Nanobubble generating system and method

Abstract

Described herein are a method (100) and a system (200) for the generation of a high concentration of nanobubbles by hydrostatic cavitation within a pressurised liquid. The method (100) includes: - applying a substantially transverse pressure (102) to a nanoporous conduit containing a liquid to compress the conduit and pressurise the liquid; wherein the compression is configured to force the liquid (104) within the nanoporous conduit into a plurality of connected nanopores within the inner surface of the nanoporous conduit; and - releasing (106) the substantially transverse pressure from the nanoporous conduit and depressurising the liquid to generate nanobubbles via hydrostatic cavitation within the liquid. The system (200) includes a reservoir (202) configured to store a liquid, a conduit (204) and a constriction device (206), preferably a peristaltic pump, configured to exert a substantially transverse pressure to the conduit (204) in order to pressurise the liquid within the conduit (204).

Description

[0001] NANOBUBBLE GENERATING SYSTEM AND METHOD

[0002] TECHNICAL FIELD

[0003] The present invention relates to an efficient, economical, and scalable method of producing nanobubbles. Also described herein is a system for the generation of nanobubbles.

[0004] BACKGROUND

[0005] Nanobubbles are gas bubbles with a diameter of approximately 200 nm or less. Nanobubbles are formed of a gas core and an outer shell. Nanobubbles are neutrally buoyant and can remain suspended in a liquid, commonly water, for long periods of time without rising to the surface, off-gassing, or suffering from degradation in their activity.

[0006] Nanobubbles generated in water comprise a gas core composition that reflects the ratio of diffused gases in the liquid, in addition to water vapour. The composition of the nanobubble surface / shell is based on hydroxide ions. Nanobubble colloids are beneficial for disinfection and organic-contaminant oxidative degradation.

[0007] Nanobubbles can arrange themselves to surround contaminants suspended in solution in a process that is comparable to the self-assembly formation of lipid micelles. Once surrounded, contaminants become more easily degradable.

[0008] The activity of nanobubbles has a wide range of commercial and industrial applications in various sectors, which include wastewater treatment, pharmaceuticals, brewing, agriculture, horticulture, functional material manufacturing, decontamination, and medical imaging.

[0009] Commonly employed methods of generating nanobubbles include ultrasonic cavitation via quasi-static compression and pressure release in the bulk liquid, and electrolysis. Nanobubbles are also produced as a secondary product during microbubbles’ formation using methods such as sonication, gas-liquid mixing, microfluidics, and mechanical agitation. A key drawback of sonication, a commonly used method of producing microbubbles and nanobubbles, is its incompatibility with hard water at room temperature. Sonication can cause cross-contamination and leaching of material from the effector in contact with the liquid. Sonication can also lead to the precipitation of amorphous calcium carbonate, leading to the formation of aragonite, thereby making the method impractical in areas with hard water sources.

[0010] Existing approaches are typically used to make small batches of nanobubbles. Many existing methods, such as those described above, cannot be feasibly scaled to an industrial level due to the amount of energy required, the generation method itself, or a combination thereof. Agricultural usage, for example, may require several thousand litres of high-concentration nanobubble solution per day, which is not attainable using current production methods due to a lack of energy efficiency, or because they do not generate nanobubbles populations having the desired distribution profiles or stability characteristics. Providing the required volumes / concentrations of nanobubbles using these methods would be prohibitively expensive, energy inefficient, and otherwise impractical.

[0011] In view of the limitations discussed above, there is a need for a versatile, compact, energyefficient, and scalable method for generating high populations of nanobubbles suspended in various media. The current invention aims to address some of these limitations.

[0012] STATEMENT OF INVENTION

[0013] According to the present invention, there is provided a system and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

[0014] According to a first aspect of the invention, there is provided a method of generating nanobubbles. The method of generating nanobubbles involves applying a substantially transverse pressure to a nanoporous conduit containing a liquid to compress the conduit and pressurise the liquid. The compression is configured to force the liquid that is contained within the nanoporous conduit into a plurality of connected nanopores within the inner surface of the nanoporous conduit. The method further comprises the step of releasing the substantially transverse pressure from the nanoporous conduit and depressurising the liquid to generate nanobubbles via hydrostatic cavitation within the liquid. Nanobubbles are generated via hydrostatic cavitation.

[0015] Advantageously, the method may provide an inexpensive and scalable way of generating high concentrations of nanobubbles stabilized by negative ions that are applicable for use in a variety of industrial applications, including agriculture / horticulture, manufacturing of functional materials, decontamination, pharmaceutical delivery, medical imaging, and disinfecting food and other products.

[0016] In another aspect of the invention, the nanoporous conduit may be repeatedly exposed to pressurisation and depressurisation to increase the yield of nanobubbles that are generated.

[0017] Advantageously, this may increase the amount of nanobubbles that can be generated. This arrangement may further enable the concentration of nanobubbles produced via this method to be increased. This arrangement may further enable the production of nanobubbles to be easily scaled.

[0018] The conduit may be a flexible polymer tubing (e.g., Tygon®).. The flexible tubing may have a compressibility from > 0.6 MPa to < 3.0 MPa. Preferably, the flexible polymer conduit or tubing may have a compressibility of < 3 MPa.

[0019] Advantageously, the flexibility of the tubing may be directly linked to the extent of hydraulic cavitation that may occur within the liquid. For example, a more flexible tubing may undergo more deformation during the pumping of the liquid, inducing a larger amount of hydraulic cavitation. As such, this type of conduit or tubing may provide an appropriate amount of flexibility to enable pressurisation of the liquid to the required extent for optimal nanobubble generation. This may in turn provide improved nanobubble generation.

[0020] In another aspect of the invention, nanobubbles may be generated at a concentration on the order of 109nanobubbles per millilitre. For example, the method may be operable to generate a nanobubble population with a concentration from > 5.0 x 107to < 5.0 x 109.

[0021] Advantageously, a higher concentration of nanobubbles in the solution may improve the useful (e.g., cleaning, disinfection germination etc.,) properties of the nanobubble solution. The higher concentration of nanobubbles in a given volume may also increase the efficacy of the nanobubble water, as used in agriculture, irrigation, fertigation, wastewater treatment, pharmaceuticals, brewing and any other uses whereby a higher concentration may provide a benefit.

[0022] In another aspect of the invention, nanobubbles may be generated with a mean average hydrodynamic diameter of < 200 nanometres. For example, the method may be operable to generate a nanobubble population with an average nanobubble diameter from > 120 nm to < 200 nm. The average nanobubble diameter may be an average nanobubble hydrodynamic diameter.

[0023] Advantageously, nanobubble production using this method may be scaled to be suitable for various purposes in the fields of agriculture, irrigation, fertigation, wastewater treatment, pharmaceuticals, and brewing. The skilled person will appreciate that nanobubbles may have further uses in other fields not listed here.

[0024] In another aspect of the invention, the average energy consumption for generating nanobubbles at a concentration on the order of 109nanobubbles per millilitre with a mean average size of approximately 200 nanometres may be between 0.1 KWh / m3and 0.5 KWh / m3.

[0025] Advantageously, this arrangement enables nanobubbles to be produced in an energyefficient way, thereby enabling the method to be scaled to produce larger volumes of nanobubble water. The disclosed method may also be used in remote locations with a limited energy supply.

[0026] In another aspect of the invention, liquid may be forced through the nanoporous conduit at a linear velocity of between 0.01 metres per second and 100 metres per second.

[0027] Advantageously, this liquid flow rate may provide the optimal conditions for effective and efficient nanobubble generation.

[0028] In another aspect of the invention, the method may further comprise the step of increasing the Zeta potential of the nanobubble containing liquid via exposure to a non-uniform permanent magnetic field acting transverse to the direction of the liquid flow.

[0029] Advantageously, exposure to a magnetic field may confer beneficial changes in the zeta potential of the colloid, thereby improving the longevity of the generated nanobubble samples. The liquid may be water. The liquid may be a buffer, an appropriate buffer may be a neutral phosphate buffer or a basic alkaline borate buffer.

[0030] Advantageously, water may be an inexpensive liquid in which successful nanobubble generation may be performed. Water may further provide an adequate storage liquid for the generated nanobubbles. Further, water is present in a wide range of locations. This may allow nanobubbles to be generated locally relative to their intended place of use. The use of an appropriate buffer may increase the stability, lifespan, and / or half-life of the generated nanobubbles.

[0031] The method may be applied to a volume of liquid that has already passed through the system, as discussed in more detail below. In other words, hydrostatic cavitation may be applied to the same volume of liquid repeatedly.

[0032] Advantageously, this arrangement may enable the same volume of liquid to be subject to hydrostatic cavitation multiple times, thereby increasing the number of nanobubbles generated and increasing the resulting concentration of nanobubbles in solution.

[0033] The method may be operable to generate a nanobubble population with an average zeta potential from < -20 mV to > -35 mV.

[0034] In order to achieve a flow rate sufficient to induce cavitation and also fill the nanoporous inner surface of the conduit. The liquid flow may be forced through the nanoporous conduit at a flow rate > 1 to < 14 mLs-1cm'2.

[0035] Advantageously, the electrically charged liquid-nanobubble interface may create a sufficient short-range repulsive force to impede bubble coalescence and stabilize the nanobubble colloid, thereby increasing the longevity of the generated nanobubble population.

[0036] The liquid may be passed around a nanobubble generating system, as discussed in more detail below, as many times as required to provide the necessary volume and / or concentration of nanobubble for the intended application.

[0037] According to a second aspect of the present invention, there is provided a system for generating nanobubbles. The system comprises a reservoir; a conduit, and a constriction device. The construction device may be a pump. As used herein, the term pump and constriction device are used interchangeably. The pump may be any suitable pump. Preferably, the pump may be a peristaltic pump configured to provide both compression or constriction of the nanoporous conduit forthe generation of nanobubbles, in addition to providing movement of the liquid contained within said conduit. The reservoir is configured to store a liquid. The conduit is configured to provide fluid communication between the reservoir and the pump. The pump is configured to exert a substantially transverse pressure to the conduit to pressurise the liquid within the conduit. The substantially transverse pressure exerted upon the conduit is configured to induce hydrostatic cavitation within the liquid to generate nanobubbles. The induced hydrostatic cavitation may arise due to the surface porosity of the conduit. The hydrostatic cavitation may thereby generate nanobubbles within the liquid.

[0038] Advantageously, this arrangement may provide an inexpensive and scalable way of generating high concentrations of nanobubbles that are applicable for use in a variety of industrial applications, such as agriculture / horticulture, manufacturing of functional materials, decontamination, pharmaceutical delivery, medical imaging, and disinfecting food and other products.

[0039] According to another aspect of the present invention, the conduit may comprise a nanoporous inner surface comprising a plurality of connected nanopores. The pressure acting upon the conduit by the pump may force pressurised liquid into the plurality of connected nanopores. This may then induce hydrostatic cavitation to generate nanobubbles on the release of pressure.

[0040] According to another aspect of the present invention, the inner surface of the conduit may comprise an average pore size of < 300 nm.

[0041] Advantageously, pores having a size mentioned above may produce nanobubbles with an average diameter within a preferred size range.

[0042] In another aspect of the present invention, the conduit may comprise a multi-wall tube, such as a flexible multi-walled tube. The multi-walled tubing may comprise at least an inner tube and an outer tube. The inner tube may be configured to enable generation of nanobubbles as described herein. In other words, the inner tube may comprise an inner surface comprising a plurality of connected nanopores that enable nanobubble generation as described herein. Meanwhile, the outer tube may be configured to protect the inner tube.

[0043] Advantageously, the use of a multi-wall conduit or tube may provide a conduit having an increased lifetime, because the outer wall may shield and / or protect the inner tubing.

[0044] According to another aspect of the present invention, the conduit may be arranged to provide fluid communication between the reservoir and pump in a loop. The looped arrangement may provide repeated circulation of the liquid. In use, the system may be operable to continuously generate nanobubbles as long as the pump is active. In other words, while the pump is switched on, the liquid may pass from the reservoir, through the pump, back to the reservoir, and continue as described until the pump is switched off. When in use, the apparatus may be operable to provide continuous nanobubble generation. The apparatus may also be configurable to work without the reservoir, for example, the apparatus could be connected directly to a water source.

[0045] Advantageously, this arrangement of the system may enable continuous nanobubble generation to be applied. This may be particularly useful when producing larger volumes of nanobubbles, or higher concentrations of nanobubbles. Recirculation of the liquid within the system may therefore provide samples containing nanobubbles that provide enhanced effects. According to another aspect of the present invention, the substantially transverse force exerted by the constriction device on the conduit induces a pressure in the entrapped liquid that may be substantially isotropic.

[0046] Advantageously, this may provide more consistent nanobubble generation.

[0047] According to another aspect of the present invention, the system may further comprise a magnetic unit. The magnetic unit may be configured to apply a non-uniform magnetic field to the liquid passing through the conduit. Application of the magnetic field to the liquid may be configured to provide an enhancement to the zeta potential of the nanobubbles. The magnetic field may be applied in a direction that is transverse to the flow of the liquid passing through the conduit. In other words, the magnetic unit may be adapted to apply the magnetic field at an angle substantially orthogonal to a longitudinal axis of the conduit, and therefore, substantially orthogonal to the direction of liquid flow through the conduit.

[0048] Advantageously, increasing the zeta potential of the generated nanobubbles may enhance stability and longevity of the nanobubbles.

[0049] Preferably, the constriction device may be configured to constrict the conduit, pressurising the fluid contained therein, whilst also translating a volume of fluid along the conduit to provide a pumping action. The constriction and release of the conduit by the constriction device provides the relevant conditions for nanobubble generation when applied to a conduit having a nanoporous or blistered inner surface. Any suitable type of peristaltic pump may be used that can provide the necessary surface contact with the conduit and liquid contained therein to deliver the necessary pressure for nanobubble generation. In some cases, a dual or multiple peristaltic pump arrangement may be used. The peristaltic pump may be a linear or rotary peristaltic pump.

[0050] Advantageously, constriction devices, such as peristaltic pumps, may be capable of providing efficient pumping, whilst also providing optimal liquid pressures for the generation of nanobubbles.

[0051] The conduit may be arranged to pass through a portion of the pump comprising a moveable component and at least two rollers, herein referred to as the pump head. The movable component may be, for example, a rotor. The pump may comprise a plurality of rollers. The conduit may be arranged along a linear, curved, or part-circular path through the pump head. The rotor may be coupled to the roller, wherein movement of the rotor is configured to move, or translate, the roller relative to the conduit. The translation of the roller may bring the roller into contact with the outside of the conduit. The contact between the roller and the conduit may compress the conduit, thereby pressurising the liquid contained therein. The roller may pass along a portion of the conduit that passes through the pump head. The compression of the conduit may have a pinching effect to prevent the backward flow of a volume of liquid in the conduit. The translation of the roller along the conduit moves the portion of the conduit that is compressed by the roller, thereby displacing the liquid therein in a forward direction along the conduit. The forward direction may be dependent upon the arrangement of the pump and how the conduit is connected to the pump. Typically, the pump may comprise an inlet and an outlet. The translation of the rollers within the pump will result in liquid being taken in via the inlet and forced towards and out of the outlet. The rollers may be configured to concurrently roll along a conduit, thereby ensuring that the liquid within the conduit does not flow backwards for part of the pumping cycle.

[0052] As described, the pump may be configured to exert a transverse pressure on the conduit. The transverse pressure may be applied to the conduit in a linear motion, a sweeping or curved motion, or a combination thereof.

[0053] Advantageously, the exertion of a transverse pressure on the conduit may provide pressurisation of the liquid and may induce hydraulic cavitation and nanobubble generation.

[0054] As used herein, the term conduit may refer to a tube, or a plurality of tubing sections that may be connected together via a series of other components. For example, a plurality of tubing sections may be provided to provide fluid communication between the reservoir, the pump, and a magnetic unit.

[0055] The conduit may comprise an outer surface and an inner surface, the inner surface being in contact with a liquid contained therein. The conduit may comprise a nanoporous inner surface. The nanoporous inner surface may comprise a plurality of micron-sized blisters or pores beneath the surface. During fluid communication between components, the pressurised liquid may pass across the nanoporous surface of the conduit. The conduit may be adapted to induce hydrostatic cavitation upon passage of the pressurised liquid across the nanoporous surface of the conduit. The hydrostatic cavitation may generate nanobubbles within the liquid.

[0056] Hydrostatic cavitation, as described herein, refers to a process whereby the nanobubbles are nucleated by a ‘nano-sponging’ effect, which occurs when water is squeezed into, and then released from, connected nanopores in the conduit.

[0057] Advantageously, the nanoporous inner surface of the conduit may be adapted to enable hydrostatic cavitation to occur within the pressurised liquid passing through the conduit, thereby providing a secondary source of nanobubble generation. This technique of applying hydrostatic cavitation for nanobubble generation may improve the efficiency and scalability of nanobubble generation using the system described herein.

[0058] The nanoporous inner surface of the conduit may comprise a plurality of micron-sized blisters, and / or open pores. The pores may comprise an average pore size, measured across their diameter. The average size of the surface blisters or pores may be from > 100 nm to < 500 nm, such as > 200 nm to < 400 nm, such as > 250 nm to < 350 nm, such as < 300 nm.

[0059] The nanoporous inner surface may comprise an average number of pores within a unit area. In other words, the surface blisters or pores may have an average density. The inner surface of the conduit may have an average surface blister or pore density from > 10 to < 20 pm-2, or > 5 to < 10 pm-2, or > 0.1 to < 0.5 pm-2.

[0060] The conduit may be a permeable conduit. The conduit may have a gas permeability coefficient from > 1 x10-3cm2s-1Pa-1to < 6 cm2s-1Pa-1. The permeability coefficient may be a nitrogen or oxygen permeability coefficient. Advantageously, the permeability of the conduit may correlate with the amount of dissolved gas available within the liquid that is available to form the gaseous core of nanobubbles. The flexible tubing may be Tygon® tubing, such as, GY-96410-18: Platinum-Cured Silicone, GY-96400-18: Peroxide-Cured Silicone, GY-96440-18: BioPharm Plus Platinum-Cured Silicone, G Y-96419-18: Puri-Flex®, GY-06434-18: C-Flex® ULTRA.

[0061] In some examples, the optimal porosity of the conduit may be dependent upon the amount of dissolved gasses that are available within the liquid.

[0062] The conduit may comprise an inner diameter from > 8.0 mm to < 12.0 mm, such as 9 mm.

[0063] The liquid may be pressurised by the peristaltic pump to a pressure of > 0.5 bar to < 1 .5 bar, such as > 0.5 bar to < 1 .0 bar. Advantageously, this pressure range may provide optimal nanobubble generation.

[0064] The magnetic unit may be adapted to apply a static magnetic field. In other words, the magnetic field may be substantially stationary and may not change direction in time. The static magnetic field may have a constant field strength. The static magnetic field may have a constant magnetic flux density. The static magnetic field may have a variable field strength. The static magnetic field may have a variable magnetic flux density. The magnetic field may be non-uniform. The magnetic field may have an average magnitude 100 mT that is directed transverse relative to the flow. The magnetic field may have a field gradient of ± 10Tm-1at each end, using permanent magnets (made of ferrite, NdFeB or SmCo) a length of 0.5 - 15 cm each. The magnetic unit may be adapted to apply the magnetic field using a permanent magnet. The magnetic unit may be operable to expose the liquid flow to a magnetic flux density of from > 100mT to < 400 mT.

[0065] BRIEF DESCRIPTION OF DRAWINGS

[0066] The invention will now be described by way of example only with reference to the figures, in which:

[0067] Figure 1 shows a method for generating nanobubbles according to a first aspect of the present invention. Figure 2 shows a schematic of a nanobubble generation system according to a second aspect of the present invention.

[0068] Figure 3 shows a schematic of a nanobubble generation system according to a second another of the present invention.

[0069] Figure 4 shows a schematic of a nanobubble generation system according to another aspect of the present invention.

[0070] Figure 5a shows a plot of experimental data showing the relationship between Zeta potential of a nanobubble sample, produced using tap water using the nanobubble generation system of the present invention, and the flow rate of the liquid, with or without exposure to a magnetic field.

[0071] Figure 5b shows a plot of experimental data showing the relationship between Zeta potential of a nanobubble sample, produced using Evian™ water (hard water) using the nanobubble generation system of the present invention, and the number of passes through the nanobubble generation system, with or without exposure to a magnetic field.

[0072] Figure 6a shows a plot of experimental data showing the relationship between Zeta potential of a nanobubble sample for deionised water, produced using the nanobubble generation apparatus of the present invention, and the number of passes through the nanobubble generation system, with or without exposure to a magnetic field.

[0073] Figure 6b shows a plot of experimental data showing the relationship between Zeta potential of a nanobubble sample, produced using the nanobubble generation system of the present invention, and the time elapsed after production of a nanobubble sample.

[0074] Figure 6c shows a plot of experimental data showing a concentration and size distribution of a nanobubble sample produced using the nanobubble generating method and system of the present invention.

[0075] Figures 7a and 7b show scanning electron microscope images of the inner surface of a new tube and the surface after nanobubble generation, respectively.

[0076] DETAILED DESCRIPTION

[0077] Hereinafter, various examples will be described with reference to the accompanying figures. The examples described below may be modified and implemented in various forms. In order to more clearly describe features of the examples, detailed descriptions of matters well known to those skilled in the art which the following examples belong will be omitted.

[0078] In the present disclosure, when an element is described as ‘connected’ or ‘coupled’ with another element, this includes not only ‘directly connected’ or ‘directly coupled’, but also ‘connected with another element there between’ or ‘coupled with another element there between’. In addition, when one element is described to ‘include’ another element, this means that, unless specifically stated otherwise, the one element may further include other elements rather than excluding other elements.

[0079] Figure 1 shows a method 100 of generating nanobubbles according to a first aspect of the present invention. The method 100 is compatible with the system 200 discussed above and below with reference to Figures 2-4.

[0080] The method 100 includes the step 102 of applying a substantially transverse pressure to a nanoporous conduit 204 containing a liquid, to compress the conduit and pressurise the liquid.

[0081] The method 100 further includes the step 104 of forcing the liquid into a plurality of connected nanopores. The plurality of connected nanopores exist across and within the inner surface of the conduit.

[0082] The method 100 further includes the step 106 of releasing the substantially transverse pressure and depressurising the liquid. This combination of effects generates nanobubbles via hydrostatic cavitation.

[0083] The method 100 may be extended to repeat steps 102-106 to continuously produce nanobubbles for a given volume of liquid. This may enable larger volumes of nanobubbles to be generated or may allow higher concentrations of nanobubbles to be generated.

[0084] The method 100 may be applied to liquid samples containing waste products for removal. For example, method 100 may be applied to samples of wastewater. The method 100 may be capable of reducing the total organic content e.g., for the removal of agro-chemicals, dyes or solvent traces from wastewater. One important example is the removal of dioxane from drinking water, which has a maximum contaminant tolerance of 1 .0 part per billion for dioxane. The method 100 may be capable of efficiently eliminating dioxane from wastewater, exposure to which can cause damage to the central nervous system, liver, and kidneys.

[0085] Figure 2 shows an aspect of the nanobubble generating system 200 of the present invention. The system 200 comprises a reservoir 202. The reservoir 202 stores a liquid, such as water. The skilled person will appreciate that the reservoir 202 may be used to store other fluids that are compatible with nanobubble generation. This skilled person will further appreciate that the size of the reservoirs used may be scaled up or down according to the system and the intended use. For example, larger reservoirs may be used for scaled up operations where larger quantities of nanobubbles are required.

[0086] The system 200 further comprises a pump 206. The pump 206 may be a peristaltic pump. The peristaltic pump is configured to compress a conduit to pressurise a fluid contained within the conduit.

[0087] The pump 206 is configured to exert a transverse pressure on a conduit 204. The conduit 204 is arranged to provide fluid communication between the reservoir 202 and the pump 206 in a continuous or recirculating loop. In other words, the liquid flows from the reservoir 202, to the pump 206, and back to the reservoir 202.

[0088] As shown in Figure 3, the system 200 may further comprise at least two T-valves 210 or equivalent components. The T-valves 210 may be arranged along sections of conduit 204 on either side of the pump 206. A bypass route 208 may be provided and arranged between the at least two T-valves 210, as shown in Figure 3. When using this arrangement, a volume of liquid can be recycled directly, without dilution, to the inlet of the pump 206 though the bypass 208. In other words, liquid can be immediately passed back through the pump 206 without being passing via the reservoir 202. The bypass route 208 may be formed using tubing, for example, the same tubing used for the conduit 204.

[0089] The liquid may be recirculated around the system a plurality of times. For example, the liquid may be recirculated around the system > 5 times, such as > 10 times, or > 20 times.

[0090] The conduit 204 is a tubing comprising an inner and outer surface. The inner surface of the conduit 204 is nanoporous. In other words, the inner surface of the conduit 204 comprises a plurality of nanoscale pores, covered with surface pores or blisters. The pores / blisters may be popped out during rolling by pump, such as a peristaltic pump, to open-up the network of nanopores in the conduit.

[0091] The surface blisters / pores of the inner surface of the conduit 204 may have an average spatial density of 15 pores per pm2.

[0092] The conduit 204 is formed of a permeable material. The conduit 204 has a gas permeability coefficient of < 9.0 cm2s-1Pa'1. The conduit 204 has an oxygen permeability coefficient of < 6.0 cm2s-1Pa'1. The conduit 204 is a flexible tubing with a compressibility of < 3 MPa. The conduit has an inner diameter of 9.0 mm.

[0093] The pump 206 is configured to exert a transverse pressure on a conduit 204. The transverse pressure increases the pressure of the liquid within the conduit 204, forcing the liquid to flow and provide the required pressure to facilitate nanobubble generation within the connected nanoporous walls of the conduit. The transverse pressure is exerted at an angle of approximately 90° to the longitudinal axis of the conduit 204.

[0094] The exertion of the transverse pressure on the conduit induces hydrostatic cavitation within the liquid to generate nanobubbles within the liquid. The passage of the pressurised liquid across the nanoporous surface of the conduit 204 further induces hydrostatic cavitation to generate nanobubbles within the liquid.

[0095] The pump 206 may provide a liquid flow rate around the system 200 of > 1 to < 14 mLs- 'em'2, preferably a flow rate of 4 mLs 'cm-2. The pump 206, in combination with the diameter of the conduit 204 may be capable of providing a liquid flow velocity from 0.01 ms-1to 100 ms-1. The nanobubble population that is produced as described herein is collected in the reservoir 202. The generated nanobubbles have a mean average hydrodynamic diameter between 120 nm and 200 nm. The generated nanobubbles have a Zeta potential of -30 ± 10 mV.

[0096] Figure 4 shows another aspect of the system 200 according to the present invention. This aspect of the system 200 is identical to the arrangement shown in Figure 2, except that the system 200 further comprises a magnetic unit 212, as shown in Figure 4.

[0097] The magnetic unit 212 may be adapted to apply a magnetic field to the liquid passing through the conduit 204 in a direction that is transverse or orthogonal to the direction of liquid flow through the conduit 204.

[0098] The magnetic unit 212 comprises a permanent magnet that is adapted to apply a static magnetic field to the liquid flow passing through the conduit 204 with a fixed or substantially constant magnetic field strength and / or magnetic flux density. The field in the device may or may not be substantially constant, but the liquid experiences a magnetic field gradient upon passing through the device, regardless of field direction.

[0099] The magnetic unit 212 is configured to provide the liquid flow in the conduit 204 with a magnetic flux density of approximately 200 mT. Exposure to the magnetic field is operable to increase the Zeta potential of the liquid between 10% and 20%.

[0100] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word ‘about’, even if the term does not expressly appear. The term ‘about’ when used herein means + / - 10% of the stated value.

[0101] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0102] 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.

[0103] 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.

[0104] Reference in the specification to ‘an example’, ‘an embodiment’, ‘an aspect’ or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example, but not necessarily in other examples. The various instances of the phrase ‘in one example’ or similar phrases in various places in the specification are not necessarily all referring to the same example. In describing and claiming examples disclosed herein, the singular forms ‘a’, ‘an’, and ‘the’ include plural references unless the context clearly dictates otherwise.

[0105] While several examples have been described in detail, it is to be understood that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting. It should be understood that the examples described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each example should typically be considered as available for other similar features or aspects in other examples. While one or more examples have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made.

[0106] EXPERIMENTAL DATA

[0107] In the experimental examples provided below, Tyndall light scattering using a 532 nm green laser was used to detect the presence of nanobubbles in a liquid. The stability of the nanobubbles was measured in triplicate 100-fold measurements of Zeta potential using Malvern Zetasier with DTS1070 plastic cells. The nanobubble populations were measured using a Malvern Zetasizer Ultra. Where appropriate, measurements were taken over the course of several days to monitor long-term stability. Nanobubbles were produced using the system 200 via the method 100 as described herein.

[0108] Experiment 1 - Effect of tubing

[0109] Identical experiments were performed using five different types of Tygon® tubing including: GY-96410-18: Platinum-Cured silicone (T1), GY-96400-18: Peroxide-Cured Silicone (T2), GY- 96440-18: BioPharm Plus Platinum-Cured Silicone (T3), GY-96419-18: Puri-Flex® (T4), GY- 06434-18: C-Flex® ULTRA (T5). Each tubing used herein had an inner diameter of 9 mm. Deionised water was flowed around the system at a flow rate of 4.0 mL / s for a total of 20 passes per tubing variant. The results are shown below in Table 1.

[0110] Table 1. O2Gas permeability and Zeta potential characteristics of five Tygon® tubing types (T1-T5) resulting from 20 passes of deionised water.

[0111] The circulated liquid was collected and the generated nanobubbles were measured using the characterisation techniques described above.

[0112] Tubing types T1 and T2, which have the highest inner surface nanoporosity, were found to provide the largest nanobubble concentrations of the order 109nanobubbles per millilitre. Tubing T 1 and T2 were also identified to produce a consistent diameter of approximately 200 nm. This is reflected by the increase in measured Zeta potential. In contrast, tubing types T3-T5 did not provide a significant increase in Zeta potential. This study confirmed that the nanoporosity of the inner surface of the tubing significantly contributes to nanobubble generation occurring.

[0113] Figures 6a and 6b show scanning electron microscope (SEM) images of the inner surface of a new T1 type tube before (Figure 7a) and after (Figure 7b) nanobubble generation circulation experiments.

[0114] Experiment 2 - Effect of liquid flow rate

[0115] Tap water was circulated through the system with T2 tubing used as the conduit. Liquid flow rates from 1 to 14 mLs-1cm'2were applied to identify the effects on the Zeta potential and nanobubble generation. This was performed for a total of 20 passes. Samples were exposed or not exposed to a magnetic field with a magnetic flux density of up to 200 mT. The relevant results are shown in Figures 5a and 5b. Figures 5a shows the zeta potential for tap water with 0.05% surfactant after 20 passes as a function of flow rate through the pump using T2 tubing. Figure 6b shows the Zeta potential for tap water with nanobubble after adding various concentration of surfactant and monitored with time. The data shows magnetic enhancement of approximately 15% at a flow velocity of 4 mLs-1(5.3 cm / s).

[0116] Table 2 shows the Zeta potential for magnetic and non-magnetic exposed samples for tap water, deionised water (Millipore™ water), and Evian™ mineral water. Table 2. Zeta potential for Millipore™ , tap, and Evian™ water after 20 passes with T2 tubing at a flow rate of 4.0 ml_s'1.

[0117] Experiment 3 - Effect of water type

[0118] Tap water, Millipore™ water, and commercial Evian™ mineral water were circulated through the nanobubble generation system using a flow rate of 4.0 mL / s for a total of 40 passes. Samples were exposed or not exposed to a magnetic field with a magnetic flux density of 200 mT. The initial Zeta potential measurements for Evian™ and tap water were between -6 to -12 mV before circulation, respectively. A Zeta potential of -30 ± 6 mV was achieved for all three water types irrespective of the hardness of the water. The differences in developed Zeta potential for various water samples are within experimental error. Samples that were exposed to the magnetic field showed an enhancement for tap water (soft) and Millipore™ water (deionised), but not in the hard Evian™ mineral water. In the latter case (for the hard water), magnetic exposure of nanobubble water can be found to modify the stability of existing nanometre-size pre-nucleation clusters of amorphous hydrate CaCCh.

[0119] Experiment 4 - Energy usage for NB generation

[0120] Nanobubbles were generated using a well-known ultrasonic cavitation method. The temperature of the liquid (water) was measured during the generation of nanobubble samples. The water temperature was measured to rise from 20 °C to 50 °C during ultrasonic cavitation. In contrast, the same measurements performed on nanobubble samples that were produced using the nanobubble generation system of the present invention, and associated method, exhibited a minor increase in the water temperature from 18 °C to 21 °C after 20 passes. The nominal electrical input required to create a single millilitre of nanobubble water was also compared. The ultrasonic method was run for 20 minutes at 100 W to produce a 20 mL nanobubble sample, giving an approximate energy input of 6 megajoules per litre of nanobubble water with an average concentration of 109nanobubbles per millilitre and an average size of approximately 200 nm. In contrast, the method 200 is capable of generating 3 L of nanobubble water within 11 minutes using a pump at 36 Watt of electrical power, giving an approximate energy input of 8 kJ / L of nanobubble water. Experiment 5 - Effect of magnetic field exposure

[0121] Tap water, Millipore™ water, or Evian™ mineral water was passed through the nanobubble generation system 40 times. The appropriate water type was passed through an arrangement as shown in Figure 4, the liquid flow was not exposed to a magnetic field. The resulting nanobubble populations were measured as previously described. A separate sample of the appropriate water type was passed through an arrangement as shown in Figure 4, the liquid flow was exposed to an inhomogeneous magnetic field with a flux density of ~ 200 mT. The results demonstrated that the Zeta potential of the measured nanobubble solution was enhanced for samples produced using Millipore and soft tap water, but not for those produced in the Evian™ mineral water samples. The magnetic field profile of the Ecocamel™ device used has a square arrangement of 10 x 10 x 100 mm ferrite magnets configured to generate a transverse field with a magnitude of 100 mT over most of the length, with a field gradient VB = 20 Tm-1at the ends.

[0122] Experiment 6 - General properties of generated nanobubbles

[0123] Nanobubble samples were produced in Millipore™ water using the nanobubble generation system of the present invention, the results are shown in Figures 6a-c. The liquid was circulated 20 times at a flow rate of 4.0 mLs-1using T2 tubing. During circulation, the samples were exposed or not exposed to a magnetic field with a magnetic flux density magnitude of 200 mT and field gradient of 20T / nr1. As Figure 6a shows the effect on Zeta Potential of the number of passes (up to 20) for Millipore through T2 tubing. Water flowed through the system once exhibited a Zeta potential of -10.2 mV, suggesting that a single pass has little effect on the Zeta potential. Millipore water attained a saturation Zeta potential of -33 ± 5 mV when not exposed to a magnetic field, and a Zeta potential of -35 ± 5 mV when exposed to a magnetic field. The enhancement in the magnetic channel is small but systematic, approximately 4 - 7% after multiple passes. Figure 6b shows a time decay curve illustrating the change in Zeta potential over a period of 100 hours for a nanobubble sample produced in Millipore™ water using the system of the present invention. The stability of Millipore nanobubbles was monitored by the decay of Zeta potential, which fell of half its initial value after 80 hours. Figure 6c shows nanoparticle tracking analysis data for nanobubbles produced in Millipore™ water. The results illustrate that the circulated solution has nanobubbles with a hydrodynamic diameter of 200 nm and a bubble population of ~ 109nanobubbles per millilitre.

Claims

CLAIMS1. A method (100) of generating nanobubbles, comprising: applying a substantially transverse pressure (102) to a nanoporous conduit containing a liquid to compress the conduit and pressurise the liquid; wherein the compression is configured to force the liquid (104) within the nanoporous conduit into a plurality of connected nanopores within the inner surface of the nanoporous conduit; releasing (106) the substantially transverse pressure from the nanoporous conduit and depressurising the liquid to generate nanobubbles via hydrostatic cavitation within the liquid (106).

2. The method (100) of claim 1 , wherein the nanoporous conduit is repeatedly exposed to pressurisation and depressurisation (102, 106) to increase the yield of nanobubbles that are generated.

3. The method (100) of claims 1 or 2, wherein nanobubbles are generated at a concentration on the order of 109nanobubbles per millilitre.

4. The method (100) of any preceding claim, wherein nanobubbles are generated with a mean average hydrodynamic diameter of < 200 nanometres.

5. The method (100) of claims 3 and 4, wherein the average energy consumption for generating nanobubbles at a concentration on the order of 109nanobubbles per millilitre with a mean average size of approximately 200 nanometres is between 0.1 KWh / m3and 0.5 KWh / m3.

6. The method (100) of any preceding claim, wherein the liquid is forced through the nanoporous conduit at a linear velocity of between 0.01 metres per second and 100 metres per second.

7. The method (100) of any preceding claim, further comprising the step of increasing the Zeta potential of the nanobubble containing liquid via exposure to a non-uniform permanent magnetic field acting transverse to the direction of the liquid flow.

8. A system (200) for generating nanobubbles, comprising: a reservoir (202), configured to store a liquid; a conduit (204), a constriction device (206); and wherein the conduit is configured to provide fluid communication between the reservoir (202) and the constriction device (206); wherein the constriction device (206) is configured to exert a substantially transverse pressure to the conduit (204) to pressurise the liquid within the conduit (204), wherein the transverse pressure exerted upon the conduit (204) is configured to induce hydrostatic cavitation within the liquid to generate nanobubbles.

9. The system (200) of claim 8, wherein the conduit (204) comprises a nanoporous inner surface comprising a plurality of connected nanopores, wherein the pressure acting upon the conduit (204) is configured to force pressurised liquid into the plurality of connected nanopores to induce hydrostatic cavitation to generate nanobubbles.

10. The system (200) of claim 9, wherein the inner surface of the conduit (204) comprises an average pore size of < 300 nm.11 . The system (200) of any one of claims 8-10, wherein the conduit (204) is arranged to fluidly connect the reservoir (202) and constriction device (206) in a recirculating loop, wherein, in use, the system (200) is operable to continuously generate nanobubbles.

12. The system (200) of any one of claims 8-11 , wherein the conduit (204) is a flexible polymer tubing, with a compressibility of < 3.0 MPa.

13. The apparatus (100) of any one of claims 8-12, wherein the liquid is pressurised to < 1.0 bar.

14. The system (200) of any one of claims 8-13, further comprising a magnetic unit (212) configured to apply a non-uniform magnetic field to the liquid passing through the conduit (204), wherein the application of the magnetic field to the liquid is configured to provide an enhancement to the zeta potential of the nanobubbles.

15. The system (200) of claim 14, wherein the magnetic field is applied transverse to the flow of the liquid passing through the conduit (204).

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