respective heater areas 1876a-1876d are thermally connected to each other by connecting portions similar to the connecting portion 881 of Figure 12 (although in other examples, at least one of the heater areas 1876a-1876d need not be thermally connected to at least one other heater area 1876a-1876d and may be operated independently). Each respective heater shim portion 1831 is configured to be heated upon supplying a current to the respective heater shim portion 1831. The metal shim portion 1878 acts as a heat spreader, similarly to the metal shim 878 of Figure 12, to spread heat generated by the heater shim portions 1831 across the heater 832, e g. to spread heat to areas of the metal shim portion 1878 in regions between adjacent heater shim portions 1831. There is a gap in the heater shim portion 1831 and the electrical insulator portion 1879 between adjacent heater areas 1876a-1876d to permit airflow generated within the heater 1832 to flow towards an interior surface of the metal shim portion 1878 in the regions between the adjacent heater areas 1876a-1876d. The metal shim portion 1878 is perforated in the regions between adjacent heater areas 1876a-1876d. An airflow 1880 can flow out of the gaps between the heater shim portion 1831 and the electrical insulator portion 1879 of adjacent heater areas 1876a-1876d, and through the perforations in the metal shim portion 1878 in these regions. The airflow 1880 can then flow out of the barrel, through an exterior surface of the barrel, and be applied to hair wrapped round the barrel of the haircare appliance comprising the heater 1832 during styling of the hair. If at least one of the heater areas 1876a-1876d is on, and the heater shim portion 1831 of the respective heater area is heated to a temperature above room temperature, heat will be transferred from the heater shim portion(s) 1831 in an on state to the metal shim portion 1878. The airflow 1880 passing through the perforations in the metal shim portion 1878 in this case is heated by the metal shim portion 1878 so as to generate a heated airflow. If the heater areas 1876a-1876d are off, the airflow 1880 passing through the perforations in the metal shim portion 1878 is unheated by the metal shim portion 1878, e.g. to generate a cooling airflow. Figure 14 shows an example of a barrel 902 comprising a heater 932 according to a further example. The barrel 902 of Figure 14 comprises a blade and slot arrangement comprising barrel blades that are solid (eg. non-perforated), with airflow 982 passing between longitudinal slots between neighbouring barrel blades. Each barrel blade comprises a heater 932, an absorbent layer 934 disposed on the heater 932 and a cover layer 936 disposed on the absorbent layer 934, which may be otherwise similar to the heater 132, absorbent layer 134 and cover layer 136 described in other examples (except for a lack of perforations). As explained with reference to Figure 1, a predefined amount of liquid may be delivered to the absorbent layer, for example to obtain a desired styling effect without hair becoming too wet to hold the style. Various arrangements may be used to store and deliver the liquid within the haircare appliance in a simple manner, as will now be explained with reference to Figures 15 to 20. In some examples, in which a reservoir is used to store the liquid to be delivered to the absorbent layer, the reservoir comprises a chamber for storing the liquid, and a volume of the chamber is adjustable in dependence on a volume of fluid stored in the chamber. The fluid for example comprises the liquid and may also comprise further fluid (such as gas introduced into the chamber). Figures 15, 16, 18 and 19 show examples with chambers with adjustable volumes. In Figure 15, a reservoir 1110 comprises a rigid case 1184 and a deformable pouch 1186 disposed within the rigid case 1184. The deformable pouch 1186 forms the chamber for storing the liquid and is coupled to an outlet 1188, which is fluidly connected to at least one further component of a liquid delivery arrangement for delivering liquid to the absorbent layer (such as the pump 116 of Figure 1, or pipes 122 for delivering the liquid to a pump). The deformable pouch 1186 is for example a soft pouch, which expands or contracts in volume depending on the quantity of fluid stored within the pouch. The deformable pouch 1186 is protected by the rigid case 1184, which for example forms a solid outer shell. The deformable pouch 1186 can be refilled by adding additional liquid to the deformable pouch 1186 via the outlet 1188. After refilling the deformable pouch 1186, some gas (e g. air) may remain in the deformable pouch 1186, which may reduce the amount of liquid delivered, eg. upon operation of a pump for a predefined rate for a predefined amount of time. To mitigate this, the haircare appliance 100 may comprise a gas removal arrangement operable to remove gas from within the reservoir 1110 (in this case, from within the deformable pouch 1186) and / or the liquid delivery arrangement. For example, the gas removal arrangement may comprise the same pump 116 that is used to deliver the liquid to the absorbent layer 134, which may be a peristaltic pump or a diaphragm pump, which may be used to pump gas out of the reservoir 1110 (although in other cases, separate pumps may be used for gas and liquid). To remove trapped gas, a gas removal procedure (referred to as a purge procedure) may be performed in which the haircare appliance 100 (or a portion thereof) is maintained at a position to allow the gas to be removed from the deformable pouch 1186, e g. using the gas removal arrangement or by manually or otherwise mechanically deforming the deformable pouch 1186 until the gas is expelled. For example, the gas removal arrangement may comprise the liquid detection system 138 to detect whether liquid is present within the reservoir 1110 and / or the liquid delivery arrangement. The gas removal arrangement may be operable to commence removal of the gas in dependence on the liquid detection signal. For example, if the liquid detection signal indicates that liquid is not present in a particular part of the liquid delivery arrangement, this may indicate that gas is trapped in the liquid delivery arrangement, and that the gas is to be removed. The gas removal arrangement may be controlled by the control system 114, which may receive the liquid detection signal and generate a control signal in response, for controlling the gas removal arrangement. The control system 114 may further determine when to cease operation of the gas removal arrangement to remove gas, based on the liquid detection signal at a subsequent time. For example, if the liquid detection signal senses liquid within the conduits of the liquid delivery arrangement for delivering liquid from the pump 116 to the absorbent layer 134, it may be inferred that liquid is present in the reservoir 1110 and that gas is substantially absent from within the reservoir 1110. In other cases, though, the amount of liquid present in the reservoir 1110 may be detected directly by the liquid detection system 138 (which may comprise a component, or be located, within the reservoir 1110). In these cases, the liquid detection system 138 may be or comprise a capacitive sensing system to detect liquid within a conduit or the reservoir 1110, or a load detector (e.g. to sense current drawn by a motor of the pump 116) to detect liquid in the pump 116. With the gas removal arrangement operable in this manner, the liquid detection system may be operated continuously or intermittently so as to purge gas from the liquid delivery arrangement and / or reservoir 1110 as desired. In other cases, though, the haircare appliance 100 may be configured with a dedicated automated process that a user can initiate, for example via operating a manual switch on the haircare appliance 100 (such as the gas removal button 117 of Figure 1) or by selecting a particular menu item on a smartphone application for controlling the haircare appliance 100 remotely. In this way, the user can instruct the operation of the gas removal arrangement (such as the pump 116) for a predefined amount of time, which may be an amount of time that is selected to be sufficient to adequately remove gas from the haircare appliance 100. Figure 16 shows a reservoir 1210 comprising a chamber 1290 comprising a first end 1292 and a second end 1294 opposite to the first end 1292. An outlet 1288 is disposed in the second end 1294 and is fluidly coupled to the liquid delivery arrangement. The second end 1294 is moveable along a direction 1296 parallel to a longitudinal axis of the barrel. The second end 1294 forms a plunger, which is moveable closer to and further from the first end 1292 along the direction 1296 so as to decrease and increase the volume of the chamber 1290, respectively. The reservoir 1210 may comprise an inlet 1298 to avoid resetting the position of the second end 1294 between refills of the chamber 1290, by allowing the chamber 1290 to be refilled from the opposite side and reinserted into the haircare appliance 100 from the opposite direction. With this reservoir 1210, the liquid detection system may comprise a limit switch connected to the second end 1294, to allow a position of the second end 1294 (and hence a likely amount of gas within the reservoir 1210) to be inferred. Figure 17 shows a further example of a reservoir 1310 that may be used as the reservoir 110 of the haircare appliance 100 of Figure 1. In Figure 17, a weighted tube 1397 is disposed within the reservoir 1310. The weighted tube 1397 is fluidly connected to the liquid delivery arrangement by an outlet 1388 of the reservoir 1310. The weighted tube 1397 is formed of a thin tube with a weight 1399 at an end distal from the outlet 1388. The weight 1399 enables an inlet to the weighted tube 1397 (at the end comprising the weight 1399) to remain closer to the ground than an outlet of the reservoir 1310, irrespective of an orientation of the reservoir 1310. This increases a likelihood that the liquid within the reservoir 1310 (rather than gas) will be delivered to the absorbent layer. The reservoir 1310 also includes a oneway valve 1398 at an opposite end to the outlet 1388, to allow gas to enter the reservoir 1310 to avoid creating a vacuum in the reservoir 1310. Figures 18 and 19 show further examples of reservoirs 1210a, 1210b that may be used as the reservoir 110 of the haircare appliance 100 of Figure 1. The reservoirs 1210a, 1210b of Figures 18 and 19 each comprise chambers 1290a, 1290b with an adjustable volume, similarly to the reservoir 1210 of Figure 16. Features of Figures 18 and 19 that correspond to features of Figure 16 are labelled with the same reference numeral but appended by an “a” and a “b” respectively; corresponding descriptions are to be taken to apply. The chambers 1290a, 1290b of the reservoirs 1210a, 1210b of Figures 18 and 19 are each partly formed by a rigid container, which each have a generally cylindrical form. The chambers 1290a, 1290b each comprise an outlet 1288a, 1288b in an end (the top end of the chambers 1290a, 1290b in the Figures) which is fluidly coupled to the liquid delivery arrangement to allow the liquid to be delivered from the reservoirs 1210a, 1210b to the absorbent layer 134. Each of the reservoirs 1210a, 1210b comprises a deformable element 1295a, 1295b. The deformable elements 1295a, 1295b are each deformable to adjust the volume of the respective chamber 1290a, 1290b. In Figure 18, the deformable element 1295a is a pouch, which is collapsible along a direction 1269a to increase the volume of the chamber 1290a and expandable to decrease the volume of the chamber 1290a. In Figure 19, the deformable element 1295b is a flexible membrane, which curves inwardly, within the chamber 1290b, towards the outlet 1288b in the arrangement shown in Figure 19. The curvature of the flexible membrane is adjustable depending on the amount of fluid in the chamber 1290b so as to adjust the volume of the chamber 1290b. In Figure 19, the extent of curvature of the flexible membrane decreases to increase the volume of the chamber 1290b (and vice versa). This causes a portion of the flexible membrane closest to the outlet 1288b to move towards the outlet 1288b (along a direction 1296b shown in Figure 19) to decrease the volume of the chamber 1290b, and to move away from the outlet 1288b along the direction 1296b to increase the volume of the chamber 1290b. The chamber 1290b comprises an air inlet 1293b to allow air to be inserted into the rigid container at an opposite side of the flexible membrane to the liquid (which is between a side of the flexible membrane facing the outlet 1288b and the outlet 1288b itself). The amount of air disposed at this side of the flexible membrane can be used to adjust the extent of curvature of the flexible membrane, e.g. by supplying more air to increase the extent of curvature of the flexible membrane towards the outlet 1288b to decrease the volume of the chamber 1290b, and vice versa. At least one conduit for delivering liquid from a pump, such as the pump 116 of Figure 1, to the absorbent layer may be arranged in various ways, as shown in Figures 20 and 21, which show schematic unwrapped views of conduits according to various examples (with generally tubular heaters, and conduit(s) disposed therein, unwrapped and presented in plan view). Figure 20 shows an example of a conduit 1438 comprising parallel conduit sections distributed circumferentially about a surface of a heater 1432, which each comprise outlets 1452 and are connected to the pump 116 via a common inlet pipe 1431. Figure 21 shows a further example of a conduit 1538 comprising outlets 1552. The conduit 1538 of Figure 21 comprises conduit sections in series across a surface of a heater 1532. The conduit sections in series are formed by a single conduit 1538, which is wound up and down the length of the barrel, with respective conduit sections at different respective positions circumferentially about the barrel (and about a circumference of the heater 1532). Figure 22 shows an unwrapped view of outlets 1652 of a conduit according to examples. An area 1633 around a given outlet 1652 is also illustrated in Figure 22. The outlets 1652 are shown schematically in Figure 22 as cross-shaped. However, in practice, the outlets 1652 may take a different form, such as a circular perforation or a slit. In this example, the outlets 1652 are arranged in a plurality of longitudinal rows 1635a-1635c that extend along the length of the conduit. Outlets of a given row of the longitudinal rows are offset from the outlets of at least one adjacent row, e.g. so that outlets of the second row 1635b are offset, e.g. staggered, from the outlets of the first and second rows 1635a, 1635c, to more efficiently distribute the liquid. Evenly spaced adjacent outlets and evenly spaced radial distances between adjacent conduits may further aid in evenly distributing the liquid to the absorbent layer. Figure 23 is a flow diagram 1700 of a predefined styling procedure according to an example, which may be implemented by a haircare appliance 100 in accordance with examples herein. The predefined styling procedure is an automated process comprising multiple phases which are controlled automatically by a control system 114 of the haircare appliance 100 after the predefined styling procedure is initiated (as indicated by a start box 1702 in the flow diagram 1700). To use the predefined styling procedure of the flow diagram 1700, hair is first wrapped around a barrel 102 of the haircare appliance 100. The hair is wrapped manually around the barrel 102. In some examples, the haircare appliance 100 comprises a tong (not shown in Figure 1) to clamp hair to the barrel 102, in use, which may provide flexibility for users desiring to curl their hair using tongs. A tong may be mechanically connected to the body 104 of the haircare appliance 100. A spring system (or other functionally similar system) may be biased to maintain the tong in a closed position in which the tong is clamped to the body 102. The spring system allows the tong to be opened and automatically closes the tong to the barrel 102 upon release of the spring system, so as to clamp the hair to the barrel 102 while styling the hair. The hair is typically dry when it is wrapped around the barrel 102 to reduce the risk of the hair being overwetted during the predefined styling procedure. The predefined styling procedure is started 1702 for example via a suitable interface such as the start button 113 or via a software application (“app”) configured to send an initiation signal to the network interface 115 in response to initiation of the predefined styling procedure using the app. This is described in more detail with reference to Figure 1. After initiation of the predefined styling procedure, a vapour generation phase 1704 of the predefined styling procedure is performed. In the vapour generation phase 1704, the haircare appliance 100 is configured, by the control system 114, to generate a vapour, using the vapour generation system, and to emit the vapour out of the haircare appliance 100 through outlets of the barrel 102 (such as perforations in the barrel 102 or gaps between barrel blades as described in examples above). The vapour generation phase 1704 may begin with a wetting phase, in which the liquid (e.g. a predefined amount of the liquid) is delivered from the reservoir 110 to the absorbent layer 134, via the liquid delivery arrangement. There may be a small amount of variation in an amount of liquid deposited on the absorbent layer 134, for example so that the predefined amount of the liquid is deposited within acceptable operational tolerances (which may define the predefined amount of the liquid as a range of acceptable liquid amounts corresponding to the predefined amount of the liquid). The haircare appliance 100 may comprise a sensor, such as the liquid detection system 138, to detect the amount of the liquid delivered to the absorbent layer 134. The liquid detection system 138 may generate a liquid retention signal indicative of an amount of the liquid retained by the absorbent layer 134, in addition to or instead of the liquid detection signal discussed above. The liquid retention signal may be sent to the control system 114 and used by the control system 114 to adjust a respective length of at least one phase of the predefined styling procedure to account for variations in the amount of liquid delivered to the absorbent layer 134, e.g. so that the liquid is substantially fully evaporated during the vapour generation phase 1704. For example, the control system 114 may determine a length of time for the configuring the haircare appliance 100 in the wetting phase based on the liquid retention signal, so as to enable a desired amount of the liquid to be retained by the absorbent layer 134. In other examples, though, the wetting phase may be omitted from the predefined styling procedure or may be performed prior to the predefined styling procedure. For example, the haircare appliance 100 need not comprise a reservoir 110. In such cases, liquid may be delivered to the absorbent layer 134 via a manual or automated spray or by passing a wet brush over a surface of the barrel 102. The user may wet the absorbent layer 134 in between each curl (e g. in between each execution of the predefined styling procedure). After the wetting phase (if present), the vapour generation phase 1704 comprises a vaporisation phase, which may be referred to as a heating phase and can be considered to be a rapid or flash steaming phase in which vapour is rapidly applied to the hair by vaporising the liquid retained by the absorbent layer 134. In the vaporisation phase, the control system 114 configures the heater 132 in an on state, in which the heater 132 is heated to a sufficiently high temperature to vaporise the liquid retained by the absorbent layer 134. The control system 114 may control an electrical power supplied to the heater 132 so as to heat the heater 132 to a desired temperature for vaporisation. The desired temperature may be selected to reduce power consumption without unduly compromising styling performance. For example, the control system 114 may maintain the heater 132 so that a surface of the barrel 102 in contact with hair (such as the cover layer 136 for the haircare appliance 100 of Figure 1), is at a temperature of less than or equal to about 120 degrees Celsius. The heater 132 may be heated to a temperature (referred to as a set barrel temperature) which is above a desired temperature for the surface of the barrel 102. However, the absorbent layer 134 typically acts as a buffer, and absorbs energy by evaporating the liquid into vapour so that the temperature of the surface of the barrel 102 (referred to in the context of Figure 23 as the surface temperature) is less than the set barrel temperature. For example, whereas the set barrel temperature may be above 100 degrees Celsius, the surface temperature may remain below 100 degrees Celsius during the vapour generation phase 1704. The relatively high temperature of the heater 132 rapidly heats liquid in contact with or adjacent to the heater 132 (e g. due to proximity between the heater 132 and the absorbent layer 134 retaining the liquid) so as to rapidly vaporise the liquid. This may allow the vapour generation phase 1704 to be performed in less than around 10 seconds. The vapour produced moves outwards, through the cover layer 136 and onto the hair. The vapour penetrates and condenses onto the hair, delivering condensation energy and moisture, which both aid in styling by breaking hydrogen bonds in the hair and making the hair more malleable. The airflow generator 112 is typically in an off state during the vapour generation phase 1704 to avoid blowing the vapour away from the hair before the vapour has had an opportunity to condense on the hair. As the vapour generation phase 1704 comes to an end, e.g. as substantially all of the liquid is evaporated from the absorbent layer 134, the surface temperature of the barrel 102, e.g. corresponding to the temperature of an outer surface of the cover layer 136, begins to rise, and may rise gradually above 100 degrees Celsius. Once the surface temperature reaches the set barrel temperature, the power drawn by the heater 132 may drop noticeably. These phenomena can be detected by appropriate sensor(s) of the haircare appliance 100, and used by the control system 114 to trigger a subsequent, airflow generation phase 1706 of the predefined styling procedure. For example, the control system 114 may be configured to control the operation of the haircare appliance 100 to end the vapour generation phase 1704 based on a power signal indicative of a power consumed by the heater 132, as obtained by the power sensor 139. The control system 114 may be configured to control the operation of the haircare appliance 100 to end the vapour generation phase 1704 in response to the power signal indicating that a reduction in the power consumed by the heater 132 exceeds a threshold power reduction and / or the power consumed by the heater 132 is less than a threshold power. The control system 114 may also or instead be configured to end the vapour generation phase 1704 based on a temperature signal indicative of temperature of the barrel 102, e.g. as sensed by a temperature sensor such as the barrel temperature sensor 140. For example, the control system 114 may end the vapour generation phase 1704 in response to the barrel temperature sensor 140 exceeding a threshold temperature. In other examples, though, the control system 114 may configure the haircare appliance 100 in the vapour generation phase 1704 for a predefined length of time, which may be more straightforward than adjusting a length of time of the vapour generation phase 1704 based on at least one sensor output. After the vapour generation phase 1704, the control system 114 configures the haircare appliance 100 in the airflow generation phase 1706, in which the haircare appliance 100 is configured to generate an airflow, using the airflow generator 112, and to emit the airflow out of the haircare appliance 100 through the outlets of the barrel. The airflow may be used to dry the hair and / or set the style. The control system 114 is configured to turn the airflow generator 112 on in the airflow generation phase 1706. In an example, the airflow generation phase 1706 comprises a heated airflow generation phase, in which the heater 132 is configured, by the control system 114, to heat the airflow generated by the airflow generator 112. This may aid drying of the hair, to remove excess moisture from the hair so as to improve the longevity of the style. In the heated airflow phase, the airflow generator 112 may provide an airflow through the barrel 102 and out of the outlets in the barrel 102 with a flow rate (referred to as a first flow rate) of between around 1 and 5 litres per second. The heater 132 serves as an air heater during the heated airflow generation phase, due to heat exchange with the air. As a result, air exiting the outlets of the barrel 102 are sufficiently warm to dry the hair. The airflow emitted out of the haircare appliance 100, through the outlets of the barrel 102, may be at a temperature (referred to as a first temperature) of between around 75 and 120 degrees Celsius, which is sufficient to dry the hair without damaging the hair. To determine whether the hair is sufficiently dry to end the heated airflow generation phase, a capacitive sensor may be used to detect a liquid content of the hair in contact with the capacitive sensor. The heated airflow generation phase may be ended by the control system 114 in response to a signal obtained by the capacitive sensor (and provided to the control system 114) indicating that the hair is sufficiently dry. In other examples, the predefined styling procedure may comprise a predefined length of time in the heated airflow generation phase or the control system 114 may be configured to determine a length of time for configuring the haircare appliance 100 in the heated airflow generation phase based on a length of time in the vapour generation phase 1704. For example, a duration of the heated airflow generation phase may be proportional to a duration of the vapour generation phase 1704, as a longer vapour generation phase 1704 typically delivers more water to the hair, so will take longer to dry. In addition or instead, the heated airflow generation phase may be ended based on a temperature signal obtained by a temperature sensor (such as the barrel temperature sensor 140) indicative of the temperature of the barrel, e g. if the temperature of the barrel exceeds a first predefined threshold temperature, such as 120 degrees Celsius. For example, a desired amount time for the heated airflow generation phase may be obtained by the control system 114 (eg. as a predefined length of time, or a length of time that is proportional to a length of time in the vapour generation phase 1704). The actual length of time the control system 114 configures the heated appliance 100 in the heated airflow generation phase may, however, be less than this, for example if the temperature of the barrel exceeds the first predefined threshold temperature before the desired amount of time has elapsed. The heated airflow generation phase (which may be referred to as a “hot shot”) may be followed by a cooling airflow generation phase of the airflow generation phase 1706 (which may be referred to as a “cold shot”). A first temperature of the airflow emitted out of the haircare appliance 100 in the heated airflow generation phase is higher than a second temperature of the airflow emitted out of the haircare appliance 100 in the cooling airflow generation phase. For example, the cooling airflow generation phase may be used to lower a temperature of the hair to help set the style by reducing the malleability of the hair. Lowering the temperature of the haircare appliance 100 may also make the haircare appliance 100 safer to handle. The first temperature may be at least 25 degrees higher than the second temperature. For example, for a first temperature of between 75 and 120 degrees Celsius, the second temperature may be less than 60 degrees Celsius. To obtain the cooling airflow generation phase, the control system 114 may configure the heater 132 in an off state and the airflow generator 112 in an on state, so as to draw room temperature air into the haircare appliance 100, through the barrel 102 and out of the outlets in the barrel 102 so as to cool the barrel 102 and the hair wrapped around the barrel 102. The control system 114 may configure the airflow generator 112 to generate an airflow at a second flow rate in the cooling airflow generation phase, which may be higher than the first flow rate in the heated airflow generation phase. For example, the second flow rate may be greater than 5 litres per second. The predefined styling procedure may comprise a predefined length of time in the cooling airflow generation phase and / or the cooling airflow generation phase may be ended based on a temperature signal obtained by a temperature sensor (such as the barrel temperature sensor 140) indicative of the temperature of the barrel, eg. if the temperature of the barrel is less than a second predefined threshold temperature, such as 60 degrees Celsius. Following termination of the cooling airflow generation phase, the predefined styling procedure finishes 1708. The hair may be released, e g. unwrapped, from the barrel 102. The control system 114 may configure the heater 132 to attain a predefined temperature (e.g. corresponding to a predefined idle temperature) upon ending the airflow generation phase 1706. The predefined temperature may be selected to enable a rapid transition to vaporising the liquid, while still allowing comfortable handling of the haircare appliance 100 during wetting and / or wrapping of the hair for subsequent styling. The predefined temperature may for example be between around 40 and 60 degrees Celsius. The control system 114 may configure the heater 132 to attain the predefined temperature immediately prior to the vapour generation phase 1704, during the vapour generation phase 1704 and / or upon ending the airflow generation phase 1706 to improve the efficiency of performing the predefined styling procedure. Figure 24 is a plot 1800 showing a surface temperature of the barrel 102 (e.g. corresponding to a temperature of the airflow emitted out of the outlets of the barrel 102 such as the outlets in the cover layer 136 of the barrel 102 and which also corresponds to a hair temperature if hair is wrapped around the barrel 102) over time, during performance of the predefined styling procedure. Initially, the hair appliance 100 is configured in an off state, in which the heater 132 is off and power is not supplied to the hair appliance 100. At this time, a surface of the barrel 102 is at a first temperature, Ti, which corresponds to room temperature, such as around 25 degrees Celsius. At a time -t, the power to the hair appliance is switched on, and the heater 132 is heated to the predefined temperature (corresponding to the idle temperature), so as to generate a surface temperature of T2, which is a second temperature of e g. around 60 degrees Celsius. With the surface of the barrel 102 at the second temperature, the hair is wrapped around the barrel by the user. The predefined amount of the liquid may be delivered to the absorbent layer 134 at this time so that the absorbent layer 134 is pre-loaded with the liquid. The vapour generation phase of the predefined styling procedure thus need not comprise the wetting phase, which may be performed prior to the predefined styling procedure. In other examples, though, the vapour generation phase may include the wetting phase, which may be performed while the heater 132 is being heated to a particular temperature to achieve vaporisation of the liquid. At a time 0, the predefined styling procedure is initiated. At this time, the heater 132 is configured in the on state, to heat the surface of the barrel 102 to a third temperature, T3, of around 95 degrees, and the airflow generator 112 is in the off state. It takes around 2 seconds for the surface of the barrel 102 to be heated to the third temperature. At this time, the liquid retained by the absorbent layer 134 is vaporised to generate a vapour. The vapour generation phase lasts for a predefined length of time of 10 seconds in this case, from time = 0, to time = 10 seconds. At time = 10 seconds, the airflow generation phase begins, and the airflow generator 112 is configured in the on state by the control system 114. In this example, the airflow generation phase comprises a heated airflow generation phase from time = 10 seconds to time = 15 seconds, and a cooling airflow generation phase from time = 15 seconds to time = 20 seconds. In the heated airflow generation phase, airflow is drawn through the barrel 102 at the first flow rate of around 4 litres per second, and the heater 132 is maintained at the same temperature as in the vapour generation phase so as to maintain the surface temperature of the barrel at around 95 degrees Celsius. In the cooling airflow generation phase, airflow is drawn through the barrel 102 at the second flow rate of around 5 litres per second, and the heater 132 is configured in the off state by the control system 114 to cause the surface temperature of the barrel to fall to around the second temperature of 60 degrees. The predefined styling procedure ends after 20 seconds (at time = 20 seconds), at which point the haircare appliance 100 is sufficiently cool to be safe to handle by a user. A user can then perform the predefined styling procedure again for a subsequent section of hair to be styled. Figure 25 shows a plot 1900 of surface temperature and heater power consumption over time, during execution of a predefined styling procedure according to an example. The plot 1900 illustrates a set temperature 1902 of the barrel 102, a surface temperature 1904 of the heater 132 and a surface temperature 1906 of the absorbent layer 134 over time. The plot 1900 also shows a power consumed 1908 by the airflow generator 112 and a power consumed 1910 by the heater 132 overtime. Between time = -t and time = 0, the haircare appliance 100 is in an idle mode, in which the barrel 102 is comfortable to touch and not warm enough to start vaporising liquid within the absorbent layer 134. In this mode, the heater 132 is at a high enough temperature to allow vaporisation to be begin rapidly after starting the predefined styling procedure. In this phase, the user can section their hair, wet the barrel 102 and wrap their hair on the barrel 102. At time = 0, the predefined styling procedure is initiated, for example by the user pressing the start button 113. This causes initiation of the vapour generation phase. The heater 132 is configured by the control system 114 to reach a temperature of 150 degrees Celsius. However, due to evaporation of the liquid, the heater 132 typically attains a temperature of less than the temperature set by the control system 114 (referred to as a set temperature) and, in this case, reaches a temperature of just below 150 degrees Celsius. The surface temperature of the absorbent layer 134 attains a temperature of just below 100 degrees Celsius while the bulk of the liquid on the barrel 102 is evaporated. As the heater 132 is typically unable to attain the set temperature, the power consumed by the heater 132 will tend to a maximum power available, which is 300 Watts (W) in Figure 25. In other examples, though, the maximum power available may be greater than 300W, such as 400W, 450W or higher. In general, a higher maximum power results in an improved styling performance of the haircare appliance, although the relationship between performance improvement and maximum power is typically not linear. As most of the liquid in the absorbent layer 134 is vaporised, the surface temperature of the barrel 102 (e g. corresponding to the absorbent layer 134 temperature) starts to rise towards the set temperature and the power demand on the heater 132 decreases as it takes less energy to maintain temperature of the barrel 102 than to evaporate the liquid from the absorbent layer 134. The increase in the surface temperature of the barrel 102 and / or the decrease in the power consumed by the heater 132 are detected. It can then be inferred that a sufficient quantity of the vapour has been vaporised from absorbent layer 134 (e g. based on surface temperature of the barrel 102, power consumption of the heater 132 and / or time elapsed) that the vapour generation phase can be ended at time = ti. At this time, the airflow is turned on and the heated airflow generation phase commences. The airflow gradually cools the barrel 102 and the power consumed by the heater 132 ramps up, to attempt to return the heater 132 to the set temperature. However, due to the airflow, the heater 132 does not attain the set temperature of 150 degrees Celsius, but the power consumption is pushed, by the control system 114, to a maximum of 300W in total (e.g. around 250W for the heater 132 plus around 50W for the motor of the airflow generator 112). This typically produces an airflow temperature of airflow emitted from the surface of the barrel 102 of just below 100 degrees Celsius, depending on the rate of the airflow (such as around 95 degrees Celsius for a flow rate of around 3 litres per second). After drying the hair, the cooling airflow generation phase is commenced at time = t2. At this time, the heater power is turned to zero and the airflow generator 112 remains in an on state and may increase the flow rate to speed up cooling. When the temperature of the surface of the barrel 102 is below a desired temperature (such as below around 40 to 60 degrees Celsius), the airflow is turned off at time = t3, at which point the predefined styling procedure ends. The user may then unwrap their hair from the device. At this point, the haircare appliance reverts to the idle mode of time = -t, so it is ready for a subsequent iteration of the predefined styling procedure at time = U In an example in accordance with Figure 25, the predefined styling procedure comprises a vapour generation phase of 10 seconds, with a set temperature of 150 degrees Celsius, an actual heater temperature of less than 150 degrees Celsius, an absorbent layer temperature of around 100 degrees Celsius, and 0.5g to 1g of liquid delivered to the absorbent layer 134. Subsequently, the predefined styling procedure of this example comprises a heated airflow generation phase of 8 seconds, with a set temperature of 150 degrees Celsius, an actual heater temperature of around 110 degrees Celsius, an absorbent layer temperature of around 100 degrees Celsius, and an airflow temperature of airflow emitted out of the barrel 102, through the outlets in the barrel, of around 95 degrees Celsius. The predefined styling procedure of this example comprises, subsequently, a cooling airflow generation phase of 2 seconds, with a set temperature of 0 degrees Celsius, an actual heater temperature of less than 70 degrees Celsius, an absorbent layer temperature of less than 70 degrees Celsius, and an airflow temperature of airflow emitted out of the barrel 102, through the outlets in the barrel, of less than 60 degrees Celsius. Further examples are envisaged. In Figure 1, the haircare appliance 100 comprises the absorbent layer 134 and the cover layer 136. An absorbent layer and a cover layer are examples of an absorbent portion and a cover portion, respectively. A “portion” as referred to herein may be a layer or another structure. In examples, a material that is laminated may be considered to comprise two layers (a material layer and a laminate layer), which may each correspond to one of the absorbent or cover layers (or portions). The absorbent layer 134 and the cover layer 136 need not be continuous around a circumference of the barrel 102. For example, the absorbent layer 134 may have gaps therein over which the cover layer 136 is disposed. Figures 26 to 28 illustrate a barrel 2002 of a haircare appliance 2000 according to a further example. The barrel 2002 may be attached to a body which is the same as or similar to the body 104 of Figure 1 to form the haircare appliance 2000. The structure of the barrel 2002 is similar to the structure of the barrel 102 of the example of Figure 1, except that the barrel 2002 does not comprise the cover layer 136 of Figure 1. The barrel 2002 comprises an absorbent layer 2034 comprising a plurality of holes 2051, which allow an airflow to escape from within the barrel 2002. The holes 2051 are arranged in elongate rows along a longitudinal axis 2041 of the barrel 2002. The rows are evenly distributed circumferentially about the barrel 2002. The absorbent layer 2034 is configured to absorb a liquid provided thereto (e.g. by a liquid delivery arrangement or by manual application), which can be vaporised by a heater of the barrel 2002 (not shown in Figures 26 to 28) beneath the absorbent layer 2034 to generate a vapour. For example, the absorbent layer 2034 may be a porous layer as described with reference to Figure 1. Upon generating the vapour, the vapour is emitted from the absorbent layer 2034 and out of the haircare appliance 2000. The vapour is generated within the absorbent layer 2034 and passes through an exterior surface of the absorbent layer 2034 (facing away from the heater) in exiting the haircare appliance 2000. The barrel 2002 also comprises a plurality of ribs (one of which is labelled with the reference numeral 2037 in Figures 26 to 28). The ribs 2037 extend along the longitudinal axis 2041 of the barrel 2041. In this example, the ribs 2037 are rigid axial splines which are formed of plastic. Although the ribs 2037 extend generally longitudinally along the longitudinal axis 2041, they need not be precisely aligned with the longitudinal axis 2041 and may, for example, be disposed at a small angle with respect to the longitudinal axis 2041. The ribs 2037 are substantially evenly distributed about a circumference of the barrel 2002, e g. evenly distributed within manufacturing and / or measurement tolerances. In Figures 26 to 28, an exterior surface 2035 of the barrel 2002 is formed by an exterior surface of the absorbent layer 2034 between the ribs 2037 and a portion of an exterior surface of the ribs 2037 that is exposed to an exterior of the haircare appliance 2000. The ribs 2037 protruding from the barrel 2002 form a relatively smooth surface to aid in withdrawing a tress of hair from the barrel 2002 once styling is complete. Figures 27 and 28 show a simplified cross-sectional view of the barrel 2002 of Figure 26 along the line A-A. In Figures 27 and 28, only the absorbent layer 2034 and five of the ribs 2037 are shown, for ease of illustration, and underlying layers of the barrel 2002, beneath the absorbent layer 2034 and ribs 2037, are omitted. In the example of Figures 26 to 28, the ribs 2037 are retractable relative to the absorbent layer 2034 to adjust an extent to which a cross-section of each of the ribs 2037, in a plane parallel to the longitudinal axis 2041 of the barrel 2002 (e g. the plane in which the cross-section is shown in Figures 27 and 28), protrudes from the absorbent layer 2034. Figure 27 shows the ribs 2037 with a first extent of protrusion and Figure 28 shows the ribs 2037 with a second extent of protrusion. In Figure 27, the ribs 2037 are retracted such that there is a negligible extent of protrusion of the ribs 2037 relative to the absorbent layer 2034. In other cases, though, the first extent of protrusion, which may be a minimum extent of protrusion, may be a zero protrusion or a non-zero amount that is nevertheless smaller than the second extent of protrusion. The second extent of protrusion is greater than the first extent of protrusion and e g. corresponds to a maximum extent to which the ribs 2037 protrude from the absorbent layer 2034. As the skilled person will appreciate, the barrel 2002 may further comprise a suitable actuation mechanism for actuating the retraction of the ribs 2037. The actuation mechanism may be electronically controllable, e.g. via a control system as described herein, and / or manually controllable, e g. via a suitable button or slider. The extent of protrusion of the ribs 2037 may be controlled based on an intended use of the barrel 2002. For example, the second extent of protrusion may be used when hair is to be removed from around the barrel 2002 and / or the first extent of protrusion may be used during use of the barrel 2002 to apply a vapour and / or airflow to hair wrapped around the barrel 2002. In further examples, a barrel may have the same structure as the barrel 2002 of Figures 26 to 28, but with a cover layer, e.g. a mesh layer, disposed over the absorbent layer 2034 and the ribs 2037. Alternatively, a barrel may have the same structure as the barrel 2002 of Figures 26 to 28 but with a cover portion disposed over respective portions of the absorbent layer 2034 (which may be considered absorbent portions) between adjacent pairs of ribs 2037. In such cases, the cover portion may not cover or be otherwise disposed over the ribs 2037 (or at least one of the ribs 2037). The ribs 2037 need not be retractable. Instead, the ribs 2037 may provide surface texture, for example, and may correspond to moulded ribs or bumps adhered to a base of the absorbent layer 2034 (e g. adhered to a plastic base layer of the absorbent layer 2034, which itself may comprise a plurality of layers). The ribs 2037, whether retractable or not, may protrude up to 1 millimetre from the absorbent layer 2034 (i.e. so that a distance between a furthest point on a given rib 2037 and the absorbent layer 2034, in a direction perpendicular to a surface of the absorbent layer 2034 facing the given rib 2037 is less than or equal to 1 millimetre). The ribs 2037 may for example be formed of a single material and may be in the form of generally cylindrical rods.

Claims

1. A barrel for a haircare appliance, the barrel comprising:a heater;an absorbent layer for retaining liquid, the absorbent layer disposed over the heater; andan exterior surface,wherein the heater is operable to heat the absorbent layer to vaporise the liquid retained thereby to generate a vapour for transmission out of the barrel, through the exterior surface, andthe heater, the absorbent layer and the exterior surface are configured to permit air within the barrel to flow out of the barrel, through the heater, the absorbent layer and the exterior surface.

2. The barrel of claim 1, wherein the heater is configured to be heated from an off state to a temperature of at least 100 degrees in less than 5 seconds.

3. The barrel of claim 1 or claim 2, wherein the heater is configured to be heated from a temperature of 60 degrees to a temperature of at least 100 degrees in less than 2 seconds.

4. The barrel of any one of claims 1 to 3, wherein the heater has a thickness of less than 0.6 millimetres in a plane perpendicular to a longitudinal axis of the barrel.

5. The barrel of any one of claims 1 to 4, wherein the barrel comprises a cover layer disposed over the absorbent layer and the exterior surface comprises an exterior surface of the cover layer.

6. The barrel of any one of claims 1 to 4, wherein the exterior surface comprises an exterior surface of the absorbent layer.

7. The barrel of claim 6, wherein the barrel comprises at least one rib disposed over the absorbent layer and extending along a longitudinal axis of the barrel, the exterior surface further comprising at least a portion of an exterior surface of each of the at least one rib.

8. The barrel of claim 7, wherein each respective rib of the at least one rib is retractable relative to the absorbent layer to adjust an extent to which a cross-section of the respective rib in a plane parallel to the longitudinal axis of the barrel protrudes from the absorbent layer.

9. The barrel of any one of claims 1 to 8, wherein the heater comprises perforations for the air to flow through.

10. The barrel of claim 9, wherein the perforations are disposed circumferentially abouta longitudinal axis of the barrel.

11. The barrel of claim 9 or claim 10, wherein the heater comprises a plurality of first longitudinal rows of heater element portions, each of the first longitudinal rows extending along a longitudinal axis of the barrel, and the perforations are arranged in a plurality of second longitudinal rows, each of the second longitudinal rows extending along the longitudinal axis of the barrel, each of the first longitudinal rows interleaved between a respective pair of adjacent second longitudinal rows of the plurality of second longitudinal rows.

12. The barrel of claim 11, wherein each of the heater element portions has a generally sinusoidal profile in a plane of the heater parallel to a surface of the heater facing the absorbent layer.

13. The barrel of any one of claims 9 to 12, when dependent upon claim 5, wherein the perforations are first perforations and at least one of:the absorbent layer comprises second perforations for the air to flow through; and the cover layer comprises third perforations for the air to flow through.

14. The barrel of claim 13, where the absorbent layer comprises the second perforationsand the cover layer comprises the third perforations, and the second perforations are aligned with at least one of: the first perforations and the third perforations.

15. The barrel of claim 13 or claim 14, wherein the cover layer comprises the third perforations, each perforation of the third perforations having a diameter of less than 70 micrometres.

16. The barrel of claim 15, wherein the cover layer is a mesh.

17. The barrel of any one of claims 1 to 16, comprising a conduit for transporting theliquid to the absorbent layer, the conduit being elongate along a longitudinal axis of the barrel and comprising outlets arranged along a length of the conduit for the liquid to flow through to reach the absorbent layer, wherein optionally the outlets are evenly distributed along the length of the conduit.

18. The barrel of any one of claims 1 to 17, comprising a plurality of conduits for transporting the liquid to the absorbent layer, wherein the heater comprises a plurality of heating areas, and a respective conduit of the plurality of conduits is disposed between each pair of adjacent heating areas of the plurality of heating areas.

19. The barrel of claim 18, wherein the plurality of conduits are substantially evenly distributed about a circumference of the barrel.

20. The barrel of any one of claims 1 to 19, wherein the absorbent layer is removeable from the barrel.

21. A haircare appliance comprising the barrel of any one of claims 1 to 20, and a body.

22. The haircare appliance of claim 21, wherein the body comprises an airflow generatorfor generating an airflow out of the barrel, through the heater, the absorbent layer and the exterior surface.

23. The haircare appliance of claim 22, wherein the heater is operable to heat the airflow generated by the airflow generator.

24. The haircare appliance of any one of claims 21 to 23, wherein the body comprises a reservoir for storing the liquid, wherein the reservoir is fluidly connected to the absorbent layer to provide the liquid from the reservoir to the absorbent layer, wherein optionally the haircare appliance comprises a liquid delivery arrangement for delivering a predefined 5 amount of the liquid from the reservoir to the absorbent layer.

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