The present disclosure relates to systems and methods for producing uniform droplets. More particularly, the present disclosure relates to systems and methods for producing uniform droplets using a piezoelectric actuator.
The demand for improved coatings and powder particle products in the thermal spray industry has been relentless as the technology suffers from compositional non-homogeneity of injected solution particles. One solution to achieve homogeneity in coatings and particle products is aimed at repeatedly producing uniform droplets with uniform diameter. Precise control of the size of the solution droplets injected into a thermal spray system achieves more precise control of the particle melt for successful and improved coatings and powder generation. The methods of droplet generation using capillary streams involve the use of a piezoelectric device impinging a pressure pulse on the walls of a reservoir vessel full of a liquid solution. In general, one such method is the imposition of amplitude modulated sinusoidal carrier disturbance on the piezoelectric device. These methods generally involve piezoelectric devices (“piezo”) in direct contact with the liquid source. One method involves using an oscillating crystal in direct contact with a liquid source to impart a disturbance and initiate capillary instability responsible for stream break up into droplets. The disturbance is imposed in a compressive fashion at the top of the liquid volume and propagated downstream to the capillary nozzle. Another method imparts this disturbance on the side wall of a columnar liquid contained in a radially contracting piezoelectric cylinder that forces liquid through a capillary nozzle and is said to produce uniform stream of droplets. These droplet generation methods are, in general, limited to high droplet diameter and/or work at frequencies no higher than 10 KHz.
Applications of droplet apparatuses known in the art have the piezo in direct contact with the liquid. For example, in a typical printer design, the piezo is immersed in the printing liquid and serves as a gate to allow or forbid droplet exit as the piezo stretches or contracts under electrical drive. In another application, the piezo oscillations are transmitted directly to the liquid so that the piezo is in contact with the liquid or, if not in contact, the transmission is done through an elastic membrane. Furthermore, the effect of oscillations involves only a small volume of liquid directly near the nozzle.
In one broad embodiment of the present disclosure, the systems for producing droplet streams with the droplets having uniform diameter, comprise: a solution dispenser in fluid communication with a fluid reservoir contained in a fluid reservoir vessel, a separation membrane disposed in the fluid reservoir vessel, the fluid reservoir adjacent to and in contact with one side of the separation membrane, a piezoelectric actuator in contact with the separation membrane on a side opposite that in contact with the fluid reservoir and disposed away from the separation membrane, and one or more capillary nozzles for receiving fluid from the fluid reservoir and ejecting a droplet stream from the one or more capillary nozzles.
In another broad embodiment, the systems for producing droplet streams with the droplets having uniform diameter, comprise: an electronics driver circuit for driving a piezoelectric actuator which acts as a capacitor, an operational amplifier (OP-AMP), a transformer stage, and a loading stage having a choke inductor. The choke inductor is in series configuration with a piezoelectric capacitor. This is intended to reduce the current requirements of the actuator operated alone by adding the inductor which in the ideal case make a resonant LC circuit with the actuator (capacitor) at the desired drive frequency. It has been found that, absent this inductor, the current requirements of the drive electronics become increasingly difficult to meet as the frequency is increased. The electronics driver circuit comprises a signal generator.
In another broad embodiment, the methods of the present disclosure for producing droplet streams with the droplets having uniform diameter, comprise: providing a solution to a fluid reservoir vessel, filling the fluid reservoir vessel with the solution to form a fluid reservoir, contacting the fluid reservoir disposed in the fluid reservoir vessel with one side of a separation membrane, contacting a piezoelectric actuator with the other side of the separation membrane, causing the piezoelectric actuator to send at least one perturbation pulse to the separation membrane and the fluid reservoir to create at least one perturbation wave through the fluid reservoir, receiving fluid from the fluid reservoir by one or more capillary nozzles disposed away from the separation membrane, and ejecting one or more droplet streams from the one or more capillary nozzles.
In another broad embodiment, the methods of the present disclosure for producing droplet streams with the droplets having uniform diameter, further comprise: actuating the piezoelectric actuator capacitor with a sinusoidal wave to produce perturbations on the separation membrane, and transmitting the perturbations through the separation membrane to the solution in the fluid reservoir.
A specific embodiment of the present disclosure will now be more fully described in conjunction with the drawings which follow, in which:
a and 4b show a schematic view of a multi-capillary nozzle for making multiple jets of uniform droplets according to the present disclosure.
Referring to the drawings and, in particular, to
Referring to
where dj is the jet diameter, η is the fluid viscosity, ρ is the fluid density, and σ the surface tension. The droplets produced are uniform and their diameter, dd, is 1.89 that of the jet diameter, dj.
Referring to
Referring to
According to the present disclosure, the concept of the membrane separating the actuator and the disturbed liquid is unique since the membrane is made of stainless steel or other rigid material and is very rigid with a prescribed thickness. The selection of the membrane thickness is based on the stiffness with the membrane being sufficiently flexible to transmit a suitable amount of deflection from the actuator into the fluid. This leads to a wide range of possible choices of membrane thicknesses and in-plane dimensions. In general for such a concentrated load from an actuator acting, for example, on a circular membrane (which behaves as a circular plate) the stiffness of a circular membrane is proportional to E h3/R2 where R is the membrane radius, E is the Young's modulus of the membrane material and h is the membrane thickness. Similar relations apply to other membrane shapes such squares and rectangles, etc. Thus, a broad range of designs are possible depending on the force capabilities of the actuator and the properties of the fluid to be expelled. The geometry may include all geometries with a suitable stiffness range which, in turn, is dependent on the actuator chosen and the chamber design and the fluid properties. The design thus can be calculated for any particular application by one of ordinary skill in the art. For the actuator used in the example and Figures, R=0.35″, h=0.02846″ and E=26×106 psi (approximately) and has been found to be usable for a range of fluids used in the exemplified actuator/chamber combination. Thus, using the above equation and the actuator and chamber exemplified, the present example employed a stainless steel membrane having a thickness of 21 gauges (0.723 mm). The membrane acts as a protective barrier for the piezo actuator from hostile liquids, and transmits the perturbation pressure pulse(s) of the piezo actuator to the liquid on the other side of the membrane.
In the specific embodiment described in connection with the Figures, the droplet maker can utilize hostile liquids such as acids (and bases) because the housing, including the reservoir, has an integrated “functional” rigid and chemical-resisting membrane made of corrosion resistant material, such as stainless steel, titanium, or a rigid material that is coated with a chemical-resistant material such as Teflon. Furthermore, the capillary nozzle is made of a dielectric that is chemically stable and can handle similar hostile liquids. Such configuration and construction of the reservoir separates the piezo actuator from the liquid. The separation membrane serves as a protective barrier for the piezo actuator. The piezo actuator is not in direct contact with the liquid. Instead, the vibrations of the piezo actuator are transmitted as perturbation pressure pulses through the rigid membrane to the liquid. Stainless steel housing has been tested with precursors containing citric acid resulting in solution with a pH of about 4. For an even more hostile environment with more acidic or basic pH, hastalloy, or other material resistant to the pH, can be used.
It is believed that the use of ceramic capillaries is unique for longitudinal actuation of the perturbation pressure pulse(s). Known systems and methods use glass capillaries, similar in shape to those capillaries of the present disclosure, but have been used for radial actuation instead which differs from the longitudinal actuation of the present disclosure.
In a multi-capillary nozzle configuration of the present disclosure, a configuration that may include 2, 3, 4, 5 or more capillaries, a symmetrical topology may preferably be used to position the capillaries to distribute evenly the liquid perturbation pressure pulse(s) for uniform droplet breakdown across all capillaries. As the piezo actuator is a disk of, e.g., 10 mm and doughnut shaped, the perturbation pressure pulse(s) is/are cylindrical in shape with a circular cross section. The capillaries are placed on a generally circular configuration smaller than the diameter of the doughnut-shaped piezo actuator.
While use of capillaries with a small diameter may generally be prone to clogging, according to the present disclosure a purging scheme has been devised to minimize or avoid clogging due to hardening of acid and/or metallic salt-based solution(s). In the present disclosure, the inlet to the liquid reservoir is run through a tunnel (channel 222 in
The OP-AMP with the transformer circuit configuration driving the LC loading stage is designed as “resonant” for optimum drive of the LC circuit. The droplet making frequency regime is chosen to be below the natural resonant frequency of the piezo capacitor to increase its lifetime. Also, the present configuration uses a small piezo ring (doughnut) shaped disk with a small capacitance (on the order of 15 nanoFaraday (nF)) which pushes the frequency bandwidth of the drive circuit to higher frequencies.
Preferably, the fluid reservoir vessel is generally or substantially cylindrical in shape, having a bottom surface and a top surface which are generally or substantially circular in shape and a columnar side portion disposed between the bottom surface and the top surface. Preferably, the solution dispenser is in communication with the fluid reservoir vessel via a fluid transfer line between the solution dispenser and the fluid reservoir vessel, with the transfer of fluid from the solution dispenser to the fluid reservoir vessel effected with a pump, preferably a peristaltic pump or pressurized tank vessel. Also preferably, the fluid is transferred from the solution dispenser to the fluid reservoir vessel via a channel that causes the fluid to enter the fluid reservoir vessel at or near the bottom surface of the fluid reservoir vessel. Also preferably, the fluid reservoir vessel has an outlet disposed generally at or near the top surface of the fluid reservoir vessel.
Preferably, as mentioned above, the reservoir vessel is made of a relatively corrosion resistant material, such as stainless steel, or steel coated with stainless steel, vanadium, titanium, and the like, but may also be made of plastic coated material, and the coating may be of, e.g., Teflon or another corrosion resistant material. The separation membrane may be part of the fluid reservoir vessel or may be part of the piezo actuator structure. In any event, the separation membrane should have characteristics which provide suitable mechanical properties to the separation membrane. The separation membrane should be of sufficient thickness or made of suitable material to allow for deflection of the separation membrane by the piezo actuator, thus imposing perturbation pressure pulse(s) on the fluid reservoir. Thus, the stiffer the separation membrane, it is likely the thinner the separation membrane will need to be. In addition, the separation membrane should have sufficient but adequately low stiffness so as to allow for adequately proper preloading of the piezo actuator. Therefore, the characteristics of the separation membrane are, in general, related but to some degree of opposite nature. The membrane where the deflections occur provides perturbation pressure pulse(s) to the liquid in the reservoir vessel and allows deflection transmission without direct physical contact between the piezo actuator and the liquid.
Capillary nozzles are generally known in the art. The capillary nozzle is generally cylindrical in shape with an inner bore diameter of from less than about 10 micrometers up to about 100 micrometers. Preferably, the inner bore diameter is between about 5 micrometers to about 100 micrometers. More preferably, the inner bore diameter is between about 1-2 micrometers to about 100 micrometers. The length of the capillary nozzle is preferably no less than 5 mm and can be up to about 30 mm or longer. In an alternative embodiment, the nozzle holder is configured to hold a plurality of similarly-sized and shaped capillary nozzles in order to produce multiple stream jets of uniform droplets. The capillary nozzle(s) may be made of stainless steel, ceramic material and the like, but may also be made of any other sufficiently rigid and chemically resistant material, so as to withstand any corrosive nature of the fluid.
The size and configuration of the nozzle(s) allows for droplet streams having uniform diameters smaller than about 200 micrometers, preferably smaller than about 150 micrometers, more preferably smaller than 100 micrometers, and most preferably smaller than about 50 micrometers. For smaller droplets with diameter size below about 100 micrometers, it has been found that higher frequency and power drives are generally useful. The present disclosure aims at producing droplets with diameters as low as 5 micrometers for which higher frequency (higher than 10 KHz) may be used. This present disclosure can achieve even smaller diameters, as low as 1 micrometer, if capillaries with similar diameter are used. Also, contrary to the known methods and apparatuses, according to the present disclosure, the membrane on which the piezoelectric actuator impacts can be far away from the liquid input entry to the capillary nozzle, or nozzles. Specifically, distances up to 4 inches or more are possible. On the other hand, configurations with an actuator close to the exit orifice may also be used. Depending upon the application, performance may be enhanced for a specific frequency if the chamber length is chosen such that a standing wave is produced with its maximum pressure located near the exit orifice.
In a particularly preferred embodiment, the system of the present disclosure for producing droplet streams with, the droplets having uniform diameter. The system comprises: a reservoir vessel as a containment for solution precursors, a dismountable housing with strain relief for a piezoelectric device to generate displacement following a pressure pulse on the fluid volume of reservoir vessel, a high frequency and high power electronics drive that generates a continuous oscillating voltage pulse, one or more capillary nozzle(s) to discharge one or more jet(s) of uniform droplets after perturbation of volume of liquid in reservoir vessel, and a nozzle holder for a single or multiple capillary nozzles. The piezoelectric device is electronically energized to expand and contract under a sinusoidal voltage drive. In another particularly preferred embodiment, the reservoir vessel is a cylindrical chamber with at least one inlet input and one purge output. In still another particularly preferred embodiment, the housing chamber of the piezoelectric device includes: a sealed chamber including a cylinder with a screw on cap, a screw on bolt, and a cylindrical sleeve. Also preferably, the piezoelectric device is axis??—symmetrically positioned with the cylindrical sleeve and held in place against the bottom of the cylinder by the screw on bolt for mounting and preloading. Still preferably, the voltage drive can deliver square, triangular, and sinusoidal signal pulses of 0 to 50 volts in amplitude at frequencies up to 100 KHz.
In additional particularly preferred embodiments, the systems of the present disclosure for producing droplet streams with the droplets having uniform diameter, the piezoelectric device or other device is capable of delivering perturbation pressure pulses which give rise to displacements of the separation membrane of few micrometers or more. For example, the displacement of the membrane may be 1-5 micrometers, preferably less than 5 micrometers, more preferably less than 3 micrometers, and more preferably from less than 1 to about less than 3 micrometers. The displacement range to be produced is to include displacements of a size sufficient to induce droplet break up which may vary based on the properties of the fluid being expelled. Also in this embodiment the high frequency and high power electronics includes a signal generator, a high voltage and high current OP AMP stage, a transformer, and a loading stage with a choke inductor in series with piezoelectric capacitive load device operating at a lower frequency than the resonant frequency of the choke-piezo capacitor load. Efficient driving of the piezo actuator without the use of very large current supplies is achieved by LC resonance tuning or near tuning of the LC circuit made with the actuator capacitance and the selected inductor. Also especially preferable, the capillary nozzles are held in a nozzle holder that is made of stainless steel and comprises a steel cap to seal the reservoir vessel and hold and align the capillary nozzles. Also preferably, the signal generator has a frequency of between 0 and 1 MHz or higher, and produces an output voltage of between 0 and 10 volts or higher. The amplifier and transformer together convert the output voltage to a voltage of at least about 20 volts, preferably at least 30 volts, more preferably of from about 30 to about 50 volts, especially preferably from about 40 volts to about 50 volts, and most preferably from about 50 to about 60 volts. Also, the amplifier and transformer together convert frequencies at or above 10 KHz, preferably at or above 20 KHz, more preferably at or above about 30 to about 40 KHz, most preferably at or above about 50 KHz, up to about 70 MHz or higher, such as up to about 100 KHz to about 200 KHz.
Because the piezoelectric device of the presently disclosed methods and systems is not in direct contact with the liquid source, this allows for flexible and simple piezoelectric mounting. The piezoelectric device can be mounted anywhere convenient in association with the solution precursors of the droplet stream, and allows for use of solution precursors for the droplet stream that can be corrosive. As stated above, preferably the perturbation pressure pulses are produced in a sinusoidal fashion and, more preferably, the sinusoidal wave is produced by a signal generator that transmits a source voltage to an amplifier to amplify and modulate the source voltage to produce an amplified and modulated voltage, which amplified and modulated voltage is then transmitted to a transformer which steps up the voltage to produce a stepped up voltage. The stepped up voltage is then transmitted to a piezo capacitor which, in turn, transmits a pressure pulse to separation membrane. Further, the pressure pulse is transferred through separation membrane to the solution in the fluid reservoir. Still further, the pressure pulse is repeatedly transferred to the solution through the separation membrane and propagates through the solution and forces the solution into the capillary, thereby ejecting the solution through the capillary and producing a stream of uniform droplets.
While the present disclosure has been described with reference to particular embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt the teaching of the present disclosure to particular use, application, manufacturing conditions, use conditions, composition, medium, size, and/or materials without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiments and best modes contemplated for carrying out this disclosure as described herein. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present disclosure.