Method, system and fluid for droplet ejection

Abstract

According to an aspect of the invention, there is provided a method (500, 600) of ejecting a droplet of fluid from a droplet ejector (710a – 710e, 910a, 910b) comprising an actuator (830) and a nozzle (820), wherein the nozzle (820, 920) is an aperture in a membrane (815, 915) and wherein the actuator (830, 930) is a piezoelectric actuator formed in or on the membrane (815, 915), the method comprising: causing (510, 610) a first actuation of the actuator (830) to displace a first volume of the fluid through the nozzle (820); and causing (520, 620) one or more subsequent actuations of the actuator (830) to displace a second volume of the fluid through the nozzle (820) and to increase the momentum of the first volume of the fluid, wherein break off of the droplet from the nozzle (820) occurs after the final actuation of the one or more subsequent actuations (304, 314, 404, 414).

Description

[0001] METHOD, SYSTEM AND FLUID FOR DROPLET EJECTION

[0002] Field of the invention

[0003] The present invention relates to a method of ejecting a droplet of fluid from a droplet ejector, a fluid droplet ejection system and a fluid suitable for use in the method and the system.

[0004] to the invention

[0005] It is known to eject a droplet of fluid from a droplet ejector by pushing a volume of fluid through a nozzle using a single actuation, for example by introducing a positive pressure into a fluid chamber using a piezoelectric actuator. This causes a cylindrical fluid jet to be expelled from the nozzle. The fluid in this jet is typically still connected to the fluid in the nozzle after the single actuation but has a positive momentum away from the nozzle. After the single actuation, a perturbation occurs which causes the jet to break off from the fluid in the nozzle, thereby causing a droplet. This perturbation may happen naturally due to surface tension as the main jet moves away from the nozzle, or it may be introduced by a pressure in the nozzle region itself. In some cases, this known method can be preceded by a method step of drawing fluid into the nozzle or creating a negative pressure in the fluid chamber.This method is employed for standard fluids having a Newtonian viscosity. A fluid has true Newtonian viscosity if it has a constant viscosity regardless of the stress or force that the fluid is subjected to. These fluids are also often typically “low” in viscosity (e.g. in the range 4 to 20 centipoise (cP)). 1 cP is equal to 1 mPa-s. Very low viscosity fluids have a viscosity of 4cP to 0.3cP.

[0006] There are other classes of fluids, such as “high viscosity” fluids which typically have a viscosity greater than 20cP and non-Newtonian fluids. Non-Newtonian fluids exhibit a varying viscosity depending on the stress or force that the fluid is subjected to. High viscosity fluids often have a viscosity of over 20cP. It is difficult to eject stable droplets of high viscosity or non-Newtonian fluids from the nozzle of a droplet ejector using known methods of droplet ejection due to the complex properties of these fluids when a force is exerted upon it. They may for example be modified by excessive pressure gradients or fail to move with sufficient speed to be cleanly ejected.

[0007] It is in this context that the present inventions have been devised.

[0008] Summary of the invention

[0009] In accordance with an aspect of the present invention, there is provided a method of ejecting a droplet of fluid from a droplet ejector. The droplet ejector may comprise an actuator. The droplet ejector may comprise a nozzle. The method may comprise causing a first actuation of the actuator. The first actuation may displace a first volume of the fluid through the nozzle. The method may comprise causing one or more subsequent actuations of the actuator. The one or more subsequent actuations may displace a second volume of the fluid through the nozzle. The one or more subsequent actuations may increase the momentum of the first volume of the fluid. It may be that break off of the droplet from the nozzle occurs after the final actuation of the one or more subsequent actuations.

[0010] In accordance with another aspect of the invention, there is provided a fluid droplet ejection system. The system may comprise a plurality of droplet ejectors. It may be that each of the plurality of droplet ejectors comprises a nozzle. It may be that each of the plurality of droplet ejectors comprises an actuator. The actuator may be configured to cause displacement of a fluid through the nozzle. The system may comprise a drive circuit. The drive circuit may be configured to apply a signal to each of the actuators.The system may comprise a controller. The controller may be configured to cause a first actuation of the actuator. The first actuation of the actuator may displace a first volume of the fluid through the nozzle of at least one of the plurality of droplet ejectors. The controller may be configured to cause one or more subsequent actuations of the actuator. The one or more subsequent actuations may displace a second volume of the fluid through the nozzle of the at least one of the plurality of droplet ejectors. The one or more subsequent actuations may be configured to increase the momentum of the first volume of the fluid. It may be that the amplitude and timing of the first actuation and the one or more subsequent actuations are selected to cause break off of the droplet from the nozzle to occur after the final actuation of the one or more subsequent actuations.

[0011] Advantageously, by using a plurality of actuations to eject a single fluid droplet, this enables ejection of fluid which cannot be ejected by a single actuation. For example, this may be because there is no single actuation that can provide sufficient momentum to a volume of fluid to cause break off, without the fluid becoming too viscous from the force applied by said single actuation to be ejected through the nozzle. The plurality of actuations allows the fluid in a droplet to accumulate across multiple actuations before break off occurs. The fluid in a droplet can be accelerated over more than one actuation, rather than in a single actuation. This is particularly useful when ejecting droplets of fluids which have a variable or high viscosity.

[0012] In another aspect of the present invention, there is provided a fluid suitable for use in the method or in the fluid droplet ejection system described herein.

[0013] According to another aspect there may be provided a printer comprising the fluid droplet ejection system described herein. It may be that the printer comprises the fluid described herein. It may be that the fluid comprises printing liquid.

[0014] According to another aspect there may be provided a method of printing comprising providing the printer described herein, and operating the drive circuit of the fluid droplet ejection system to eject one or more droplets of printing liquid from the nozzle of at least one of the droplet ejectors.It may be that the method of printing comprises controlling at least one of the plurality of droplet ejectors such that the actuator is caused to flex a nozzle-forming layer, whereby to eject printing liquid from the droplet ejector to print using the printing liquid.

[0015] Typically, the method is a method of ejecting a droplet of a non-Newtonian fluid from a droplet ejector. It may be that the fluid is a non-Newtonian fluid.

[0016] Typically, the method is a method of ejecting a droplet of a high viscosity fluid from a droplet ejector. It may be that the high viscosity fluid has a viscosity of greater than 20cP. It may be that the fluid is a high viscosity fluid.

[0017] Advantageously, by using a plurality of actuations to eject a single (e.g. discrete) fluid droplet, this reduces placing high shear rate stresses on the non-Newtonian or high viscosity fluid, which in turn avoids very high ink viscosities in a shear thickening fluid.

[0018] Advantageously, by using a plurality of actuations to eject a single (e.g. discrete) fluid droplet, this avoids placing very high shear rates that may damage long molecules in non-Newtonian fluids or high viscosity fluid.

[0019] Advantageously, by using a plurality of actuations to eject a single (e.g. discrete) fluid droplet, this introduces high shear rate stresses to reduce ink viscosities in a shear thinning fluid to assist with ejection of a droplet of a non-Newtonian fluid or high viscosity fluid.

[0020] It may be that causing the first actuation comprises generating one or more first pressure pulses to displace the first volume of fluid. It may be that causing the one or more subsequent actuations comprises generating one or more subsequent pressure pulses. It may be that break off of the droplet from the nozzle occurs after the final pressure pulse of the one or more subsequent pressure pulses.

[0021] A first actuation causes one or more first pressure pulses which start fluid moving through the nozzle, causing the fluid to be displaced, and one or more subsequent actuations (e.g. a second actuation) cause one or more subsequent pressure pulses which accelerate the already moving fluid, thereby achieving a greater velocity for a given maximum pressure gradient.As mentioned above, non-Newtonian fluids exhibit a varying viscosity depending on the stress or force that the fluid is subjected to. The stress is often caused by shear forces (which may be time dependent). Typically, non-Newtonian fluids have a nontime dependent change in viscosity with respect to applied shear force. Typically, some non-Newtonian fluids may have a time dependent change in viscosity with respect to shear forces. Typically, these fluids are complex in that they are made up of multiple components which have different characteristics, such as shear thinning or shear thickening, e.g. some non-Newtonian fluids include solids that behave in a different manner at high shear rates. Some non-Newtonian fluids comprise “long chain” molecules that may exist in a folded state at room pressure or low strain rates but physically break at high strain rates or uncurl at high strain rates, which changes the fluid viscosity. Some non-Newtonian fluids may change chemical composition at high strain rates or have a viscosity which varies depending on temperature.

[0022] These fluids may used in high-value applications. For example, polymeric fluids can often be used for coatings, additive manufacturing, adhesives and long chain molecules are often used in pharmaceuticals.

[0023] It may be that the high viscosity fluid has a viscosity of for example at least 30cP, such as at least 50cP, for example at least 70cP.

[0024] Typically, the system is an inkjet printhead. The inkjet printhead may be a piezoelectric printhead. The plurality of droplet ejectors may be droplet ejectors for (e.g. configured for use in) an inkjet printhead. The plurality of droplet ejectors may be inkjet droplet ejectors. The printhead may be configured to print fluids (i.e. liquids), such as functional fluids, for use in the manufacture of printed electronics. The printhead may be configured to print biological fluids. Biological fluids typically comprise biological macromolecules, e.g. polynucleotides, such as DNA or RNA, microorganisms, and / or enzymes. The printhead may be configured to print other fluids used in biological or biotechnological applications, such as diluents or reagents. The printhead may be a voxel printhead (i.e. a printhead configured for use in 3D printing, e.g. additive printing). The system may be a system for dispensing metered amount of fluid, for example a nebulizer or part of a chemical (e.g. pharmaceutical) manufacturing system.

[0025] It may be that (e.g. some or each of) the plurality of droplet ejectors share a common substrate. For example, it may be that the plurality of droplet ejectors are integrated onsaid common substrate. The printhead may be an inkjet printhead. Each of the plurality of droplet ejectors may be an inkjet droplet ejector.

[0026] It may be that the nozzle is an aperture in a membrane. It may be that the actuator is a piezoelectric actuator formed in or on the membrane.

[0027] Typically, the droplet ejector comprises a fluid chamber. The fluid chamber may store the fluid to be ejected. The fluid chamber may be formed in a substrate. It may be that the droplet ejector comprises the nozzle. The droplet of fluid may be ejected through the nozzle. The fluid chamber and the nozzle may be in fluid communication with one another. The fluid chamber may be defined by side walls and a top wall and a bottom wall. By the top wall we refer to the side of the fluid chamberout of which fluid is ejected in use, irrespective of the orientation of the fluid chamber at a given time. The nozzle may be formed in the top wall. The top wall may be formed of a membrane. The membrane may form the nozzle, such that the membrane is a nozzle forming layer. The nozzle may be an aperture in the membrane.

[0028] The droplet ejector may comprise an actuator. The actuator may cause fluid to be ejected through the nozzle. The actuator may be formed on the top wall of the fluid chamber. The nozzle may extend through the actuator. It may be that the actuator is formed on the bottom wall of the fluid chamber, opposite the nozzle. It may be that the actuator is formed on one or more side walls of the fluid chamber with the nozzle on the top wall of the fluid chamber. In each of these cases, actuation of the actuator creates a pressure pulse in the fluid chamber and displacement of fluid.

[0029] The actuator may be a piezoelectric actuator. The piezoelectric actuator may comprise a piezoelectric body provided between a corresponding pair of drive electrodes. It may be that the piezoelectric actuator is substantially annular (i.e. ring-shaped). Thus, the ejectors are typically configured to eject ink composition in an inertial mode. In other examples, the actuator may be a positive displacement / pressure actuator.

[0030] The piezoelectric body may comprise (e.g. be formed from) one piezoelectric material. Alternatively, the piezoelectric body may comprise (e.g. be formed from) more than one piezoelectric material. The piezoelectric body may comprise (e.g. be formed from) a ceramic material comprising aluminium and nitrogen and optionally one or more elements selected from: scandium, yttrium, titanium, magnesium, hafnium, zirconium,tin, chromium, boron. The piezoelectric body may comprise (e.g. be formed from) aluminium nitride (AIN). The piezoelectric body may comprise (e.g. be formed from) zinc oxide (ZnO). The one or more piezoelectric materials may comprise (e.g. consist of) aluminium nitride and / or zinc oxide. Aluminium nitride may consist of pure aluminium nitride. Alternatively, aluminium nitride may comprise one or more elements (i.e. aluminium nitride may comprise aluminium nitride compounds). Aluminium nitride may comprise one or more of the following elements: scandium, yttrium, titanium, magnesium, hafnium, zirconium, tin, chromium, boron. The piezoelectric body may comprise (e.g. be formed from) scandium aluminium nitride (ScAIN). It may be that the piezoelectric body comprises one or more piezoelectric materials processable at a temperature below 450°C. Examples of piezoelectric materials that are processable at temperatures below 450°C include aluminium nitride (AIN), zinc oxide (ZnO), and / or scandium aluminium nitride (ScAIN). Some electronics (e.g., CMOS components) can be damaged at temperatures greater than 450°C. By using piezoelectric materials that are processable at a temperature below 450°C, MEMS and electronic components can be integrated whilst avoiding degradation of the electronics, thereby improving the operation and efficiency of the ejector.

[0031] Thus, typically, actuation of the actuator (e.g. piezoelectric actuator) generates a pressure pulse. Typically, the actuator (e.g. piezoelectric actuator) is actuated to generate the one or more first pressure pulses and actuated again to generate the one or more subsequent pressure pulses. The one or more first pressure pulses and one or more subsequent pressure pulses are typically pulses in fluid pressure adjacent the nozzle, thereby urging fluid through the nozzle. It may be that the one or more first pressure pulses and the one or more subsequent pressure pulses are configured to propel a volume of fluid through the nozzle. It may be that the one or more first pressure pulses and the one or more subsequent pressure pulses are configured to cause movement of a volume of fluid through the nozzle.

[0032] It may be that the drive circuit comprises at least one electronic component configured to carry a signal from the controller to the actuator. The drive circuit is typically configured to selectively apply (i.e. when actuated (e.g. when in use, connected to a power supply (e.g. a voltage signal line) and responsive to an actuation signal)) a signal to the actuator. It may be that the signal is a potential difference applied to the actuator. Typically, the signal causes the pressure pulses to be generated. The signal may be acontrol signal for the droplet ejector. The signal may be a signal which controls the actuator of the droplet ejector.

[0033] When the actuator is a piezoelectric actuator, the signal may be a potential difference between the drive electrodes to cause deflection of the piezoelectric body. When the piezoelectric body deflects, this causes deflection of the top wall membrane which causes a pressure pulse in the fluid chamber which in turn causes a volume of fluid to be forced through the nozzle. It may be that the drive circuit comprises a CMOS control circuit. It may be that the drive circuit is formed in the substrate (i.e. both the droplet ejector and the drive circuit are formed in the substrate).

[0034] Typically, the first actuation (and therefore the one or more first pressure pulses) and the one or more subsequent actuations (and therefore the one or more subsequent pressure pulses) are provided over a (e.g. time) period. The period may be such as at least 3 micro seconds (ps), for example at least 5 ps, such as at least 10 ps, for example at least 20 ps, such as at least 30 ps, for example at least 40 ps. The period may depend on the natural frequency of ejection system. Typically, there is a settling period between consecutive actuations. The settling period is typically a period of time which allows the fluid chamber to settle, for example for fluid in the fluid chamber to be replaced or for the meniscus of fluid at the nozzle to settle. The settling period may be such as at least 3 micro seconds (ps), for example at least 5 ps, such as at least 10 ps, for example at least 20 ps, such as at least 30 ps, for example at least 40 ps. The settling period may depend on the natural frequency of ejection system.

[0035] It may be that the method comprises generating an electric field through which the droplet moves to further increase the momentum of the droplet by electrohydrodynamic forces. It may be that the droplet ejected from the nozzle is subject to electrohydrodynamic (EHD) forces to further increase the momentum of the droplet. Typically, the EHD forces augment the forces already exerted on the fluid by movement of the membrane due to deformation of the piezoelectric body. Advantageously, the EHD forces increase the momentum and make it easier to eject droplets of high viscosity fluid and non-Newtonian fluid (e.g. when it is at a high viscosity).

[0036] It may be that the droplet ejector comprises an electric field generator (e.g. one or more electrodes) to generate an electric field which causes EHD force and through which the droplet moves. It may be that the electric field generator is the (e.g. drive)electrodes of the piezoelectric actuator of the droplet ejector generates the electric field which causes EHD forces and through which the droplet moves.

[0037] The electric field generator may generate a charge on a droplet, for example in addition to accelerating a droplet.

[0038] It may be that the droplet ejector comprises at least one electrode, which is separate to the piezoelectric actuator. It may be that the at least one electrode is configured to generate an electric field through which the droplet moves to further increase the momentum of the droplet by electrohydrodynamic forces

[0039] It may be that the electric field generator comprises at least one electrode, which is separate to the piezoelectric actuator (e.g. the electrodes of the piezoelectric actuator).

[0040] Typically, the electric field generator, such as the at least one electrode (or an additional pair of electrodes) is controlled using a different signal to the actuation signal of (e.g. the electrodes of) the piezoelectric actuator. The signal which controls the at least one electrode (or the additional pair of electrodes) may be an electric field control signal. It may be that the electric field control signal (e.g. a potential difference) is applied to the electric field generator (e.g. the at least one electrode or additional pair of electrodes) for a plurality of actuations of the piezoelectric actuator. That is, it may be that the electric field control signal is applied to the electric field generator (e.g. the at least one electrode or the additional pair of electrodes) for the first actuation and at least one of the one or more subsequent actuations. It may be that a constant signal is applied to the electric field generator (e.g. the at least one electrode or the additional pair of electrodes) per droplet. In other examples, the at least one electrode or the additional pair of electrodes may be controlled using the same signal to the actuation signal of (e.g. the electrodes of) the piezoelectric actuator.

[0041] The electric field control signal may repeat for each droplet ejection. The electric field control signal may have a waveform which extends during the actuation and one or more subsequent actuations. The electric field control may comprise a waveform which repeats with a period which is longer than the first actuation or subsequent actuations. The electric field control may comprise a waveform which repeats with a period equal to the time between droplet ejections.It may be that the drive circuit (e.g. the CMOS control circuit) is configured to actuate the piezoelectric body by applying an electrical potential gradient to the piezoelectric body in a first direction to cause the piezoelectric body to flex in a first sense (e.g. in a direction perpendicular to the nozzle-forming layer) and then to apply an electrical potential gradient to the piezoelectric body in the opposite direction to cause it to deform in an opposite second sense (e.g. in a direction also perpendicular to the nozzle-forming layer but in the opposite direction to the first second sense). In this way, the first and second volumes of fluid are displaced in a direction out of the nozzle and away from the fluid chamber. It may be that the fluid is displaced in a direction perpendicular to the plane of the nozzle-forming layer.

[0042] The electrical potential gradient is applied by regulating the voltages applied to the first and / or second electrodes. One electrode may remain at ground in which case only the voltage applied to the other electrode need be regulated.

[0043] By applying an electrical potential gradient to the piezoelectric body in a first direction to cause the piezoelectric body to flex in a first sense and then applying an electrical potential gradient to the piezoelectric body in the opposite direction to cause it to deform in an opposite second sense, the actuator may act as a push-pull actuator and readily implement both draw and dispense portions of an ejection cycle.

[0044] Typically, an actuation refers to a signal pulse. Typically, the actuator returns to a quiescent state for a period of time between the first actuations and each of the one or more subsequent actuations.

[0045] Typically, a signal may form part of a waveform. Typically, the signal has an amplitude which increases or decreases from a baseline value and then returns to the baseline value (e.g. a pulse). The signal may be a trigonometric, square, trapezoidal, sawtooth or triangular waveform.

[0046] The movement, or displacement, of fluid through the nozzle of the droplet ejector means that a volume of fluid passes from the fluid chamber, through the nozzle, to outside of the fluid chamber. The first actuation (and therefore the one or more first pressure pulses) may be sufficient to cause the first volume of fluid to pass through the nozzle. The one or more first pressure pulses are generated by the first actuation. The one or more first pressure pulses may comprise for example at least two pressurepulses, such as at least three pressure pulses, for example at least four pressure pulses, such as at least five pressure pulses, for example at least six pressure pulses.

[0047] The one or more subsequent actuations are actuations which temporally follow the first actuation. The one or more subsequent actuations cause one or more subsequent pressure pulses and causes a volume of fluid to be passed through the nozzle to form a single droplet of fluid together with the volume of fluid passed through the nozzle as a result of the first actuation (i.e. the one or more first pressure pulses). The one or more subsequent pressure pulses are generated by the one or more subsequent actuations. The one or more subsequent actuations may comprise for example at least two actuations, such as at least three actuations, for example at least four actuations, such as at least five actuations, for example at least six actuations.

[0048] The one or more subsequent pressure pulses may be sufficient to cause the second volume of fluid to pass through the nozzle. The second volume of fluid comprises the total volume of fluid which passes through the nozzle as a result of the total number of subsequent actuations (e.g. and the total number of subsequent pressure pulses). The one or more subsequent pressure pulses may comprise for example at least two pressure pulses, such as at least three pressure pulses, for example at least four pressure pulses, such as at least five pressure pulses, for example at least six pressure pulses.

[0049] It will be appreciated that the number of first pressure pulses, the number of subsequent actuations and the number of the subsequent pressure pulses will depend on the fluid being ejected by the droplet ejector, and the natural frequency of the ejection system.

[0050] Typically, the method may comprise causing a second actuation as the one or more subsequent actuations. It may be that break off of the droplet from the nozzle occurs after the second actuation.

[0051] The final actuation of the one or more subsequent actuations is an actuation which temporally follows the first actuation and causes break off of a single droplet of fluid having a total volume made up of the first volume of fluid (from the one or more first pressure pulse) and the second volume of fluid (from the one or more subsequent pressure pulses). The final pressure pulse of the one or more subsequent pressurepulses may be the final pressure pulse generated by the one or subsequent (e.g. second) actuation.

[0052] Typically, the first volume of fluid remains connected to fluid inside the nozzle between the first actuation and the final actuation of the one or more subsequent actuations.

[0053] Typically, the method may comprise causing the first actuation of the actuator to have a first amplitude. Typically, the method may comprise causing the one or more subsequent actuations of the actuator to each have a different amplitude to the first amplitude. Typically the first actuation has a first amplitude. Typically, the one or more subsequent actuations have a different amplitude to the first actuation. The period of the waveforms formed by the first actuation and the one or more subsequent actuations will typically be tuned to the natural frequency of the ejection system.

[0054] The first actuation (and the one or more first pressure pulse) and the one or more subsequent actuations (and the one or more subsequent pressure pulses) typically have a different purpose to one another. The first actuation (and the one or more first pressure pulses) may cause the first volume of fluid to pass through the nozzle at a relatively low velocity. This will result in a first volume of fluid outside the nozzle moving with a positive momentum away from the nozzle. As this momentum would be insufficient to cause droplet ejection, the first volume of fluid would return into the nozzle due to capillary action because the first volume of fluid is still connected to the fluid inside the nozzle. The one or more subsequent actuations (and the one or more subsequent pressure pulses) cause more fluid (i.e. the second volume of fluid) to pass through the nozzle which provides more momentum to the first volume of fluid already outside the nozzle after the first pressure pulse. The second volume of fluid contributes to the momentum of the initial displaced first volume of fluid. As a result, the total volume of fluid on the outside of the nozzle (i.e. the first volume of fluid and the second volume of fluid) moves away from the nozzle with sufficient momentum to result in the formation of a fluid stalk and a droplet break off by either a natural stalk mechanism or perturbation mechanism. Droplet break off refers to an ejection event of a droplet of fluid in which a single droplet of fluid separates from the fluid inside the nozzle.

[0055] In some examples, at least one of the one or more subsequent actuations of the actuator may have the same amplitude as the first actuation.It may be that the fluid droplet ejection system comprises a predetermined fluid.

[0056] It may be that the peak amplitude of each of the first actuation and the one or more subsequent actuations are typically less than the peak amplitude associated with a single actuation. The relatively low peak amplitude results in lower shear rates and lower peak pressures, thus minimizing the effect of shear thickening or increased viscosity. Therefore, this is particularly advantageous when ejecting non-Newtonian fluids and high viscosity fluids.

[0057] The first volume may be for example at least 1 picolitre, such as at least 2 picolitres, for example at least 5 picolitres, such as at least 7 picolitres. The first volume may be such as no more than 10 picolitres, for example no more than 8 picolitres, such as no more than 5 picolitres, for example no more than 3 picolitres.

[0058] The second volume may be for example at least 1 picolitre, such as at least 5 picolitres, for example at least 10 picolitres, such as at least 15 picolitres, for example at least 20 picolitres, such as at least 25 picolitres. The second volume may be such as no more than 50 picolitres, for example no more than 45 picolitres, such as no more than 40 picolitres, for example no more than 35 picolitres.

[0059] Typically, the droplet ejected by the method has a volume of at least 3 picolitres. It may be that the droplet has a volume of such as at least than 4 picolitres, for example at least 5 picolitres, such as at least 10 picolitres, for example at least 15 picolitres, such as at least 20 picolitres.

[0060] Advantageously, the method results in larger droplets of fluid than the droplets of fluid associated with a single actuation method for droplet formation.

[0061] Typically, each of the one or more subsequent actuations has a greater amplitude than the first amplitude. Typically, at least one of the one or more subsequent actuations may have a greater amplitude than the first amplitude.

[0062] Typically, each of the one or more subsequent actuations has a lower amplitude than the first amplitude. Typically, at least one of the one or more subsequent actuations may have a lower amplitude than the first amplitude.It may be that the amplitude of the first actuation is equal to the amplitude of at least one of the one or more subsequent actuations.

[0063] Where there is more than one subsequent actuation, it may be that the amplitude of the actuations is interlaced. That is, consecutive actuations may not have an increasing or decreasing amplitude. Instead, consecutive actuations may increase and decrease in amplitude across adjacent actuations. For example, the second amplitude of the second actuation may be greater than the first amplitude of the first actuation and a third amplitude of a third actuation may be less than the second amplitude of the second actuation.

[0064] In general, the amplitudes of the actuations will depend on the natural frequency of the system and the predetermined fluid properties.

[0065] Advantageously, the peak amplitude of each individual actuation may be less than the peak amplitude of a single actuation ejection. Therefore, the shear rates and peak pressures are lower using the multiple actuation method compared to the single actuation method. This is particularly advantageous when ejecting non-Newtonian fluids and high viscosity fluids which become high in viscosity when a force is exerted upon them.

[0066] The amplitudes of the first and one or more subsequent actuations may be determined by empirical testing. According to the disclosure there may be provided a method for determining amplitudes (e.g. voltages) of actuations (e.g. voltage signals), such as te first actuation and the one or more subsequent actuations. The method may comprise determining the number of actuations at a starting amplitude (e.g. voltage) which are required to cause break off of a droplet. The method may comprise determining one or more target parameters of the droplet. The parameters may include droplet mass and droplet velocity in flight. The method may comprise modifying the amplitude of the actuations (e.g. positively or negatively) to achieve one or more target parameters of the droplet (e.g. a target droplet mass and target droplet velocity). The target parameters, or actuation control parameters derived therefrom (e.g. the duration and amplitude of actuator control signals) may be stored in memory to be read by the one or more processors of the controller during operation, to determine droplet mass and velocity.Advantageously, this provides a method to determine an appropriate amplitude for the first and one or more subsequent actuations to achieve a droplet having one or more target parameters.

[0067] The controller may be configured to perform any steps of the methods described herein. The controller may comprise one or more processors. The controller may comprise a non-transitory computer readable memory storing instructions. The instructions, when executed by the one or more processors may cause the controller to operate at least one of the droplet ejectors as described herein. The one or more processors may be located in a single unit. In other examples, where the one or more processors is a plurality of processors, the controller may be distributed.

[0068] Description of the Drawings

[0069] An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:

[0070] Figure 1 is a graph showing the properties of fluids;

[0071] Figure 2 is a graph of a single actuation method according to the prior art; Figure 3 is a graph of a multiple actuation method according to the invention; Figure 4 is a graph of a multiple actuation method according to the invention; Figure 5 is a flow chart of a method according to the invention;

[0072] Figure 6 is a flow chart of a method according to the invention;

[0073] Figure 7 is a diagram of an exemplary system according to the invention; Figure 8a and 8b are diagrams of an exemplary droplet ejector according to the invention;

[0074] Figure 9 is a diagram of a controller according to the invention;

[0075] Figure 10 is a flow chart of a method according to the invention;

[0076] Figures 11a and 11b are diagrams of exemplary droplet ejector according to the invention;

[0077] Figure 12 is a flow chart of a method according to the invention; and Figure 13 is a schematic diagram of a stalk formed during the droplet ejection process, shown at a time between the first and subsequent actuations.

[0078] Detailed Description of an Example EmbodimentFigure 1 is a graph 100 showing the properties of fluids. Shear strain rate dY / dt, is shown on the x axis and Shear stress r, is shown on the y axis. The graph shows how the shear stress changes and the shear strain rate increases for different types of fluids. The greater the gradient of the line, the higher the viscosity of the fluid. An ideal fluid would have a line overlapping the x axis. A pseudoplastic fluid is represented by line 110 which has a higher viscosity at low shear strain rates and a lower viscosity at high shear strain rates. Line 120 represents a Newtonian fluid which has a constant viscosity across all shear strain rates. Line 130 represents a dilatant fluid which has a fairly high viscosity at low shear strain rates and an even higher viscosity at higher shear strain rates. Line 140 represents an ideal plastic (Bingham plastic) with a very high viscosity at very low shear strain rates and a lower viscosity at most shear strain rates. An ideal solid would have a line overlapping the y axis.

[0079] Figure 2 is a graph of a single actuation method according to the prior art. In the single actuation method, a single actuation causes break off of a droplet of a Newtonian fluid from the nozzle (i.e. an ejection event). In the signal 200, there is a first waveform 210 to cause a first (and only) actuation which ejects a first droplet and a second waveform 220 to cause a second (and only) actuation which ejects a second droplet. The first waveform 210 and the second waveform 220 are signals of a single voltage pulse of alternating current in which the voltage increases from zero to a positive peak, then to a negative peak before returning to zero. The single waveforms each have a period of oscillation aligned with oscillation of the membrane forming the nozzle of the droplet ejector (see Figure 8).

[0080] As will be appreciated, these waveforms 210, 220 each cause actuation of an actuator of the droplet ejector which in turn each cause one or more pressure pulses in the fluid chamber. The settling period is represented by dual sided arrow 230 and is the time between single waveforms 210, 220. In the single actuation method, the volume of Newtonian fluid passed through nozzle from the pressure caused by each of the single waveform 210, 220 moves with sufficient momentum to break off from the nozzle to form a single droplet of fluid ejected from the droplet ejector. Each of the actuations 210, 220 are single actuations which may each cause a plurality of pressure pulses, but there is only one actuation per droplet. This is because the amplitude of the pressure caused by the single waveform 210, 220 is sufficiently high to provide enough energy to the Newtonian fluid. The amplitude of the pressure caused by single waveform can be sufficiently high because the Newtonian fluid has a constantviscosity, regardless of the force applied to it, so there is no need to reduce the amplitude to account for the change in viscosity when applying a large force to the Newtonian fluid.

[0081] Figure 3 is a graph of a multiple actuation method according to the present invention. In the multiple actuation method, multiple actuations are needed to cause break off of a single droplet of a non-Newtonian fluid (or a high viscosity fluid) from the nozzle (i.e. an ejection event). In the signal 300, there is a first waveform 310 to eject a first droplet and a second waveform 320 to eject a second droplet. The first waveform 310 includes two (first and second) actuations 302, 304. The second waveform 320 includes two (first and second) actuations 312, 314. The first actuations 302, 312 can cause one or more first pressure pulses in the fluid chamber. The second actuations 304, 314 (which are the only subsequent actuation in this example) can cause one or more subsequent pressure pulses in the fluid chamber. Each of the actuations 302, 304, 312, 314, are signal of alternating current in which the voltage increases from zero to a positive peak, then to a negative peak before returning to zero. The peak amplitude of the second actuations 304, 314 in each waveform 310, 320 is greater than the amplitude of the first actuations 302, 312 in each waveform 310, 320. The waveforms 310, 320 each have a period of oscillation aligned with oscillation of the membrane forming the nozzle of the droplet ejector (see Figure 8). Each of the first actuations 302, 312 and the second actuations 304, 314 may cause a plurality of pressure pulses, and there are multiple actuations per droplet.

[0082] In other examples, the peak amplitude of the second actuations in each waveform may be less than or the same as the amplitude of the first actuations in each waveform. In some examples, there may be first, second and third actuations in each waveform, where the first, second and third actuations cause a single droplet to be ejected.

[0083] As will be appreciated, these waveforms 310, 320 are made up of two actuations, where each actuation of the actuator of the droplet ejector which causes one or more pressure pulses in the fluid chamber. The settling period is represented by dual sided arrow 330 and is the time between the first actuations 302, 312 of each waveform 310, 320. In the multiple actuation method, the volume of non-Newtonian (or high viscosity) fluid passed through nozzle from the pressure caused by the first actuation 302, 312 is a first volume of fluid which moves at a relatively low velocity due to the low amplitude of the pressure caused by the first actuation (relative to the amplitude of the pressurecaused by the single waveform in the single actuation method). The first volume of fluid moves away from the nozzle with a positive momentum that is insufficient to cause break off of the droplet. The pressure caused by the second actuation 304, 314 causes more fluid (i.e. a second volume of fluid) to pass through the nozzle. This increases the volume of the fluid outside of the nozzle to form the total volume of the droplet of fluid. This also provides more momentum to the first volume of fluid from the first actuation 302, 312. The total volume of fluid then has enough momentum to result in the formation of a fluid stalk and a droplet break off by either a natural stalk mechanism or perturbation mechanism. In Figure 3, the amplitude of the second actuation 304, 314 is greater than the amplitude of the first actuation 302, 312 but may still be lower than the amplitude associated with the waveform of the single actuation method. The amplitude of the pressure caused by each actuation in the multiple actuation method has to be sufficiently low to avoid exerting too much force on the non-Newtonian fluid (or the high viscosity fluid) such that the viscosity increases to such an extent that it cannot be passed through the nozzle. Since this amplitude is so much lower than the single actuation method, the momentum of the non-Newtonian (or high viscosity) fluid is not enough to cause break off, thus this is why more than a single actuation is needed to eject a single droplet.

[0084] Figure 4 is a graph of a multiple actuation method according to the present invention. Figure 4 is similar to Figure 3 and like features are given similar reference numerals. For example, the waveforms 310, 320 in Figure 3 are referred to as waveforms 410, 420 in Figure 4. The waveforms 410, 420 are formed of a first actuation 402, 412 to cause one or more first pressure pulses and a second actuation 404, 414 to cause one or more second pressure pulses (as one or more subsequent pressure pulses). However, in this signal 400, the amplitude of the first actuation and the pressure caused by the first actuation 402, 412 is greater than the amplitude of the second actuation and the pressure caused by the second actuation 404, 414 but still lower than the amplitude associated with the pressure pulse of the single actuation method.

[0085] Figure 5 is a flow chart of a method 500 according to the invention. Figure 6 is a flow chart of a method according to the invention. In particular, method 600 is a particular embodiment of method 500. In both methods 500, 600 the first method step is causing 510, 610 a first actuation, which causes one or more first pressure pulses. The one or first pressure pulses cause the first volume of fluid to be displaced and pass through the nozzle. The second method step of method 500 comprises causing 520 one ormore subsequent actuations, which cause one or more subsequent pressure pulses, whilst the second method step in method 600 comprises causing 620 a second actuation as the one or more subsequent actuations. In other words, in method 600, there is only one subsequent actuation. Each of the first and second actuations may generate one or more pressure pulses. The one or more subsequent pressure pulse cause the second volume of fluid to pass through the nozzle and increase in the momentum of all the fluid outside the nozzle. As a result, break off of the droplet from the nozzle occurs after method steps 520, 620. Figures 3 and 4 represent the signal produced by performing the method 600 twice (with the difference between the two signals 300, 400 being the relative amplitudes of the first and second actuations).

[0086] Figure 7 is a diagram of an exemplary system 700 according to the invention. The system 700 in this example is an inkjet printhead including a controller 800 and a plurality of droplet ejectors 710a, 710b, 710c, 71 Od, 71 Oe. The controller 800 transmits signals to the plurality of droplet ejectors 710a - 71 Oe through signal lines in one or more electronic components (not pictured).

[0087] Figure 8a is a diagram of an exemplary droplet ejector 710a - 71 Oe according to the invention. The droplet ejector 710a is formed in a substrate 801. The droplet ejector 710a includes a side wall 805 which is annular to form a fluid chamber 810. The top wall membrane 815 include an aperture in the centre which forms a nozzle 820. A piezoelectric actuator 830 surrounds the nozzle 820. The piezoelectric actuator 830 is formed of two drive electrodes 822, 826 sandwiching a piezoelectric body 824. The drive electrodes 822, 826 apply a potential difference across the piezoelectric body 824 which causes it to deflect. The deflection of the piezoelectric body 824 causes deflection of the top wall membrane 815 which causes fluid from the fluid chamber 810 to be pushed out of the nozzle 820. Control electronics are also part of the substrate 801 though this is not illustrated in Figure 8. The drive electrodes 822, 826 may be provided in a layer (not shown) of the substrate 801.

[0088] In Figure 8b, the generation of a pressure pulse occurs when a potential difference is applied to the piezoelectric actuator 830. The piezoelectric actuator 803 deforms downward with the membrane 815 which increases the pressure in the fluid chamber 810, thereby causing a pressure pulse.The piezoelectric actuator 803, particularly the bias of the drive electrodes 822, 826, generates an electric field. The droplet ejected by the droplet ejector 710a passes through that electric field and is subject to electrohydrodynamic (EHD) forces. The EHD forces cause the droplet to have additional momentum which assists with break off of the droplet from the nozzle 820.

[0089] Figure 11a is a diagram of an exemplary droplet ejector 910a according to the invention. Similar to the droplet ejector 710a, the droplet ejector 910a is formed in a substrate 901. The droplet ejector 910a includes an annular side wall 905 forming a fluid chamber 910a. The top wall membrane 915 includes an aperture which forms a nozzle 920. A piezoelectric actuator 930 surrounds the nozzle 920. The piezoelectric actuator 930 is formed of two drive electrodes 922, 926 with a piezoelectric body 924 therebetween. The drive electrodes 922, 926 apply a potential difference across the piezoelectric body 924 which causes it to deflect. The deflection of the piezoelectric body 924 causes deflection of the top wall membrane 915 which causes fluid from the fluid chamber 910a to be pushed out of the nozzle 920. Control electronics are also part of the substrate 901 though this is not illustrated in Figure 11a.

[0090] The droplet ejector 910a has an additional pair of electrodes 932, 936 which are annular around the nozzle 920. An insulating material 934 is provided between the additional electrodes 932, 936. The additional pair of electrodes 932, 936 generates an electric field which a droplet ejected by the droplet ejector 910a passes through. The droplet becomes subject to electrohydrodynamic (EHD) forces. The EHD forces cause the droplet to have additional momentum which assists with break off of the droplet from the nozzle 920. The additional pair of electrodes 932, 936 provide a greater electric field than just the drive electrodes 822, 826 do. By applying a potential difference to the additional pair of electrodes 932, 936 at the right time and of the right amount, the electric field can apply a desired EHD to the droplet at the right moment in time to assist with break off of the droplet from the nozzle 920. The signal controlling the additional pair of electrodes 932, 936 may be different to the actuation signals of the piezoelectric actuator 930. The drive electrodes 922, 926 and the additional electrodes 932, 936 are provide in layer 940 of the substrate 901.

[0091] Figure 11b is a diagram of an exemplary droplet ejector 910b according to the invention. The droplet ejector 910b includes all of the same features as the dropletejector 910a. However, the additional pair of electrodes 932, 936 in the droplet ejector 910b are coaxial with the drive electrodes 922, 926.

[0092] Figure 12 is a flow chart of a method 1200 according to the invention. The method 1200 includes the method step of generating 1210 an electric field. The droplet moves through the electric field and experiences electrohydrodynamic forces which further increase the momentum of the droplet. The method 1200 may be performed by the drive electrodes 822, 824 of the piezoelectric actuator 830 as in Figures 8a and 8b, or the additional pair of electrodes 932, 936 as in Figure 11a, or both the additional pair of electrodes 932, 936 and the drive electrodes 922, 926.

[0093] Figure 9 is a controller 800 according to the invention. The controller 800 is integrated into the substrate 801 along with the droplet ejector 710a. The controller 800 includes one or more processors 1010 and a storage medium 1020. The controller 800 transmits control signals to the droplet ejector 710a (or any other droplet ejector 710b - 71 Oe) using wires 1025 (i.e. a drive circuit). The controller may generate the control signals, or selectively switch signals generated elsewhere (e.g. off the substrate).

[0094] Figure 10 shows a flowchart of a method 1000 according to the invention. The method 1000 is a method to determine amplitudes of the first actuation and the one or more subsequent actuations. In particular, the method 1000 is used to determine amplitudes of actuations, which are voltage signals in this example. The method involves determining 1010 the number of actuations at a starting amplitude, e.g. a starting voltage, which are required to cause break off of a droplet. The method 1000 then involves determining 1020 one or more target parameters of the droplet. The method step 1020 may happen before method step 1010. As an example, the target parameters include target droplet mass and target droplet velocity in flight. The method 1000 includes modifying 1030 the amplitude of the actuations to achieve one or more target parameters of the droplet.

[0095] Figure 13 shows a droplet ejector during the droplet ejection process, just before the subsequent actuation starts. The image shows a droplet 1750 from a previous droplet ejection moving in a first direction 1702 and a stalk formed by a first volume of fluid, shown generally as 1704, where a new droplet is being ejected by the first actuation. A new forming droplet has a portion 1708 which is being ejected in the first direction and a portion 1706 which will return to the ejector fluid chamber 1724. In between thetwo portions there is the stagnation point 1710 where there is no net fluid flow in the first direction. In the image, the actuator, formed by piezoelectric body 1714 sandwiched between actuator drive electrodes 1716 and 1718, is located within a flexible membrane 1720 which defines the nozzle 1722 that moves in the second direction (opposite the first direction 1702 shown) and then back in the first direction during each of the first and second actuations. Also shown is substrate 1726 which defines the fluid chamber 1724 in combination with the membrane 1720. During the subsequent actuation, a second volume of fluid is displaced through the nozzle to increase the momentum of the first volume of fluid.

[0096] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to and do not exclude other components, integers, or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0097] Features, integers, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

23Claims1. A method (500, 600) of ejecting a droplet of fluid from a droplet ejector (710a - 71 Oe, 910a, 910b) comprising an actuator (830, 930) and a nozzle (820, 920), wherein the nozzle (820, 920) is an aperture in a membrane (815, 915) and wherein the actuator (830, 930) is a piezoelectric actuator formed in or on the membrane (815, 915), the method comprising:causing (510, 610) a first actuation (302, 312, 402, 412) of the actuator (830, 930) to displace a first volume of the fluid through the nozzle (820, 920); andcausing (520, 620) one or more subsequent actuations (304, 314, 404, 414) of the actuator (830, 930) to displace a second volume of the fluid through the nozzle (820, 920) and to increase the momentum of the first volume of the fluid,wherein break off of the droplet from the nozzle (820, 920) occurs after the final actuation of the one or more subsequent actuations (304, 314, 404, 414).

2. The method of claim 1, wherein the piezoelectric actuator comprises a piezoelectric body provided between a corresponding pair of drive electrodes, wherein the piezoelectric body comprises scandium aluminium nitride.

3. The method of claim 1 or claim 2, wherein the momentum of the first volume of fluid caused by the first actuation is insufficient to cause droplet ejection.

4. The method (500, 600) of any of claims 1 to 3, wherein causing (510, 610) the first actuation (302, 312, 402, 412) comprises generating one or more first pressure pulses to displace the first volume of fluid and causing (520, 620) the one or more subsequent actuations (304, 314, 404, 414) comprises generating one or more subsequent pressure pulses, and wherein break off of the droplet from the nozzle (820, 920) occurs after the final pressure pulse of the one or more subsequent pressure pulses (304, 314, 404, 414).

5. The method (500, 600) of any of claims 1 to 4, wherein the first volume of fluid remains connected to fluid inside the nozzle (820, 920) between the firstactuation (302, 312, 402, 412) and the final actuation of the one or more subsequent actuations (304, 314, 404, 414).

6. The method (500, 600) of any of claims 1 to 5, comprising causing the first actuation (302, 312, 402, 412) of the actuator (830, 930) to have a first amplitude and causing the one or more subsequent actuations (304, 314, 404, 414) of the actuator (830, 930) to each have a different amplitude to the first amplitude.

7. The method (500, 600) of any preceding claim, wherein each of the one or more subsequent actuations (304, 314, 404, 414) has a greater amplitude than the first amplitude.

8. The method (600) of any preceding claim, comprising causing a second actuation (304, 314) as the one or more subsequent actuations of the actuator (830, 930) and wherein break off of the droplet from the nozzle (820, 920) occurs after the second actuation (304, 314).

9. The method (500, 600) of any preceding claim, wherein the method (500, 600) is a method (500, 600) of ejecting a droplet of a non-Newtonian fluid from a droplet ejector (710a - 710e, 910a, 910b).

10. The method (500, 600) of any of claims 1 to 9, wherein the method (500, 600) is a method (500, 600) of ejecting a droplet of a high viscosity fluid from a droplet ejector (710a - 71 Oe, 910a, 910b) and wherein the high viscosity fluid has a viscosity of greater than 20cP.

11. The method (500, 600) of any preceding claim, comprising generating (1210) an electric field through which the droplet moves to further increase the momentum of the droplet by electrohydrodynamic forces.

12. A fluid droplet ejection system (700) comprising:a plurality of droplet ejectors (710a - 71 Oe, 910a, 910b), wherein each of the plurality of droplet ejectors (710a - 710e, 910a, 910b) comprises a nozzle (820, 920) and an actuator (830, 930) configured to cause displacement of a fluid through the nozzle (820, 920) wherein the nozzle (820, 920) is an aperturein a membrane (815, 915) and wherein the actuator (830, 930) is a piezoelectric actuator formed in or on the membrane (815, 915);a drive circuit (1025) configured to apply a signal to each of the actuators (830, 930); anda controller (800), the controller (800) configured to:cause a first actuation of the actuator (830, 930) to displace a first volume of the fluid through the nozzle (820, 920) of at least one of the plurality of droplet ejectors (710a - 71 Oe, 910a, 910b); and cause one or more subsequent actuations of the actuator (830, 930) to displace a second volume of the fluid through the nozzle (820, 920) of the at least one of the plurality of droplet ejectors (710a- 71 Oe, 910a, 910b) and to increase the momentum of the first volume of the fluid, the amplitude and timing of the first actuation and the one or more subsequent actuations (304, 314, 404, 414) selected to cause break off of the droplet from the nozzle (820) to occur after the final actuation of the one or more subsequent actuations (304, 314, 404, 414).

13. The fluid droplet ejection system (700) of claim 12, wherein the system (700) is an inkjet printhead.

14. The fluid droplet ejection system (700) of claim 12 or claim 13, wherein the droplet ejector (910a, 910b) comprises at least one electrode (932, 936), which is separate to the piezoelectric actuator (930) and is configured to generate an electric field through which the droplet moves to further increase the momentum of the droplet by electrohydrodynamic forces.

15. The fluid droplet ejection system (700) of any one of claims 12 to 14, wherein the piezoelectric actuator comprises a piezoelectric body provided between a corresponding pair of drive electrodes, wherein the piezoelectric body comprises scandium aluminium nitride.

16. The fluid droplet ejection system (700) of claims 12 to 15, wherein the momentum of the first volume of fluid caused by the first actuation is insufficient to cause droplet ejection.2617. A fluid suitable for use in the method (500, 600) of claims 1 to 11 , or in the fluid droplet ejection system (700) of claims 12 to 16.

18. The fluid of claim 17, when suitable for use in the method (500, 600) of claim 8 or any claim dependent thereon, wherein the fluid is a non-Newtonian fluid.

19. The fluid of claim 17, when suitable for use in the method (500, 600) of claim 9 or any claim dependent thereon, wherein the fluid is a high viscosity fluid having a viscosity of greater than 20cP.

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