WO2006047453A2

WO2006047453A2 - Process for high throughput electrohydrodynamic spraying of fluids

Info

Publication number: WO2006047453A2
Application number: PCT/US2005/038260
Authority: WO
Inventors: Charles P. Beetz, Jr.; Thomas A. Chasteen; David R. Salem; James Agresta
Original assignee: Charge Injection Technologies, Inc.
Priority date: 2004-10-22
Filing date: 2005-10-21
Publication date: 2006-05-04
Also published as: WO2006047453A3

Abstract

A method for atomizing a fluid comprising subjecting an electrically conductive fluid (14) to an electric potential, and simultaneously forcing said fluid through a nozzle (22) having dielectric walls defining a nozzle opening. The subjecting and forcing steps desirably are performed so that said fluid is substantially atomized after passing out of the nozzle. Most preferably, the fluid exiting the nozzle exhibits a stable whipping mode of atomization.

Description

PROCESS FOR HIGH THROUGHPUT ELECTROHYDRODYNAMIC SPRAYING OF FLUIDS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. Provisional Application Serial No. 60/621,598, filed October 22, 2004, the disclosure of which is hereby incorporated by reference herein. FIELD OF THE INVENTION

[0002] The present invention relates to the methods and apparatus for atomizing a conductive fluid. BACKGROUND OF THE INVENTION

[0003] Electrohydrodynamic ("EHD") atomization refers to a method of dispersion of a fluid due at least in part to the action of electrical forces. In conventional EHD atomization, an apparatus forces a conductive fluid through an electrically conductive nozzle. The nozzle is connected to a high negative voltage, whereby an electric field is created between the conductive nozzle and a ground electrode. This field is strongest at the tip of the nozzle. As the fluid exits the nozzle, electrical and mechanical effects cause the fluid jet to break up into droplets. The fluid jet that forms can take on many forms, depending on the properties of the fluid such as surface tension, electrical conductivity, viscosity, density and viscoelastic behavior as well as operational variables such as the rate of flow and the local electric field. When the fluid jet is sent in the direction of a target that is grounded, or is at a potential of opposite polarity, or a potential lower than that of the jet, the droplets are attracted to the target and will deposit on the target.

[0004] In order to sufficiently atomize a fluid through the conventional methods of EHD atomization, a small diameter nozzle is required, and the fluid must pass through the nozzle at very low flow rates. For many applications, the achievable flow rates are inadequate. There have been attempts to avoid some of these as, for example, by using an apparatus with numerous nozzle tips. One EHD atomizer of this type is shown in U.S. Pat. No. 4,613,075 to Owen. [0005] However, despite the significant effort devoted to development of electrostatic atomization in the art, still further improvement would be desirable. In particular it would be desirable to provide high-throughput atomization of electrically conductive liquids. One industrially-important application is electrically conductive latex paint. SUMMARY OF THE INVENTION

[0006] The present invention addresses these needs. [0007] In accordance with one aspect of the present invention, a method for EHD atomization of a conductive fluid comprises subjecting an electrically conductive fluid to an electric potential relative to the surroundings, and simultaneously forcing the fluid through a nozzle having dielectric walls defining a nozzle opening, the subjecting and forcing steps being performed so that said fluid is substantially atomized after passing out of said nozzle. Most preferably, the subjecting and forcing steps are performed so that fluid passing out of said nozzle exhibits a high flow rate, stable whipping mode. Most preferably, the fluid has a conductivity greater than 1 μ Siemens/cm under the conditions prevailing in the system.

[0008] As further discussed below, preferred embodiments of the present invention incorporate the discovery that, in the case of electrically conductive liquids flowing from a nozzle with dielectric walls defining the nozzle opening, there is a particularly advantageous operating regime. When the liquid is subjected to a given voltage, it typically will exhibit an unstable behavior if it exits the nozzle opening at a low velocity and low flow rate, and normally will not atomize in a predictable manner. However, as the flow rate and velocity increase, the fluid exits the nozzle as a jet and breaks up into droplets within a narrow cone around the axis of the jet, having an included angle which is typically less than about 30 degrees. In this mode of operation, the fluid is atomized to a relatively narrow range of fine droplet sizes. Because this unique and advantageous mode of operation occurs at high flow rates, it allows useful atomization of conductive liquids with high throughput per nozzle.

[0009] Optionally, the step of forcing the fluid through the nozzle also includes making the fluid rotate about the axis of the fluid jet flow. The rotation or swirl will create a mechanical instability that can increase the amount of atomization that can occur.

[0010] The upper limit on the flow rate is determined by the size of the nozzle opening and the pressure available to force fluid through the nozzle. Certain preferred embodiments of the invention can achieve throughputs great enough for industrial applications of fluid atomization. For example, the method can be used for spraying paint, including electrically conductive latex paint, as well as for atomization of liquid metals as, for example, in forming metal powders.

[0011] In other applications of the method, the fluid solidifies or gels during the atomization process as, for example, by chemical reaction, by evaporation of solvent, or by freezing. In this case depending on the rheological properties of the liquid, the atomization process tends to form fibers rather than droplets. Such variants can be used, for example, in electrospinning of polymeric nanoscale fibers, and in fabrication of ceramic and composite nanofiber structures. In other cases such as a chemically reactive fluid interacting with a stationary gas phase or thermally heated region, the atomization process tends to form nanoparticles .

[0012] Further aspects of the invention provide apparatus for atomization of a fluid. Apparatus according to this aspect of the invention desirably includes a source of a conductive fluid under pressure, and a nozzle communicating with the source for discharging the fluid. The nozzle most preferably has dielectric walls defining a nozzle opening. The apparatus also includes a source of electrical potential electrically connected to the fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic view of an apparatus for atomizing a conductive fluid in accordance with an embodiment of the invention.

[0014] Figure 2 is a cross-sectional view of a nozzle according to a further embodiment of the invention, viewed in a direction transverse to the axis of the nozzle.

[0015] Figure 3 is a cross-sectional view of a nozzle as viewed along the axis of the nozzle.

[0016] Figure 4 is a cross-sectional view of a nozzle according to the viewed along the axis of the nozzle.

[0017] Figure 5 is a photograph of a fluid jet with an undesirable configuration.

[0018] Figure 6 is a photograph of a fluid jet having a desirable, stable whipping mode of atomization.

[0019] Figure 7 is a graph representing the anode target current as a function of the applied negative high voltage from the power supply, using a nozzle with an 800μm diameter opening.

[0020] Figure 8 is a graph representing the anode target current as a function of the applied negative high voltage from the power supply, using a nozzle having a 250 μm diameter opening. DETAILED DESCRIPTION

[0021] In describing the preferred embodiments of the subject matter illustrated and to be described with respect to the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific term and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

[0022] As used herein, the term "dielectric" means an insulting material with sufficiently high resistance to prevent a substantial current from forming under the given electrical potentials. The term "conductive fluid" means a fluid having a conductivity of at least 1 μ Siemens/cm under the conditions prevailing within the apparatus in the vicinity of the nozzle. [0023] An atomizing apparatus 10 in accordance with one embodiment of the invention is shown in Figure 1. The apparatus includes a liquid reservoir 12 for containing fluid 14. Reservoir 12 is connected to a set of nozzles 22. Each nozzle

22 has walls 23 defining a tapered channel 25 extending in a downstream direction (to the left as seen in Figure 1) to an opening 26. Channel 25 has cross-sectional dimensions which decrease progressively in the downstream direction. The walls

23 of the nozzle defining the opening 26 and the downstream end of channel 25 preferably are made of a dielectric material, such as a polymeric material or glass . The dielectric material desirably is substantially inert to the fluid which will be atomized. The walls of the nozzle remote from opening 26, and the conduit connecting the nozzle with reservoir 12, may be formed from dielectric or conductive materials .

[0024] The nozzle opening 26 may be circular in cross- section. The circular nozzle opening 26 typically has a diameter between approximately 100 to 3000 μm, more preferably about 250 to about 800 μm.

[0025] An electrode 20 is disposed on or within reservoir 12, in contact with the fluid in the reservoir. The electrode may form a portion or all of the vessel wall. A power supply 18 is electrically connected to an electrode 20 for biasing the fluid 14 to a negative voltage with respect to the surroundings, and, in particularly, with respect to a counterelectrode 24 disposed outside of the nozzle. Most preferably, power supply 18 is operative to apply a negative voltage of about 5,000 and 90,000V to the fluid, whereas counterelectrode 24 is maintained at ground potential. The liquid reservoir 12 may be dielectric or conductive.

[0026] The liquid reservoir 12 is connected to a pressure source 16 for pressurizing the liquid reservoir 12. The pressure source 16 may be a source of compressed gas, such as an air compressor or tank of compressive gas connected to the space within the reservoir above the fluid 12. Alternatively, the pressure source may be a pump connected between the reservoir and the nozzles. In any case, the pressure source 16, should be a device which can force the fluid 14 through the nozzles 22 at a sufficient velocity to as to cause the fluid 14 to achieve a flow rate sufficient to achieve the stable whipping mode operation discussed below, most preferably on the order of 0.5 x ICT⁵ m³/s or greater.

[0027] In a method of atomization according to one embodiment of the invention, the fluid 14 is electrically biased by the electrode 20 and is pressurized by the pressure source 16. The pressurized fluid 14 then flows from the liquid reservoir 12 through nozzles 22, and the fluid 14 is atomized upon exiting the nozzles. The atomized fluid deposits on counterelectrode 24.

[0028] The fluid exits from each nozzle opening 26 as a jet having diameter comparable to the diameter of the nozzle opening. At low flow rates, the jet develops instabilities that cause a divergent whipping mode to develop as shown in Figure 6. In this divergent whipping mode, the jet slews back and forth through a wide range of angles relative to the axis of the nozzle. This low flow rate whipping mode is an undesirable mode, creating a large distribution of particle sizes and a wide distribution of fluid particles. However, as the flow rate of the liquid is increased, the system will suddenly enter into a stable operating mode which is also a form of a whipping mode. As shown in Figure 7, the high flow rate, stable whipping mode is characterized by a narrow distribution of droplets within a cone of approximately 30 degrees. Stated another way, the jet and the droplets forming from the jet remain within a cone of about 30 degrees included angle. Moreover, the cone tends to form at a given distance downstream from the nozzle opening. This distance decreases as the voltage applied by power supply 18 increases. Typically, the stable whipping mode occurs when the velocity of the fluid in the downstream direction through the nozzle tip is at least about 10 m/s, which corresponds to a flow rate of at least about 1 ml/s for a circular nozzle of 250 μm diameter. Higher velocities can be employed, with a velocity of about 20 m/s being more preferable than that of 10 m/s, and a velocity of about 50 m/s being even more preferable. In most cases, the practical upper limit of the velocity which can be achieved is set by the capacity of the pressure source. [0029] The importance of the discovery of a stable whipping mode comes from its ability to produce a stable flow of small particles, such as paint particles, in the range of 20 to 500 μm in diameter at extremely high flow rates. The embodiments have allowed flow rates which far exceed 40 ml/min per nozzle while producing paint particles in the range of 20 to 500 μm. Specifically, a flow rate of 2760 ml/min has been achieved using an embodiment of the invention to atomize latex paint. [0030] The high flow rate whipping mode is produced in part, from the presence of a nozzle having a dielectric walls defining the nozzle opening. Although the present invention is not limited by any theory of operation, it is believed that the dielectric walls cause the fluid 14 to be exposed to a far greater electric field intensity than would be the case using a conventional, electrically conductive nozzle tip. With a conductive nozzle tip, the field tends to concentrate on the exterior of the nozzle. By contrast, with a nozzle tip having dielectric walls defining the opening, the electric field that is largely contained within the area of the nozzle opening 26. It is further believed that in operation under the conditions which produce the stable, high flow rate whipping mode, the conductive fluid carries substantial negative charge downstream from the nozzle, so that the fluid in the jet produces a substantial self-field. That is, portions of the negatively- charged fluid in the jet tend to repel one another. Here again, although the present invention is not limited by any theory of operation, it is believed that this effect may relate, at least in part, to the relationship between the velocity of the fluid passing downstream through the nozzle and the charge induced instabilities produced in the fluid.

[0031] As the relative voltage produced by the power supply is increased, the length of the fluid jet will be decreased, as the fluid 14 will atomize more quickly upon leaving the nozzle 22. In some applications, a short fluid jet may be preferable, so that a power supply 18 producing a voltage greater than the threshold voltage will be desirable. it has been observed that a maximum voltage exists past which the length of the fluid jet will no longer be greatly affected. In some applications, the maximum voltage has been observed to be around 50,000V to 60,000V; voltages beyond that range having little effect on the length of the fluid jet. However, this maximum voltage will vary on the type of fluid that is being atomized. The threshold voltage required for atomization of fluid 14 is related to the size of the nozzle opening 26. As the area in the nozzle opening 26 is decreased, so is the threshold voltage required for reaching stable whipping mode atomization. This is shown in comparing Example 1 and Example 2 below.

[0032] Although three nozzles 22 are depicted in Figure 1, apparatus according to other embodiments of the invention may include only one nozzle or any number of nozzles. Apparatus according to a further embodiment of the invention includes a nozzle 122 (Figure 2) that is not constructed entirely of a dielectric material. Instead, the nozzle 122 has walls 102 constructed of a conductive material defining the region of the flow passage 125 remote from the nozzle opening 126, but has dielectric walls 126 defining the nozzle opening. The conductive walls 102 may be connected to the power supply, and may serve as the electrode in contact with the fluid. Apparatus according to a further embodiment of the invention includes a nozzle 122 containing a channel 25 that is not tapered.

[0033] Alternatively or additionally, an electrode 120 (Figure 2) may be disposed within the nozzle 122, upstream of the nozzle opening 126. Although the electrode 20 may be placed in any position that will cause the fluid 14 to be biased to a desired potential with respect to ground, placement of the electrode within or near the nozzle can produce a greater localized electric filed on the fluid passing through the nozzle. The electrode 120 is not required to be any specific shape. However, an electrode with one or more sharp features, such as a point 127 at the downstream end of the nozzle or a rough surface with numerous points 128 can be used to increase the field. As stated above, the fluid desirably is conductive. Exposure to the high field prevailing within the apparatus can render fluids which are normally nonconductive momentarily conductive. For example, certain commercial hydrocarbon fluids such as fuels and lubricants contain detergent additives which are normally present in a unionized condition. When electrical conductivity is measured under low field conditions, the fluid has conductivity well below 1 μ Siemens/cm, and accordingly appears to be non-conductive. However, when exposed to a high electric field, the additives ionize or dissociate and the fluid becomes conductive.

[0034] In apparatus and methods according to still further embodiments of the invention, the fluid is induced to rotate about the upstream-to-downstream axis of the fluid column as it passes downstream through the nozzle to the nozzle opening. The fluid rotation may be achieved in numerous ways. For example, a 222 shown in Figure 3 incorporates one or more grooves 234 on the inner surface of the nozzle wall, as shown in Figure 4. The groove or grooves 234 twist around the upstream to downstream axis of the nozzle, and extend over at least a portion of the upstream-to-downstream extend of the nozzle. As the fluid passes downstream toward nozzle opening 226, the grooves cause the fluid to rotate around the axis. Other structures, such as vanes (not shown) may be provided within the nozzle passageway. In still other embodiments, the structures which are provided to induce rotation may include an inlet (not shown) communicating with the interior passageway of the nozzle remote from the upstream-to-downstream axis, thereby forcing the fluid to enter the interior passageway of the nozzle with angular momentum about the upstream-to-downstream axis. Although the invention is not limited by any theory of operation, it is believed that this rotation creates a mechanical instability in addition to the electrical instability produced by the potential created in the fluid. This mechanical instability can increase the degree of atomization that can occur. It has been observed that when the fluid is rotating about the downstream axis upon exiting the nozzle, the fluid may form multiple jets or nodes. These nodes can themselves enter high flow rate whipping modes, thereby- producing multiple whipping mode jets from one nozzle.

[0035] In still other embodiments, the nozzle opening, and the internal passageway of the nozzle, may be non-circular. For example, a nozzle 322 (Figure 4) has an eye shaped opening 326. The eye shaped opening most preferably has an internal width w of about 150 μm to about 500 μm and an internal length 1 of about 600 μm to about 2500 μm. Although any nozzle opening shape can be used, rectangular openings are generally less preferred. It is believed that with a rectangular opening, the fluid 14 will tend to travel at markedly unequal velocities through different portions of the nozzle opening.

[0036] Essentially any type of conductive fluid can be employed. One extremely useful application is in spraying latex paint. In other embodiments, the fluid 14 is metal which is in a liquid state. The nozzle and reservoir may be maintained at an elevated temperature, above the liquidus temperature of the metal. The atomized metal may form a fine metal powder. In a further application, the fluid is a solidifiable fluid which will form a solid or a gel as, for example, by evaporation of a solvent from the liquid, by chemical reaction as it passes downstream from the nozzle, or by freezing. If the speed of solidification is such that solidification occurs during the time required for atomization, and the rheological properties of the fluid are appropriate, the atomization process can be interrupted before the fluid is converted to discrete droplets. In this case, the fluid is converted to small fibers. This can be used, for example, in electrospinning of polymeric nanoscale fibers, or in the fabrication of ceramic and composite nanofiber structures. In the case of reacting or fast evaporating fluid systems, the atomization process is capable of producing nanometer scale particles.

[0037] The following non-limiting examples further illustrate certain features of the invention.

[0038] EXAMPLE 1

[0039] The apparatus was as shown in Figure 1, having a single nozzle. The fluid filling the liquid reservoir was tap water. The nozzle was made of cylindrical ceramic, having an overall length of 2.5 cm, with a non-tapered end section of approximately 0.5 mm. The taper angle of the nozzle was 15 degrees, and the nozzle opening had a diameter of 800μm. A water stream was ejected from the ceramic nozzle using gas pressure applied to the reservoir. Process conditions were varied so as to increase the negative voltage of the power supply progressively. Simultaneously, the current reading of the power supply and the anode current coming from the target were recorded. This process was continued until the water stream was completely atomized. The anode current and the power supply current tracked one another, and the threshold voltage was found to exist at approximately -10,000V, at which point the current coming from the target began to substantially increase as shown in Figure 8. [0040] EXAMPLE 2

[0041] The apparatus was the same as the apparatus used in Example 1, except the nozzle had a diameter of 250 μm. The water reservoir was charged by increasing the negative voltage of the power supply, and the anode current coming from the target was recorded. This process was continued until the water stream was completely atomized. The anode current and the power supply current tracked one another as they did in Example 1, however the threshold voltage had changed and was now approximately -7,000V, as shown in Figure 9. [0042] EXAMPLE 3

[0043] The apparatus was a paint gun consisting of a plastic reservoir with a stainless steel metal rod electrode for applying voltage, a pressure inlet for pressurizing the paint reservoir, a power supply connecting to the electrode. The nozzle consisted of two swirl nozzles each with an orifice diameter of 550 micrometers. The paint gun reservoir was charged with 140 ml of Promar 400 flat latex paint. The power supply voltage was set at -75,000 volts, the pressurization of the reservoir to 250 psi, the paint atomized and the reservoir emptied in 3 seconds for a volumetric flow rate of 2760 ml/min. The average particle size as measured with a particle size analyzer was between 200 to 300 micrometers. [0044] EXAMPLE 4

[0045] A low melting point Indium alloy (m.p. = 117 °F) was placed in a heated reservoir at a temperature of 140 ⁰F. The reservoir was pressurized to a 2 to 3 psi. A high voltage electrode was placed in the reservoir and connected to a high voltage power supply set at 75,000 volts. The spray nozzle consisted of 120 rectangular orifices 0.021 x 0.030 inches. The molten metal was sprayed at a mass rate of 20 lbs/min at a particle size less than 100 micrometers with the majority of particles less than 50 micrometers in diameter. [0046] EXAMPLE 5

[0047] An aqueous solution of polyvinyl alcohol 10% PVA by weight was prepared and poured into a plastic reservoir of a spray device. The conductivity of the PVA solution was 650 μ Siemens/cm. The reservoir contained a stainless steel electrode for charging the polymer solution. The reservoir was pressurized to 85 psi and held at 25°C. The electrode was connected to a power supply set at -30,000 volts. A metal mesh placed 11 inches from the nozzle was used as a target for collecting the nanofibers. The metal mesh was held at +30,000 volts. The PVA solution was sprayed from a single dielectric nozzle with an orifice diameter of 250 micrometers. Approximately 30 ml of solution was spun in 45 seconds for a flow rate of 40 ml/min. A dense deposition of fibers was collected on the metal mesh, with fiber diameters in the range 400 to 1000 nm.

Claims

1. A method for atomization of a fluid comprising: subjecting an electrically conductive fluid to an electric potential, and simultaneously forcing said fluid through a nozzle having dielectric walls defining a nozzle opening, said subjecting and forcing steps being performed so that said fluid is substantially atomized after passing out of said nozzle.

2. A method as claimed in claim 1 wherein said subjecting and forcing steps are performed so that fluid passing out of said nozzle exhibits a high flow rate whipping mode.

3. A method as claimed in claim 1 wherein said step of subjecting the fluid to a potential includes applying an electric field to said fluid sufficient to substantially increase the conductivity of said fluid.

4. A method as claimed in claim 3 wherein said step of subjecting the fluid to a potential includes applying said potential using an electrode immersed in said fluid, said electrode having one or more sharp features.

5. A method according to claim 1 wherein said forcing step includes forcing the fluid along an upstream-to-downstream axis through the nozzle and inducing rotational flow of the fluid about the axis.

6. A method according to claim 1 wherein said forcing step includes forcing the fluid through a plurality of nozzles.

7. A method according to claim 1 wherein the opening of said tip is circular.

8. A method according to claim 1 wherein said tip is non- circular.

9. A method according to claim 1 wherein said fluid forms a flat sheet of droplets after exiting said tip.

10. A method according to claim 1 wherein said fluid is paint.

11. A method according to claim 1 wherein said fluid is a latex paint.

12. A method according to claim 1 wherein said fluid is a molten polymer for producing nanofibers .

13. A method according to claim 1 wherein said fluid is a molten metal.

14. A method according to claim 1 wherein said fluid is flowing at a rate of at least 1.67*10^~6 m³/s•

15. Apparatus for atomizing a fluid comprising: a source of a conductive fluid under pressure, a nozzle communicating with said source for discharging said fluid, said nozzle having a dielectric walls defining a nozzle opening; and a source of electrical potential electrically connected to said fluid.

16. Apparatus as claimed in claim 15 wherein said source of electrical potential includes an electrode in contact with said fluid.

17. Apparatus as claimed in claim 16 wherein said electrode is within the walls of said nozzle.

18. Apparatus as claimed in claim 15 wherein said electrode has one or more sharp features.

19. Apparatus as claimed in claim 15 wherein said source of fluid includes an electrically conductive vessel and said source of electrical potential is electrically connected to said fluid by said vessel.

20. An apparatus as claimed in claim 15 wherein said nozzle is constructed and arranged to create a rotation in said liquid around the upstream-to-downstream axis through the nozzle.

21. An apparatus as claimed in claim 15 wherein a plurality of nozzles communicate with said source for discharging said fluid.