WO2024118648A1

WO2024118648A1 - Fluid transport and atomization method

Info

Publication number: WO2024118648A1
Application number: PCT/US2023/081416
Authority: WO
Inventors: James Friend; Amihai HORESH; William CONNACHER; Kha Hongmai NGUYEN
Original assignee: University of California Berkeley; University of California San Diego UCSD
Current assignee: University of California Berkeley; University of California San Diego UCSD
Priority date: 2022-11-29
Filing date: 2023-11-28
Publication date: 2024-06-06
Anticipated expiration: 2025-05-29

Abstract

An atomizer includes a substrate with piezoelectric material for generating acoustic energy. The atomizer includes a fluid reservoir attached to a first surface of the substrate. The atomizer causes acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, where the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate. The acoustic energy causes the fluid on the second surface of the substrate to be atomized. In an example, thrust may be produced by atomizing the fluid flowing through the opening onto the second surface of the substrate. The diameter of the opening may be in a range between one hundredth of a wavelength of an acoustic wave traveling in the substrate and one half of the wavelength of the acoustic wave traveling in the substrate.

Description

FLUID TRANSPORT AND ATOMIZATION METHOD

Cross-Reference to Related Applications

[001] The present application claims priority to U.S. Provisional Patent Appl. No. 63/385,290 to Friend et al., filed November 29, 2022, and entitled “Thrustonsonic: Acoustic Microthruster for Spacecraft,” and incorporates its disclosure herein by reference in its entirety. The present application also claims priority to U.S. Provisional Patent Appl. No. 63/507,592 to Friend et al., filed June 12, 2023, and entitled “Thrustonsonic: Acoustic Microthruster for Spacecraft,” and incorporates its disclosure herein by reference in its entirety.

Technical Field

[002] The present disclosure generally relates to ultrasonic atomization techniques.

Background

[003] The number of small spacecraft has greatly increased over the last ten years in performing a variety of near and deep space missions. Once lofted into a target orbit, they require small and nimble thrusters for maneuvers and stationkeeping. As spacecraft and satellites grow in number and decrease in size, the need for reliable and lightweight means of attitude control likewise grow. With thousands of launches per year required for advanced telecommunications applications, alongside interplanetary missions for tiny “Cubesaf ’ satellites, and tourism flights into low earth orbit, there have never been so many space-bound platforms in need of thrusters, particularly small thrusters. These craft do not need large thrust, their typical needs are on the order of micro to milli Newtons, but thrust application must be tightly controlled to execute maneuvers, and the mass, volume, and power constraints are restrictive. Additionally, unless there is some way to refuel them, their active lifetime is dependent on the specific impulse. There are a large number of candidates being actively researched: chemical thrusters (e g., butane), cold gas thrusters, Hall Effect and ion thrusters, and electrospray thrusters to name a few. A new technology, Film-evaporated MEMS Tunable Array (FEMTA), offers good thrust to power consumption ratios and is extremely compact and light weight compared to the more mature options above, but at the cost of relatively low absolute thrust and specific impulse.

Summary

[004] In some implementations, an atomizer includes a substrate with piezoelectric material for generating acoustic energy. The atomizer includes a fluid reservoir attached to a first surface of the substrate. The atomizer causes acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, where the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate. The acoustic energy causes the fluid on the second surface of the substrate to be atomized. In an example, thrust may be produced by atomizing the fluid flowing through the opening onto the second surface of the substrate. The diameter of the opening may be in a range between one hundredth of a wavelength of an acoustic wave traveling in the substrate and one half of the wavelength of the acoustic wave traveling in the substrate.

[005] Non-transitory computer program products (i.e., physically embodied computer program products) are also described that store instructions, which when executed by one or more data processors of one or more computing systems, causes at least one data processor to perform operations herein. Similarly, computer systems are also described that may include one or more data processors and memory coupled to the one or more data processors. The memory may temporarily or permanently store instructions that cause at least one processor to perform one or more of the operations described herein. In addition, methods can be implemented by one or more data processors either within a single computing system or distributed among two or more computing systems. Such computing systems can be connected and can exchange data and/or commands or other instructions or the like via one or more connections, including a connection over a network (e.g., the Internet, a wireless wide area network, a local area network, a wide area network, a wired network, or the like), via a direct connection between one or more of the multiple computing systems, etc.

[006] The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims.

Brief Description of the Drawings

[007] The accompanying drawings, which are incorporated in and constitute a part of this specification, show certain aspects of the subject matter disclosed herein and, together with the description, help explain some of the principles associated with the disclosed implementations. In the drawings,

[008] FIG. 1 illustrates a diagram of a focused surface acoustic wave (fSAW) device, in accordance with some example implementations of the current subject matter;

[009] FIG. 2 illustrates heat transfer and melting time equations, in accordance with some example implementations of the current subject matter;

[010] FIG. 3 illustrates a time sequence of images taken during a melting experiment, in accordance with some example implementations of the current subject matter;

[011] FIG. 4 illustrates a plot of melting times, in accordance with some example implementations of the current subject matter;

[012] FIG. 5 illustrates plots of drop and substrate temperatures, in accordance with some example implementations of the current subject matter;

[013] FIG. 6 illustrates a diagram of a focused surface acoustic wave device (fSAW) device, in accordance with some example implementations of the current subject matter; [014] FIG. 7 illustrates a table with the exit velocity of atomized droplets for different fSAW power inputs, in accordance with some example implementations of the current subject matter;

[015] FIG. 8 illustrates a side view of an atomizer, in accordance with some example implementations of the current subject matter;

[016] FIG. 9 illustrates a diagram of an atomizer, in accordance with some example implementations of the current subject matter;

[017] FIG. 10 depicts a diagram of a stacked transducer, in accordance with some example implementations of the current subject matter;

[018] FIG. 11 depicts an oblique view of a substrate of an atomizer, in accordance with some example implementations of the current subject matter;

[019] FIG. 12 illustrates an example of a process for atomizing a fluid, in accordance with some example implementations of the current subject matter; and

[020] FIG. 13 illustrates an example of a process for operating a thrust controller, in accordance with some example implementations of the current subject matter.

Detailed Description

[021] Disclosed herein are various methods and mechanisms, including techniques for producing thrust for small spacecraft. In an example, thrust is provided by continuous acoustic atomization of water from a small piezoelectric, single crystal substrate. This same transducer can be used to melt ice on the substrate. In some examples, solid-liquid phase change may be induced using a surface acoustic wave (SAW). Moreover, these methods and mechanisms may include continuous SAW atomization through a nozzle (i.e., a hole in the substrate). The devices constructed as disclosed herein may produce melting and atomization of a 1 pL drop with an input power on the order of 1 Watts. A device may be capable of producing thrust in the range 1-10 pN when the device is 12 x 20 x 0.5 mm³ in size and 0.6 g mass. An advantage of this thrust method is that, by simply adjusting the drive signals input into focused interdigital transducers (fLDTs), the direction and amplitude of thrust can be finely tuned with no moving parts.

[022] The number of small spacecraft — such as cubesats — has greatly increased over the last ten years in performing a variety of near and deep space missions. Once lofted into a target orbit, they require small and nimble thrusters for maneuvers and stationkeeping. There are many thruster options, but disclosed herein is a new alternative: acoustofluidic atomization. Acoustofluidic atomization involves high frequency acoustic waves being generated by piezoelectric transducers coupled into liquid and transferring energy in the form of both acoustic radiation and streaming to produce a directed atomized spray.

[023] A challenge in an acoustofluidic atomization system, as with most liquid-thrust systems, is the risk of phase change due to the extreme thermal environment in space, particularly in the freezing of the working fluid. Acoustic energy has also been known to produce rapid and controllable heating, for example, to enhance chemical reactions. A thrust mechanism is disclosed herein whereby water as a working fluid is allowed to freeze at the nozzle of the acoustic device. Activation of the acoustic device melts the working fluid, forming a meniscus on the surface of the device, and then allows its atomization for propulsion. The atomization produces capillary pressure sufficient to draw in fluid from a reservoir, although a simple pressure-driven pump may be used to support greater atomization rates. Disclosed herein is a simple energy conservation model to explain the acoustothermal interaction which has been validated with experiments. The specific impulse of this type of thruster is modest at 0.1 to 0.4 s, but the thrust is up to 12.3 pN, which is large considering the fact the device is 12 x 20 x 0.5 mm³ in size and is 0.6 g mass.

[024] Disclosed herein is a new approach using ultrasonic atomization that has advantages similar to those in film-evaporated MEMS tunable array (FEMTA) for small craft. In this approach, high frequency surface acoustic wave (SAW) and thickness mode transducers produce droplets from a fluid in contact with the surface of the transducer. The droplets are ejected with a velocity of 1 meter/second (m/s) order and normal to the surface from a stationary bulk liquid. This phenomenon can be used to generate thrust dependent on the flow rate and velocity of the sum of droplets. The piezoelectric transducers are low in mass and volume, and miniaturized driving electronics have already been developed. The concept is to feed liquid water to a nozzle — a small (e.g., 100 pm) hole in the substrate — and expose it to vacuum, where it will freeze and remain frozen until thrust is required. Ultrasound then melts the ice and atomizes the liquid water to produce thrust.

[025] Small liquid volumes increase in temperature when they are exposed to SAW upon a substrate. SAW devices typically use lithium niobate (LN), a single crystal media that exhibits no hysteresis, notably different than the polycrystalline ceramic lead zirconate titanate (PZT), zinc oxide (ZnO), and lead-free alter-natives to PZT, such as doped barium titanate media. This reduces its energy loss compared to its hysteretic, polycrystalline alternatives and prevents heat generation during energy transformation. If a fluid is placed upon LN with SAW being leaked into it, the heat generation is solely within the fluid. This has even been used for biological and chemical reactions.

[026] Acoustofluidics is now a well-developed field with many successful applications, including the atomization of liquids to produce sprays traveling in excess of 1 meter/second. There is a wealth of knowledge to draw on while creating a new acoustofluidic micro-thruster technology. Based on the methods and mechanisms presented herein, it is possible to continuously and linearly control the direction of droplets ejected from the source to over 50 degrees off-axis, suggesting the potential of a steerable thruster with no moving parts. However, an aspect that has not been explored in detail is the possibility of using the same acoustic device to melt the ice, enabling the subsequent operation of the thruster. While sublimation of water ice in a vacuum is certainly an important matter, the advantage here is the elimination of valves and other complexities by using a working fluid that can be allowed to freeze in the nozzle of an acoustofluidic thruster, thus rendering the thruster inert until needed, at which point the integrated heater melts the working fluid and also functions as the electrically-driven thruster element.

[027] In an example, a focused 55.5 MHz SAW (fSAW) device is employed, with the device capable of producing thrust via atomization of water. This same device can rapidly melt frozen water under a range of conditions. Furthermore, melting does not occur through resistive heating from the interdigital electrode present on the SAW device’s substrate. Atomization and ejection of the liquid are employed to produce thrust. Additionally, water may be continuously atomized through a nozzle in the substrate. Through demonstration of these basic capabilities, it is contemplated that acoustic atomization is suitable for further study as a thrust mechanism for small spacecraft.

[028] For example, spacecraft thrusters which are steerable may be designed using the methods and mechanisms that are described herein. A single thruster may be used in some embodiments, and the single thruster may be steered in any direction. In another example, aircraft engines producing atomization may allow the fuel injection to be steered into the engines to change the sound that the engines make. This would allow for some of the tones to be eliminated from the sound that aircraft engines make. In a further example, pulmonary delivery of vaccines and large- molecule drugs and cells may utilize the methods and mechanisms described herein.

[029] In an example, a thruster may be able to avoid losing the working fluid by allowing the fluid to freeze at the nozzle location when the thruster is idle. Then, when the thruster needs to transition from an idle state to an active state, the acoustic energy is activated and causes the frozen fluid to thaw. The fluid will then start to flow again and the acoustic energy will cause droplets to be atomized and ejected, allowing the thruster to transition to the active state without the use of any moving parts. Most applications of thrusters require a valve, but the methods and mechanisms described herein enable valve-less thrusters to be deployed and utilized in space.

[030] Although some of the examples disclosed herein refer to a thruster in the context of a spacecraft, the devices disclosed herein may be implemented in other systems including, but not limited to, drug delivery devices, agricultural sprayers, fuel sprayers, fragrance systems, 3D printing, wound care, painting, cosmetics, and/or the like.

[031] Referring now to FIG. 1, a diagram of a focused surface acoustic wave (fSAW) device 100 is shown, in accordance with some example implementations of the current subject matter. In an example, a sessile 1.00+0.01 micro-liter (pL) drop of deionized (DI) water may be placed at the focal spot of a 55.5 MHz fSAW device 100 constructed on 127.68° Y-rotated LN substrate in direct physical contact with a Peltier thermoelectric control module, TEC, (e.g., TECF2S, Thorlabs, AT_max = 66.4 °C, Imax=1.9 A) as shown in FIG. 1. Driven with a power supply (e.g., DC Power Supply, YH-305D, YiHUA), the TEC may be mounted upon a heat sink (e.g., ATS- 54425D-C1-R0, Advanced Thermal Solutions) that may in turn placed in a water bath. The bottom of the TEC — the “hot” side — may be maintained at near freezing, -0.8+1.3°C.

[032] The temperature on the fSAW device 100 surface may be measured with a thermocouple probe (e.g., TJ1-CAIN-IM15G-600, Omega Engineering Inc, Norwalk, CT, USA) and meter (e.g., HH91 IT, Omega) while the temperature of the sessile drop may be measured using an infrared (IR) camera, (e g., FLIR A35 FOV 13, Teledyne FLIR LLC, Wilsonville, OR, USA), calibrated with the thermocouple. The fSAW device 100 may be driven with a signal generator (e.g., WF1967 multifunction generator, NF Corporation, Yokohama, Japan) and amplifier (e.g., 5U1000, Amplifier Research Corp., Souderton, Pennsylvania, USA) and the electrical signal may be measured using an oscilloscope (e.g., InfiniiVision 2000 X-Series, Keysight Technologies, Santa Rosa, CA, USA). The vibrational displacement and velocity of the surface of the fSAW device may be measured by laser Doppler vibrometer (e g., LDV, UHF-120, Polytec, Waldbronn, Germany).

For visualization, the 1 pL droplet may be dyed with 0.005+10^-4 g of sulforhodamine B (e.g., Millipore-Sigma, St, Louis, MO, USA).

[033] In order to simulate a reduced temperature environment, the cold face of a thermoelectric cooler (TEC) module may be placed in contact with the underside of the fSAW transducer and the hot face may be coupled to a heat sink in an ice bath. The temperature of the fSAW surface depends on the current running through the TEC. A sessile drop of 1+0.1 pL DI water may be pipet-ted at the fSAW focal spot and may be frozen at several different temperatures below 0°C. After establishing these initial states, a voltage signal may be applied to the HDT at a range of input powers for each state in order to determine how easily the droplet can be melted. Experiments may assess how easily the approach could be used to melt the thruster’s working fluid without requiring other components.

[034] Consider the frozen sessile drop as a thermodynamic system in equilibrium at a temperature below freezing, 0°C, as set by the operation of the TEC. Heat influx from the laboratory environment is balanced with heat flux removed by the TEC. Now consider a new source of heat being turned on at time t = 0, in this case by applying a voltage signal to the flDT. It is assumed that some fraction of the input power to the flDT generates heat in the frozen sessile drop via some combination of mechanisms. Water ice is known to have an acoustic loss mechanism that generates heat, but this mechanism is usually considered in the context of acoustic wave propagation in ice floes and aircraft icing. The frozen sessile drop will also radiate heat to the laboratory environment and conduct heat to the substrate. Therefore the energy balance is governed from the first law of di-: thermodynamics as shown in equation 205 (of FIG. 2), where ^i:it the heat transfer into the system, Qrad is heat flux due to radiation, Q_cond is heat flux due to conduction, PIDT is the input power to the IIDT, and N accounts for any inefficiencies by which power to the fLDT does not generate heat in the system (e.g., kinetic energy, heat outside the system).

[035] Integrating equation 205 from t = 0 to the time when the drop has fully melted yields, on the left hand side, the total energy change required to melt the sessile drop, which consists of the heat capacity of ice and the latent heat of phase transformation. On the right hand side, the energy contributions from conduction, radiation, and flDT input are obtained. Dividing the power terms from the time interval yields equation 210 (of FIG. 2) for calculating the time required to melt the drop. For equation 210, where c is the heat capacity of ice, m is the mass of the drop, AT is the difference between 0°C and the initial temperature set by the TEC, A//L is the latent heat of transformation from ice to liquid water, A/ is the change in time, e is the emissivity of ice, o is the Stefan-Boltzmann constant, AD/E is the surface area between the drop and the environment, and AD/S is the surface area between the drop on the substrate.

[036] Condensation from a laboratory environment occurs across the fSAW device substrate when the surface temperature is reduced below 0°C via the TEC. The condensed water then freezes, forming a thin layer of ice across the substrate in addition to the frozen sessile drop. If the flDT were producing significant heat via resistive heating, then isotropic melting outward from the fTDT would be expected as well as heating of the bus bars. A time sequence of images during a melting experiment are shown in FIG. 3. In the melting experiment depicted in FIG. 3, a 1 pL frozen DI water droplet is melted by internal acoustic losses. The flDT can be seen at the top and the sessile drop at the center of each frame. In diagram (a), an LDV scan shows the relatively narrow region over which fSAW exists during activation, about one-third of the aperture width and progressively more narrow over the measured region. The sessile water droplet was placed at the circle marked in this image. [037] In diagram (b), initially, the droplet was frozen through the action of the TEC and maintained at -13.63+/-0.05C. The water was dyed with sulforhodamine B to ease visualization.

Frost appears across the surface of the device due to condensation. The temperatures were measured at the regions denoted with the dotted circles. In diagram (c), applying 1 10+/-0.05 W to the fTDT starts melting the water after 2.4 seconds (s). Note that frost remains outside of the fIDT aperture but becomes liquid inside the aperture. In diagram (d), after 4.8 s, the frozen droplet is partially melted, principally in the viscous boundary layer in contact with the SAW substrate. In diagram (e), the fSAW completely melts the drop after roughly 7.2 seconds. In these diagrams, the scale bar is 2 millimeters (mm) in length.

[038] FIG. 3 shows that the thin ice layer melts specifically where SAW is propagating, not in the surrounding regions of the substrate where heat would be easily transmitted. Resistive heating in the fIDT appears to be negligible in comparison to the effect of the SAW. Once the melting interface propagates to the sessile drop it fully melts the drop in a few seconds, depending on both the initial temperature and the fSAW input power. The time to melt a drop was extracted from videos of experiments and is plotted in FIG. 4 along with the melting time obtained from Equation 210 (of FIG. 2). It was found that if it is assumed that 7% (i.e., N= 14) of the fIDT input power produced heat in the drop, then the model roughly matched the data. It was also found that an additional term proportional to the TEC current was necessary to properly account for the different equilibrium temperatures. The modified model, as shown in equation 215 (of FIG. 2), where / is the current in the TEC, agrees quite well with the data with these two mechanism-based fitting parameters. In conclusion, the model represents the major thermal mechanisms at play in the experiments.

[039] At the lowest input power, the energy barrier in order to melt the drop could not be overcome except in the highest initial temperature set by the TEC. This agrees with the model since the melt time trends to infinity as the fSAW input power is reduced below about 0.5 W in the system (see FIG. 4). In cases where the drop did eventually melt, the melting time was greater for lower initial temperatures and lower fSAW input power. However, at high fSAW powers beyond about 1 W, there was little difference between the melt time for any of the four starting temperatures. This likely indicates that conduction and radiation, which depend on the temperature difference between the droplet and its surroundings, play a larger role when the fSAW power is lower. At high fSAW power, there is less time for this temperature difference to have an effect through conduction and radiation. Instead, the energy introduced by the fSAW device is overwhelming.

[040] The temperature on the substrate near the drop, but outside the fSAW path, was recorded via thermocouple at the end of each experiment along with the temperature on the drop via IR camera. These data are plotted in FIG. 5. Note that the substrate temperature outside the fSAW path remains below freezing except in the case of the lowest TEC current (associated with the highest substrate temperature), while the drop is melted in every case.

[041] Turning now to FIG. 6, a focused surface acoustic wave (fSAW) device 600 used to create a prototype thruster by causing atomization of a liquid is shown, in accordance with some example implementations of the current subject matter. A pair of fIDTs 620 and 670 are used in fSAW device 600 instead of just one fTDT as in fSAW device 100 (of FIG. 1). In an example, the pair of fIDTs 620 and 670 may be controlled by a thrust controller 610. Thrust controller 610 may also be referred to a controller or as a control unit. It is noted that in other embodiments, a fSAW device may have other numbers of fIDTs, such as three, four, five, six, and so on. As shown in FIG. 6, the pair of fIDTs 620 and 670 are both focused at a central point, and a nozzle 650 is cut into the substrate 630 at this location. The acoustic waves generated by fIDTs 620 and 670 only propagate on the top surface of substrate 630, while the bottom surface of substrate 630 is inert. [042] In an example, a small, 100 pm, capillary through-hole 645 is cut using a 1030-nm femtosecond pulsed laser at 9 mJ-cm^-2 and 60 kHz pulse rate (e.g., LightShot, Optec Laser, Frameries, Belgium) into the fSAW substrate 630. In an example, a small silicone tube 640 was attached to the underside of the substrate via a barb nipple 650 glued to the substrate 630 using ultraviolet-cured epoxy (e.g., N0A61, Norland Products, Jamesburg, NJ USA). The other end of the tube 640 may be connected to a working fluid supply 635. In an example, the working fluid supply 635 is a DI water-filled 50 mL syringe that acted as a reservoir.

[043] In the fSAW device 600 designed to produce thrust from atomization of the working fluid, with two HDTs 620 and 670 and a through hole 645 representing a nozzle 650, a potential problem is the effects of allowing water to freeze in the machined hole 645 of the fSAW device 600. The water’s expansion could cause failure of the material. Two different devices laden with water were frozen over ten times and no failure, cracks, nor other flaws during subsequent operation were found. During use, the nozzle hole 645 in the LN substrate 630 is subjected to substantial tensile and compressive stresses at 55.5 MHz, or, in other words, over 55 million cycles per second. Consequently, if there were cracks or flaws introduced into the nozzle from the freezing, these would likely lead to failure of the device in a few minutes.

[044] While the system is self-pumping, with capillary pressure at the hole 645 upon the surface of the fSAW device 600 entirely sufficient to replace atomized fluid, introducing a pump substantially increases the atomization rate. For this reason, a syringe pump may be employed with the syringe. Drive signals may be supplied to both HDTs 620 and 670 on the device 600. When the fSAW power from each side is equal, water is atomized directly upward at a velocity dependent on the total power. When fSAW power from one side is greater than the other, the direction of atomization changes. [045] Video was taken at 6400 fps, viewing the atomization from the side as it occurred vertically against gravity. At these scales and near the fluid meniscus 655, gravity has no effect on the droplet ejection. The video information was used to calculate the exit velocity of the atomized droplets 660 for different fSAW power inputs as provided in Table 700 (of FIG. 7). The flow rate, defined by the syringe pump, was used with the droplet ejection velocity to calculate the equivalent thrust, T= v_e x Q/ . Here, v_e is the exit velocity of the working fluid and Q/ is the mass flow rate. The flow rate was chosen to match the maximum atomization rate for a given fSAW power. Larger flow rates cause leaking of non-atomized fuel and smaller flow rates needlessly reduce the thrust to power ratio. This fluid supply mechanism may be replaced in other prototypes in favor of a wicking mechanism suited to zero gravity.

[046] FIG. 8 depicts an example side view of an atomizer 800 which includes a thickness mode resonator device 810 causing atomization of a liquid. In an example, thickness mode resonator device 810 is a plate which vibrates in and out. One of the challenges with ultrasound atomizers is how to provide fluid onto a vibrating surface in a way that does not disturb or negate the atomization. Accordingly, a solution to this challenge is to cut a hole (i.e., orifice 830) through the material of thickness mode resonator device 810 to allow the fluid to flow from fluid reservoir 820 to the opposite surface of thickness mode resonator device 810. Then, the fluid is atomized on the opposite surface by absorbing the acoustic energy generated by alternating an electric field applied to thickness mode resonator device 810. In an example, a size of the hole may be selected so that a diameter of the hole is in a range between one hundredth of a wavelength of an acoustic wave in the substrate and one half of the wavelength of the acoustic wave in the substrate.

[047] With the arrangement of the through-hole mechanism of thickness mode resonator device 810, a steady supply of fluid can be provided to the surface opposite fluid reservoir 820. This steady supply of fluid helps to keep the atomized droplet size substantially the same, allowing for a wide variety of applications of fluid atomization based on this approach. In the embodiment of FIG. 8, there is a single hole (i.e., orifice 830) in the substrate of thickness mode resonator device 810, supplied with fluid from a larger fluid reservoir 820 via a fluid supply tube. At the end of the fluid supply is a small porous material (i.e., wick 840) sized to be approximately one-quarter the wavelength of sound in the fluid at the frequency of the resonator's oscillation (e.g., between 1 MHz and 1 GHz). Wick 840 reduces the transmission of the ultrasound from the vibrating surface of the resonator into the fluid reservoir 820, crucially important in applications where the fluid reservoir 820 is relatively large. This is because the acoustic wave energy can be taken up by the fluid in the reservoir 820 instead of being used to produce droplets from the fluid film.

[048] It should be understood that while one hole (also referred to as an “opening”) is shown in the substrate of thickness mode resonator device 810, this is merely indicative of one implementation. In other implementations, a substrate may have multiple holes cut through the substrate for supplying fluid to the opposite surface of the substrate. In an example, each hole, of the multiple holes, could be exposed to the acoustic wave with equal amplitudes. Also, the size of each hole, and the number of holes in the substrate, may be selected based on the targeted application. Some end-use applications may benefit from having a relatively large number of holes, other applications may benefit from having a relatively smaller size of through-hole, and so on.

[049] The surfaces of thickness mode resonator device 810 undergo axial vibration to cause the fluid from fluid reservoir 820 to be atomized as the fluid passes through orifice 830 to the opposite surface of thickness mode resonator device 810. Wick 840 may be placed in between fluid reservoir 820 and orifice 830 of thickness mode resonator device 810 to prevent the vibrations from being absorbed by fluid reservoir 820. The motion of the thickness mode resonator device 810 will cause acoustic waves to appear in the fluid, and the thickness of wick 840 may be chosen so as to prevent the loss of acoustic energy into fluid reservoir 820. [050] In dashed box 850 is an expanded view of wick 840. Wick 840 prevents the loss of acoustic energy into fluid reservoir 820 which would otherwise occur based on fluid reservoir 820 being in contact with the moving surface of thickness mode resonator device 810. During operation, the acoustic energy emanates throughout the material of thickness mode resonator device 810. Accordingly, wick 840 mitigates the loss of the acoustic energy into fluid reservoir 820 from the vibrations of thickness mode resonator device 810.

[051] In an example, the thickness of wick 840 may be set to a value somewhere in the range of between a quarter- wavelength and a half-wav elength of the speed of sound in the piezoelectric material used to construct thickness mode resonator device 810. In another example, the thickness of wick 840 is selected to be between one tenth the wavelength of sound and one half the wavelength of sound in the piezoelectric material of thickness mode resonator device 810. In another example, the thickness of wick 840 is selected to be between one tenth and one half of the wavelength of sound in water. In other examples, the thickness of wick 840 may be selected based on other ranges and/or based on other factors.

[052] In an example, thickness mode resonator device 810 is made of piezoelectric material, and an electric field is produced across the piezoelectric material, and this causes the piezoelectric material to strain. The polarization of the electrical field is then flipped, causing the piezoelectric material to strain in the opposite direction. In other words, the piezoelectric material of thickness mode resonator device 810 may be caused to alternate between expanding and contracting in response to the polarization of the electrical field applied to the piezoelectric material being switched back and forth. In an example, the polarization of the electrical field alternates according to a resonant frequency, causing thickness mode resonator device 810 to resonate.

[053] In an example, the width (i.e., thickness) of thickness mode resonator device 810 is chosen to be one-half of the wavelength of the speed of sound in the piezoelectric material of thickness mode resonator device 810. Selecting the width of thickness mode resonator device 810 to be one-half of the wavelength of the speed of sound of the material may be referred to as the fundamental mode of thickness mode resonator device 810. In an example, when thickness mode resonator device 810 is constructed of lithium niobate (LN), the LN substrate will be selected to be .5 millimeter (mm) thick. In general, the thickness of thickness mode resonator device 810 may range from 100 nanometers (nm) thick to 4 mm thick. In an example, the resonant frequency may be 6.9 megahertz (MHz). In other examples, other devices may have other resonant frequencies.

[054] In an example, a goal is to produce an atomizer 800 (i.e., a droplet generator) from an arbitrarily-sized fluid reservoir 820 with ultrasound in as efficient a process as possible. The atomizer 800 may be used to produce a handheld drug delivery device that could also be useful for agricultural spraying, fuel sprays, fragrance systems (commercial), 3D printing, wound care, painting, cosmetics, and other applications. Using a single-crystal piezoelectric element in thicknessmode vibrations may be superior in atomization efficiency compared to other devices such as SAW devices, hybrid mode devices, and ultrasound devices that employ poly crystalline piezoelectric media (PZT for example). However, determining a way to provide the fluid to be atomized from the surface of the device may be challenging.

[055] Over the years, many have tried supplying the fluid onto the surface via dripping, wicks, and side-flows. These approaches have drawbacks: either they fail to produce uniform droplet sizes spatially across the surface and over time, they clog, or they are inefficient. By introducing one or more holes in the transducer, it is possible to have a collection point for the fluid provided from the outside. From this small reservoir 820 contained in the resonator itself, fluid may be drawn out upon the vibrating surface, forming a thin film that is subsequently atomized to produce the desired droplets. By having this reservoir 820, variations in the fluid volume supplying the thin fluid film are reduced, and the extent and depth of the resulting fluid film remains more constant, producing more uniform droplets. This may be important in many applications, particularly in medical applications where the droplet size defines the location of the droplet's eventual deposition: 1-5 pm into the deep lung, 10-25 pm large airways, 40-100 pm throat, and so on. It also matters in aircraft engines, where current fuel spray generators produce -50-250 pm droplets, but future applications in detonation engines for faster flight regimes demand droplets of diameter less than 5 pm.

[056] To generate the thickness-mode vibration, sheet electrodes on both faces of the single crystal piezoelectric material may be used. In the embodiment of FIG. 8, this may be 1 pm of Au plated atop 10 nm of Cr, to drive a 10 x 10 mm x 500 pm thick 127.68 Y-rotated, X-propagating cut of lithium niobate into resonance at about 7.8 MHz, although other dimensions, materials, and frequencies may be realized as well. Different materials may be used to accomplish the same or improved outcome, for example the use of langasite, quartz, or similar piezoelectric media. Moreover, multiple layers of the material may be used as well in a stack to reduce the voltage necessary to drive the resonant vibration behavior, improving safety in medical and aircraft applications, for example. Also, multiple layers of the material may provide space to introduce a centrally-mounted plate that extends beyond the boundaries of the piezoelectric plates to provide a means to mount the transducer without suppressing its vibrations, or to introduce an end-mounted plate or stacked plates to serve as a backing material to reflect the generated vibration and improve the output efficiency by transmitting most of the energy through the other face of the device. This could include a backing of quarter- wavelength thick plates, such as tungsten-acrylic-tungsten- acrylic-..., as an acoustic reflector. Moreover, the sheet electrodes could be segmented to produce different vibration amplitudes on one portion of the resonator compared to another portion, thereby steering the atomized droplets from the surface at an angle, controlled by either the relative amplitudes of the vibration or the phase between the vibrations, or both. [057] Referring now to FIG. 9, a diagram of an atomizer 910 causing atomization of a liquid is shown. In an example, a fluid reservoir 940 is attached to a first surface 925 of atomizer

910, and fluid flows from the fluid reservoir via an opening 930 in substrate 920 to a second surface 935 of atomizer 910. In this example, a surface acoustic wave propagates across the second surface 935 of substrate 920 during operation of atomizer 910. The surface acoustic wave may propagate from one side or from both sides of the second surface 935 opposite the fluid reservoir 940. In this example, the fluid is introduced into atomizer 910 via opening 930 without concern for loss of acoustic energy because the acoustic energy that is being produced is on the second surface 935. It is noted that opening 930 may also be referred to as a hole or as an orifice.

[058] Based on the ability to activate only a portion of the segmented electrodes 950, atomization may be produced such that the atomization droplets are ejected in a desired direction. It is noted that segmented electrodes 950 may also be referred to as a segmented set of electrodes. In an example, only the top part of segmented electrodes 950 may be activated, causing the droplets to be ejected in some angle off-axis in the downward direction. In another example, only the bottom part of segmented electrodes 950 may be activated, causing the droplets to be ejected in some angle off-axis in an upward direction.

[059] A front view of the second surface 935 of atomizer 910 is shown in box 970 on the left-side of FIG. 9. The front view includes segmented electrodes 975A-D with a particular pattern shown with four separate electrodes. The first surface 925 of the atomizer 910 may have a single electrode across the entirety of the surface, or across some percentage of the surface, with the percentage being at least 70% in a first embodiment, at least 80% in a second embodiment, at least 90% in a third embodiment, or any of various other percentages in other embodiments. The single electrode on the first surface 925 of the atomizer 910 may be in the shape of a square in a first embodiment, in the shape of a rectangle in a second embodiment, or any of various other shapes in other embodiments. In an example, the single electrode on the first surface 925 of the atomizer 910 is connected to electrical ground. In another example, the single electrode on the first surface 925 of the atomizer 910 may be connected to a fixed voltage level. Alternatively, the single electrode on the first surface 925 of the atomizer 910 may be connected to other voltage sources.

[060] The pattern of segmented electrodes 975A-D allows for steering the ejected droplets at a desired angle in a two-dimensional space by selectively applying an alternating electric field to specific segments while simultaneously not applying an alternating electric field to one or more other segments. In an example, the alternating electric field is applied by driving a sinusoidal voltage signal at a frequency equal to, or approximately equal to, the resonant frequency of the atomizer 910. The selections of which segments to apply the electric field to will determine in which direction the atomized droplets will be ejected. It should be understood that the particular pattern of segmented electrodes 975A-D is merely shown for illustrative purposes. Other types of segmented electrode patterns, with other numbers of segments, are possible and are contemplated.

[061] As a result of selectively applying an alternating electric field to specific segments of segmented electrodes 975A-D, there will be atomization of fluid from the central location of hole 980, and the atomized fluid will spray in the desired direction. For example, if only electrode 975C is activated (by applying the alternating electric field to only electrode 975C), then the atomized fluid droplets will spray in the upwards direction at an angle away from electrode 975C. In another example, if only electrode 975B is activated, then the atomized fluid droplets will spray in the leftwards direction at an angle away from electrode 975B. In a further example, electrodes 975C and 975D may be activated to cause the atomized fluid droplets to spray in a direction upwards and to the right. Additionally, in other examples, the frequency of the sinusoidal voltage signal as well as the voltage may be adjusted to one or more of segmented electrodes 975A-D. [062] Various droplet ejection patterns may be generated based on the sinusoidal voltage signals that are applied to one or more of segmented electrodes 975A-D over some period of time. For example, sinusoidal voltage signals may be applied to segmented electrodes 975A-D to cause the droplet ejection pattern to spell out the letters of one or more words. In other examples, sinusoidal voltage signals may be applied to segmented electrodes 975 A-D to cause the droplet ejection pattern to draw out some desired shape.

[063] Turning now to FIG. 10, a diagram of a stacked transducer 1000 is shown, in accordance with some example implementations of the current subject matter. As shown, stacked transducer 1000 includes transducer 1010 on the left-side of central mount 1040 and transducer 1015 on the right-side of central mount 1040. Orifice 1020 is cut through the middle of the stacked layers to allow a path for fluid to flow through the layers. Any number of additional layers 1030 may also be stacked together with the layers shown in FIG. 10. The additional layers 1030, which may be active or inactive, help to increase the amplitude of the motion and help to reduce the operating frequency for embodiments that would benefit from a lower operating frequency. Central mount 1040 provides a mounting method to mount stacked transducer 1000 during operation. During the operation of stacked transducer 1000, motion may be asymmetric to avoid losses.

[064] Acoustic energy may leak out from stacked transducer 1000 into the surrounding material and be wasted, and central mount 1040 provides a way to mount stacked transducer 1000 while minimizing the loss of acoustic energy. This is achieved by having a plate for transducer 1010 on the left of central mount 1040 and having a second plate for transducer 1015 on the right-side of central mount 1040. In an example, central mount 1040 is made out of a non-piezoelectric material such as brass, steel, titanium, or the like. When the entire stack is excited with an electric field, central mount 1040 does not vibrate. By increasing the thickness of stacked transducer 1000, this lowers the resonant frequency of stacked transducer 1000. This may be helpful in some applications to have a relatively low ultrasound frequency.

[065] Referring now to FIG. 11, an oblique view of a substrate 1110 of an atomizer 1100 is shown. As shown in FIG. 11 , one or more orifices 1120 (also referred to as “openings”) may be cut through the interior of substrate 1110 to provide fluid flow. The surfaces of atomizer 1100 may include electrodes which receive oscillatory input signals to generate the acoustic energy necessary to atomize the fluid flowing through orifice(s) 1120. The number and sizes of orifice(s) 1120 may vary from embodiment to embodiment.

[066] Turning now to FIG. 12, a process for atomizing a fluid is shown. At the beginning of method 1200, acoustic energy is generated in a substrate made of piezoelectric material (as shown in FIG. 1, FIG. 6, and FIG. 8 at FIG) where a fluid reservoir is attached to a first surface of the substrate (block 1205). Generally speaking, the fluid reservoir is located on a first side of the substrate, where the first side is contiguous to the first surface. The fluid reservoir may be attached to another device or structure in some embodiments. Alternatively and/or additionally, the fluid reservoir may be coupled to the first surface of the substrate via a tube. It should be understood that other methods and mechanisms for coupling, attaching, affixing, adhering, mounting, and/or connecting the fluid reservoir to the first surface of the substrate are possible and are contemplated.

[067] Next, the acoustic energy is absorbed by a fluid on a second surface of the substrate opposite to the first surface, where the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate (block 1210). Then, the fluid is atomized on the second surface of the substrate, where a rate of atomization of the fluid is based on an amount of the acoustic energy absorbed by fluid droplets on the second surface (block 1215). Next, the thrust is produced by continuously atomizing the fluid flowing through the opening onto the second surface of the substrate (block 1220). After block 1220, method 1200 ends. In cases where the acoustic energy is generated by vibrations of a thickness mode device, a wick may be placed in between the first surface of the substrate and the fluid reservoir. The wick helps to reduce the reduce the amount of acoustic energy that is absorbed by the fluid in the fluid reservoir.

[068] Referring now to FIG. 13, a process for operating a thrust controller is shown. At the beginning of method 1300, a thrust controller (e.g., thrust controller 610 of FIG. 6) enters into freeze mode by turning off or deactivating an acoustic energy source of a thruster device (block 1305). Depending on the embodiment, the acoustic energy source may be a surface acoustic wave source or a thickness mode resonator source. It is noted that the term “thruster device” may also be referred to herein more generally as an “atomization device” or as an “atomizer”.

[069] During freeze mode, the thrust controller allows the fluid at the end of the nozzle to freeze, which seals the end of the nozzle and prevents the loss of any fluid from the thruster device (block 1310). It is noted that the “end” of the nozzle may also be referred to herein as the “tip” of the nozzle or as a “release point”.

[070] If the thrust controller detects a condition or an indication for transitioning to thawing mode (conditional block 1315, “yes” leg), then the thrust controller enters thawing mode by activating the acoustic energy source of the thruster device with a first set of voltage and frequency settings (block 1325). As part of entering thawing mode, the thrust controller determines how long it will take until the frozen fluid at end of the nozzle thaws, causing fluid to flow from a fluid reservoir through a hole in a substrate of the thruster device to an opposite surface where the liquid is atomized (block 1330). In other words, in block 1330, the thrust controller determines how long it will take to transition from thawing mode into thrust mode. The determination in block 1330 may be considered an estimation or prediction of how long the thawing process will take. In an example, the thrust controller may utilize equation 210 or equation 215 (of FIG. 2) for determining how long the thawing process will take. In other examples, the thrust controller may utilize other techniques separately and/or in combination with an equation (e.g., equation 210, equation 215) to determine how long the thawing process will last. During the thawing process, acoustic energy is generated but does not result in atomization of the fluid. Once the thawing process is complete, the generated acoustic energy will cause atomization of the fluid which in turn will cause the fluid to flow from the fluid reservoir to the nozzle to generate continuous thrust via ejection of droplets. If the thrust controller does not detect a condition for transitioning to thawing mode (conditional block 1315, “no” leg), then the thrust controller remains in freeze mode (block 1320), and then method 1300 returns to conditional block 1315.

[071] Once the amount of time predicted for the thawing process has elapsed, the thrust controller enters thrust mode by driving the acoustic energy source of the thruster device with a second set of voltage and frequency settings (block 1335). In an example, the second set of voltage and frequency settings are different from the first set of voltage and frequency settings used during thawing mode. In another example, the second set of voltage and frequency settings used during thrust mode may be the same as the first set of voltage and frequency settings used during thawing mode. If, during thrust mode, a condition for entering freeze mode is detected (conditional block 1340, “yes” leg), then then method 1300 returns to block 1305 with the thrust controller deactivating the acoustic energy source. Otherwise, if a condition for entering freeze mode is not detected (conditional block 1335, “no” leg), then the thrust controller remains in thrust mode (block 1340), and then method 1300 returns to conditional block 1335. It is noted that the voltage and frequency settings used to drive the acoustic energy source may change while the thrust controller remains in thrust mode, depending on the desired thrust. The thrust controller may receive commands during operation to make adjustments to the voltage and frequency settings used to drive the acoustic energy source. [072] The subject matter described herein can be embodied in systems, apparatus, methods, and/or articles depending on the desired configuration. The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. For example, the logic flows may include different and/or additional operations than shown without departing from the scope of the present disclosure. One or more operations of the logic flows may be repeated and/or omitted without departing from the scope of the present disclosure. Other implementations may be within the scope of the following claims.

[073] In the descriptions above and in the claims, phrases such as “at least one of’ or “one or more of’ may occur followed by a conjunctive list of elements or features. The term “and/or” may also occur in a list of two or more elements or features. Unless otherwise implicitly or explicitly contradicted by the context in which it is used, such a phrase is intended to mean any of the listed elements or features individually or any of the recited elements or features in combination with any of the other recited elements or features. For example, the phrases “at least one of A and B;” “one or more of A and B;” and “A and/or B” are each intended to mean “A alone, B alone, or A and B together.” A similar interpretation is also intended for lists including three or more items. For example, the phrases “at least one of A, B, and C;” “one or more of A, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together.” Use of the term “based on,” above and in the claims is intended to mean, “based at least in part on,” such that an unrecited feature or element is also permissible.

[074] The subject matter described herein can be embodied in systems, apparatus, methods, and/or articles of manufacture depending on the desired configuration. The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. For example, the logic flows may include different and/or additional operations than shown without departing from the scope of the present disclosure. One or more operations of the logic flows may be repeated and/or omitted without departing from the scope of the present disclosure. Other implementations may be within the scope of the following claims.

[075] Although ordinal numbers such as first, second and the like can, in some situations, relate to an order; as used in a document ordinal numbers do not necessarily imply an order. For example, ordinal numbers can be merely used to distinguish one item from another. For example, to distinguish a first event from a second event, but need not imply any chronological ordering or a fixed reference system (such that a first event in one paragraph of the description can be different from a first event in another paragraph of the description).

[076] The foregoing description is intended to illustrate but not to limit the scope of the invention, which is defined by the scope of the appended claims. Other implementations are within the scope of the following claims.

[077] In the descriptions above and in the claims, phrases such as “at least one of’ or “one or more of’ may occur followed by a conjunctive list of elements or features. The term “and/or” may also occur in a list of two or more elements or features. Unless otherwise implicitly or explicitly contradicted by the context in which it used, such a phrase is intended to mean any of the listed elements or features individually or any of the recited elements or features in combination with any of the other recited elements or features. For example, the phrases “at least one of A and B;” “one or more of A and B;” and “A and/or B” are each intended to mean “A alone, B alone, or A and B together.” A similar interpretation is also intended for lists including three or more items. For example, the phrases “at least one of A, B, and C;” “one or more of A, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together.” Use of the term “based on,” above and in the claims is intended to mean, “based at least in part on,” such that an unrecited feature or element is also permissible.

[078] In view of the above-described implementations of subject matter this application discloses the following list of examples, wherein one feature of an example in isolation or more than one feature of said example taken in combination and, optionally, in combination with one or more features of one or more further examples are further examples also falling within the disclosure of this application: [079] Example 1: A method comprising: generating acoustic energy in a substrate comprising piezoelectric material, wherein a fluid reservoir is attached to a first surface of the substrate; causing the acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, wherein the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate; and atomizing, with the acoustic energy, the fluid on the second surface of the substrate.

[080] Example 2: The method of Example 1, further comprising producing thrust by atomizing the fluid flowing through the opening onto the second surface of the substrate.

[081] Example 3: The method of any of Examples 1-2, wherein the opening passes through an interior of the substrate from the first surface to the second surface, and wherein a diameter of the opening is in a range between one hundredth of a wavelength of an acoustic wave traveling in the substrate and one half of the wavelength of the acoustic wave traveling in the substrate.

[082] Example 4: The method of any of Examples 1-3, wherein a wick is situated in between the fluid reservoir and the first surface, and wherein a thickness of the wick is in a range between a quarter-wavelength and a half-wavelength of a speed of sound in the piezoelectric material of the substrate.

[083] Example 5: The method of any of Examples 1 -4, wherein the acoustic energy is generated by a segmented set of electrodes, and wherein a first portion of the segmented set of electrodes is activated so as to produce atomization droplets which are ejected in a desired direction.

[084] Example 6: The method of any of Examples 1-5, wherein a rate of atomization of the fluid is based on an amount of the acoustic energy absorbed by fluid droplets on the second surface.

[085] Example 7: The method of any of Examples 1-6, further comprising: entering, by a thrust controller, into freeze mode by deactivating a source of the acoustic energy; and allowing the fluid to freeze at an end of a nozzle to seal the nozzle and prevent fluid loss from the fluid reservoir. [086] Example 8: The method of any of Examples 1-7, further comprising: detecting, by the thrust controller, a condition for transitioning from freeze mode into thawing mode; responsive to detecting the condition, entering thawing mode by activating the source of the acoustic energy with a first set of voltage and frequency settings; determining a duration for the fluid at the end of the nozzle to thaw; and responsive to the duration elapsing, entering thrust mode by activating the source of the acoustic energy with a second set of voltage and frequency settings.

[087] Example 9: The method of any of Examples 1-9, wherein the substrate is part of a thickness mode resonator device, and wherein the method further comprising generating the acoustic energy by applying a sinusoidal voltage signal to the thickness mode resonator device.

[088] Example 10: The method of any of Examples 1-9, wherein the substrate is part of a surface acoustic wave device, and wherein the method further comprising generating the acoustic energy by applying a sinusoidal voltage signal to the surface acoustic wave device.

[089] Example 11 : An atomizer comprising: a substrate comprising piezoelectric material; and a fluid reservoir attached to a first surface of the substrate; wherein the atomizer is configured to: cause acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, wherein the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate; and atomize, with the acoustic energy, the fluid on the second surface of the substrate.

[090] Example 12: The atomizer of Example 11, wherein the atomizer is further configured to produce thrust by atomizing the fluid flowing through the opening onto the second surface of the substrate.

[091] Example 13: The atomizer of any of Examples 11-12, wherein the opening passes through an interior of the substrate from the first surface to the second surface, and wherein a diameter of the opening is in a range between one hundredth of a wavelength of an acoustic wave traveling in the substrate and one half of the wavelength of the acoustic wave traveling in the substrate.

[092] Example 14: The atomizer of any of Examples 11-13, further comprising a wick situated in between the fluid reservoir and the first surface, wherein a thickness of the wick is in a range between a quarter-wavelength and a half-wavelength of a speed of sound in the piezoelectric material of the substrate.

[093] Example 15: The atomizer of any of Examples 11-14, further comprising a segmented set of electrodes, wherein a first portion of the segmented set of electrodes is activated so as to produce atomization droplets which are ejected in a desired direction.

[094] Example 16: The atomizer of any of Examples 11-15, wherein a rate of atomization of the fluid is based on an amount of the acoustic energy absorbed by fluid droplets on the second surface.

[095] Example 17: The atomizer of any of Examples 11-16, further comprising a nozzle, wherein the atomizer is further configured to: enter into freeze mode by deactivating a source of the acoustic energy; and allow the fluid to freeze at an end of the nozzle to seal the nozzle and prevent fluid loss from the fluid reservoir.

[096] Example 18: The atomizer of any of Examples 11 -17, wherein the atomizer is further configured to: detect a condition for transitioning from freeze mode into thawing mode; and responsive to detecting the condition, enter thawing mode by activating the source of the acoustic energy with a first set of voltage and frequency settings; determine a duration for the fluid at the end of the nozzle to thaw; and responsive to the duration elapsing, enter thrust mode by activating the source of the acoustic energy with a second set of voltage and frequency settings. [097] Example 19: The atomizer of any of Examples 11-18, wherein the atomizer is further configured to generate surface acoustic waves to atomize the fluid on the second surface of the substrate.

[098] Example 20: A system comprising: a thrust controller; and an atomizer comprising: a substrate comprising piezoelectric material; and a fluid reservoir attached to a first surface of the substrate; wherein the atomizer is configured to: cause acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, wherein the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate; and atomize, with the acoustic energy, the fluid on the second surface of the substrate.

[099] The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and sub-combinations of the disclosed features and/or combinations and sub-combinations of several further features disclosed above. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. Other implementations can be within the scope of the following claims.

Claims

What is claimed:

1. A method comprising: generating acoustic energy in a substrate comprising piezoelectric material, wherein a fluid reservoir is attached to a first surface of the substrate; causing the acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, wherein the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate; and atomizing, with the acoustic energy, the fluid on the second surface of the substrate.

2. The method of claim 1, further comprising producing thrust by atomizing the fluid flowing through the opening onto the second surface of the substrate.

3. The method of claim 1, wherein the opening passes through an interior of the substrate from the first surface to the second surface, and wherein a diameter of the opening is in a range between one hundredth of a wavelength of an acoustic wave traveling in the substrate and one half of the wavelength of the acoustic wave traveling in the substrate.

4. The method of claim 1, wherein a wick is situated in between the fluid reservoir and the first surface, and wherein a thickness of the wick is in a range between a quarter- wav elength and a half-wavelength of a speed of sound in the piezoelectric material of the substrate.

5. The method of claim 1, wherein the acoustic energy is generated by a segmented set of electrodes, and wherein a first portion of the segmented set of electrodes is activated so as to produce atomization droplets which are ejected in a desired direction.

6. The method of claim 1, wherein a rate of atomization of the fluid is based on an amount of the acoustic energy absorbed by fluid droplets on the second surface.

7. The method of claim 1, further comprising: entering, by a thrust controller, into freeze mode by deactivating a source of the acoustic energy; and allowing the fluid to freeze at an end of a nozzle to seal the nozzle and prevent fluid loss from the fluid reservoir.

8. The method of claim 7, further comprising: detecting, by the thrust controller, a condition for transitioning from freeze mode into thawing mode; responsive to detecting the condition, entering thawing mode by activating the source of the acoustic energy with a first set of voltage and frequency settings; determining a duration for the fluid at the end of the nozzle to thaw; and responsive to the duration elapsing, entering thrust mode by activating the source of the acoustic energy with a second set of voltage and frequency settings.

9. The method of claim 1, wherein the substrate is part of a thickness mode resonator device, and wherein the method further comprising generating the acoustic energy by applying a sinusoidal voltage signal to the thickness mode resonator device.

10. The method of claim 1 , wherein the substrate is part of a surface acoustic wave device, and wherein the method further comprising generating the acoustic energy by applying a sinusoidal voltage signal to the surface acoustic wave device.

11. An atomizer comprising: a substrate comprising piezoelectric material; and a fluid reservoir attached to a first surface of the substrate; wherein the atomizer is configured to: cause acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, wherein the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate; and atomize, with the acoustic energy, the fluid on the second surface of the substrate.

12. The atomizer of claim 11, wherein the atomizer is further configured to produce thrust by atomizing the fluid flowing through the opening onto the second surface of the substrate.

13. The atomizer of claim 11, wherein the opening passes through an interior of the substrate from the first surface to the second surface, and wherein a diameter of the opening is in a range between one hundredth of a wavelength of an acoustic wave traveling in the substrate and one half of the wavelength of the acoustic wave traveling in the substrate.

14. The atomizer of claim 11, further comprising a wick situated in between the fluid reservoir and the first surface, wherein a thickness of the wick is in a range between a quarterwavelength and a half-wavelength of a speed of sound in the piezoelectric material of the substrate.

15. The atomizer of claim 11, further comprising a segmented set of electrodes, wherein a first portion of the segmented set of electrodes is activated so as to produce atomization droplets which are ejected in a desired direction.

16. The atomizer of claim 11, wherein a rate of atomization of the fluid is based on an amount of the acoustic energy absorbed by fluid droplets on the second surface.

17. The atomizer of claim 11, further comprising a nozzle, wherein the atomizer is further configured to: enter into freeze mode by deactivating a source of the acoustic energy; and allow the fluid to freeze at an end of the nozzle to seal the nozzle and prevent fluid loss from the fluid reservoir.

18. The atomizer of claim 17, wherein the atomizer is further configured to: detect a condition for transitioning from freeze mode into thawing mode; and responsive to detecting the condition, enter thawing mode by activating the source of the acoustic energy with a first set of voltage and frequency settings; determine a duration for the fluid at the end of the nozzle to thaw; and responsive to the duration elapsing, enter thrust mode by activating the source of the acoustic energy with a second set of voltage and frequency settings.

19. The atomizer of claim 11, wherein the atomizer is further configured to generate surface acoustic waves to atomize the fluid on the second surface of the substrate.

20. A system comprising: a thrust controller; and an atomizer comprising: a substrate comprising piezoelectric material; and a fluid reservoir attached to a first surface of the substrate; wherein the atomizer is configured to: cause acoustic energy to be absorbed by a fluid on a second surface of the substrate opposite to the first surface, wherein the fluid flows through an opening in the substrate from the fluid reservoir to the second surface of the substrate; and atomize, with the acoustic energy, the fluid on the second surface of the substrate.