Patent Publication Number: US-6656394-B2

Title: Method and apparatus for high throughput generation of fibers by charge injection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application Ser. No. 60/183,450, filed Feb. 18, 2000, the disclosure of which is hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electrostatic methods and apparatus for forming fibers from fluids. 
     BACKGROUND OF THE INVENTION 
     In conventional commercial production of low diameter fibers, a liquid material such as a liquid polymer is forced through a small orifice in an apparatus referred to as a spinneret. The liquid polymers utilized in many fibers are extremely viscous and difficult to pass through a small orifice. These methods encounter practical difficulties. 
     Certain methods of electrostatic formation of fibers from liquid polymers are known. These methods use an electrode defining an orifice. The liquid is passed through the orifice, from a first side of the electrode to a second side. An oppositely charged surface is remotely disposed with respect to the electrode, on the second side of the electrode, to attract and collect the fibers formed after the fluid issues from the orifice. These methods require large potential differences developed over the large air gap between the orifice and the charged surface on which the fibers are collected. The electric field developed over the air gap is relied upon to develop the necessary charge within the fluid and attenuate the fluid. The attenuated fluid then solidifies into fibers. For low conductivity fluids, such as liquid polymers utilized to develop fibers for commercial applications such as fabrics, the flow rates attained by these methods are unacceptable. Known methods also include the use of a capillary needle as the electrode and orifice discussed above. Fibers having diameters of 50 nanometers and up have been produced utilizing these methods. 
     Electrostatic formation of fibers has great potential and it has been known that electrostatic formation of fibers would present a much more convenient and efficient method of producing fibers. However, despite considerable effort to develop these methods, these methods have been unable to handle commercially acceptable flow rates. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these needs. 
     In accordance with one aspect of the present invention, a method of producing fibers comprises providing a stream of a solidifiable fluid, providing the stream with a net charge so as to disrupt the stream by passing the stream through a body defining an orifice so that the stream passes through an electric field before exiting the orifice, and allowing the disrupted stream to solidify to form fibers. “Solidify” as used herein, means a marked change in viscosity or change in state such that the material tends to retain a definite shape. “Solidify” as used herein includes a change in the fluid to an elastomeric fiber, rigid or semi-rigid fibers, and solid or semi-solid fibers. 
     Preferably, the step of providing the stream with a net charge includes injecting a net charge into the stream. The step of injecting a net charge preferably includes injecting a net charge so as to develop a self electric field for the stream of at least 0.5 megavolts per meter. Charge injection of the solidifiable fluid achieves a high charge density in the fluid. Charge injection creates a strong “self-field” within and in proximity to the fluid stream, and the fluid stream forms fibers under the influence of the self-field. 
     In certain preferred embodiments, a pair of electrodes is provided in the vicinity of the orifice while a potential difference is maintained between the electrodes. One of the pair of electrodes may comprise the body. An electric field is developed between the electrode and the body so that the stream is provided with a net charge. Charge injection occurs within the stream of fluid, in the space between the electrode and the body defining the orifice. 
     The self-field within and immediately surrounding the fluid stream causes the fluid stream to break into highly elongated filaments which solidify to form solid fibers. A further surface remote from the orifice such as a container or a collection reel may be used to collect the fibers. This surface may be at the same potential as the body defining the orifice, or may be at a different potential. However, there is no need to provide a large potential difference between this surface and the body. Typically, both the body defining the orifice and the collecting surface are grounded. 
     The limit on the flow rate of the solidifiable fluid is the size of the orifice so that throughput orders of magnitude greater than the known electrostatic methods is achieved. The improved throughput is surprising. Embodiments in accordance with the invention have achieved throughputs great enough for industrial production of fibers. 
     The method, in certain preferred embodiments, comprises heating the disrupted stream as it passes out of the orifice. The step of providing the stream with a net charge preferably provides the stream with a charge density of at least 0.5 coulombs per cubic meter. 
     The step of injecting a net charge, in certain preferred embodiments, comprises passing the stream past an electron gun located adjacent the orifice. 
     The step of providing a stream of a solidifiable fluid may comprise passing the solidifiable fluid through an orifice at a rate of at least 0.1 grams per second, in certain embodiments, or a rate of at least 0.5 grams per second, in other embodiments. The solidifiable fluid may be passed through an orifice at a rate of at least 1 gram per second. 
     The step of providing a stream of solidifiable fluid may include heating a polymeric material and the step of allowing the stream to solidify may comprise allowing the disrupted stream to cool. The step of providing a stream of a solidifiable fluid may comprise providing a polymeric material in a solvent and the step of allowing the stream to solidify may comprise allowing the solvent to evaporate. 
     The solidifiable fluid may comprise a liquid polymer, for example. In certain preferred embodiments, the liquid polymer comprises a molten polymer. 
     The solidifiable fluid may comprise a liquid glass, a liquid polyester, such as polytetrafluoroethylene, polyethylene terephthalate (“PET”), polybutylene terephthalate, or a liquid thermoplastic polyurethane. 
     The solidifiable fluid may comprise a liquid solution including a polymeric material, such as LEXAN® and methylene chloride, or tetrahydrofurane and urethane. 
     Another aspect of the present invention, is an electrostatically formed fiber produced by the providing a stream of a solidifiable fluid, providing the stream with a net charge so as to disrupt the stream by passing the stream through a body defining an orifice so that the stream passes through an electric field prior to exiting the orifice, and allowing the disrupted stream to solidify to form fibers. The fiber may be formed of a polyester, a polytetra fluoroethylene, polyethylene terephthalate, polybutylene terephthalate, thermoplastic polyurethane, carbon, or glass. The fibers preferably have a diameter of less than 100 micrometers, more preferably less than 10 micrometers. In certain preferred embodiments, the fiber has a diameter of less than 500 nanometers, preferably less than 100 nanometers, even more preferably less than 20 nanometers. 
     In another aspect of the present invention, a method of producing fibers comprises providing a plurality of streams of solidifiable fluid. Each of the plurality of streams is provided with a net charge so as to disrupt the streams by passing each stream through a structure defining an orifice so that the stream passes through an electric field prior to exiting the orifice. Each disrupted stream is allowed to solidify to form fibers. Orifices for multiple streams may be utilized in an assembly for generating fibers on a large scale. 
     In another aspect of the present invention, a method of forming a charged solid comprises providing a stream of a solidifiable fluid, providing the stream with a net charge by passing the stream through a body defining an orifice so that the stream passes through an electric field prior to exiting the orifice, and allowing the stream of solidifiable fluid to solidify while still charged. In certain preferred embodiments, the stream disrupts under the influence of the net charge. Preferably, the stream of solidifiable fluid has a maximum charge mobility of 10 −6  m 2 /V • sec. Preferably, the stream of solidifiable fluid has a minimum net charge of 0.1 coulombs per cubic meter. 
     In yet another aspect of the present invention, an apparatus for producing fibers comprises a feed system adapted to deliver a stream of molten polymeric material, and a charge injection device adapted to provide the stream with a net charge so as to disrupt the stream, said device comprising a body defining an orifice and being arranged so that the stream passes through an electric field prior to exiting the orifice. 
     The feed system preferably comprises at least one heater for melting the polymeric material. In certain preferred embodiments, the charge injection device comprises a pair of electrodes, in which one of the pair of electrodes comprises the body defining the orifice. In other embodiments, the charge injection device comprises an electron gun. 
     In another aspect of the present invention, a method of forming fibers comprises providing a stream of a solidifiable fluid at a rate of at least about 0.02 grams per second, injecting electrical charge into the stream of solidifiable fluid, whereby the stream will tend to disperse and form filaments, and solidifying the filaments. The method preferably comprises injecting electrical charge so as to inject at least about 1 coulomb per cubic meter. The method preferably comprises providing the stream of fluid at a rate of at least 0.1 gram per second and more preferably at least 1 gram per second. 
     In another aspect of the present invention, a method of forming fibers comprises providing a stream of a solidifiable fluid, injecting at least about 1 coulomb of electrical charge per cubic meter of fluid into the stream of solidifiable fluid, whereby the stream will tend to disperse and form filaments and solidifying the filaments. Preferably, the stream is provided at a rate of at least about 0.02 grams per second. 
     In another aspect of the present invention, a method of forming fibers comprises providing a stream of a solidifiable fluid at a rate of at least about 0.03 milliliters per second, injecting electrical charge into the stream of solidifiable fluid, whereby the stream will tend to disperse and form filaments, and solidifying the filaments. The method preferably comprises injecting electrical charge so as to inject at least about 1 coulomb per cubic meter into the solidifiable fluid. The method preferably comprises providing the stream of fluid at a rate of at least 0.1 gram per second and more preferably at least 1 gram per second. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanied drawing where: 
     FIG. 1 is a schematic cross-sectional view of an apparatus for performing a method in accordance with an embodiment of the present invention; 
     FIG. 2 is a schematic view of a fluid feed system for the embodiment of FIG. 1; 
     FIG. 3 is a view of a stream of fluid disrupted under the influence of a net charge; 
     FIG. 4 is a cross-sectional view of an apparatus for implementing a method in accordance with another embodiment of the present invention; 
     FIG. 5 is a schematic circuit diagram of a controller for the apparatus of FIG. 4; 
     FIG. 6 is a schematic front-right partial perspective view of the apparatus of FIGS. 4 and 5; 
     FIG. 7 is a schematic partial cross-sectional view of an apparatus for implementing the method in accordance with a further embodiment of the present invention; 
     FIG. 8 is a front-left partial perspective view of the apparatus of FIG. 7; 
     FIG. 9 is a partial front elevational view of the apparatus of FIGS. 7 and 8; 
     FIG. 10 is a side elevational view, partially in cross-section, of an apparatus in accordance with another embodiment of present invention; 
     FIG. 11 is a cross-sectional view taken along line  11 — 11  in FIG. 10; 
     FIG. 12 is a side-elevational view, partially in section, of an apparatus in accordance with a further embodiment of the present invention; 
     FIG. 13 is a cross-sectional view taken along line  13 — 13  in FIG. 12; 
     FIG. 14 is a graph illustrating the electrode to body current vs. operating voltage of the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     An apparatus for performing a method in accordance with an embodiment of the invention comprises a dispersing apparatus  10 , as shown in FIG.  1 . An electrically conductive metallic body  11  with a central axis  14  has a liquid supply line  19  formed therein and opens into a central chamber  12 . The body  11  shown in FIG. 1 has a generally cylindrical shape. A shape including as few corners as possible is preferred. However, the shape of the body  11  is not essential. The body  11  defines a first end  13  and a second end  15  opposite from first end  13  for the apparatus  10 . The body  11  defines a forward wall  16  at the first end  13  of the apparatus. The forward wall  16  has an orifice  22  opening therethrough on central axis  14 . An electrically insulating support  38  is disposed within the central chamber  12  of the body  11 . Insulator  38  is generally cylindrical and coaxial with the body  11 . The insulator defines a plurality of liquid distribution channels  44  extending generally radially and a set of axially extensive grooves  49  adjacent the outer periphery of the insulator. Radial channels  44  merge with one another adjacent the central axis  14  and merge with the grooves  49 . Further, the radial channels  44  and axial grooves  49  communicate with the supply line  19  of body  11 , so that the supply line is in communication, via the radial channels  44 , with all the axial grooves  49  around the periphery of insulator  38 . A fluid source  37  delivers a fluid to supply line  19  so that the fluid flows through channels  44  and grooves  49  to the chamber  12 . Insulator  38  may be formed of any substantially rigid dielectric material, such as a glass, non-glass ceramic, thermoplastic polymer or thermosetting polymer. 
     A charge injection device  21  comprises a central electrode  25 . A central electrode  25  is mounted within insulator  38  and electrically insulated from the body  11  by insulator  38 . Central electrode  25  has a pointed forward end  42  having a tip  40  disposed in alignment with orifice  22  and in close proximity thereto. Preferably, the forward tip  40  of central electrode  25  is formed from a setaceous element having numerous small points  43 . For example, the setaceous element may be formed from ytrria stabilized zirconia-tungsten eutectic. Alternatively, the electrode may comprise a metal rod. A ground electrode  52  is mounted remote from body  11  and remote from orifice  22 . Although electrode  52  is schematically illustrated as a flat plate in FIG. 1, its geometrical form is not critical. For example, the ground electrode  52  may comprise a drum. Where the atomized liquid is directed into a vessel, pipe or other enclosure, the ground electrode  52  may be a wall of the enclosure. 
     Ground electrode  52  is at a reference or ground electrical potential. The body  11  is connected via a resistor to the ground potential  47 . Tip  40  of central electrode  25  is connected to a voltage potential source  50 . The foregoing components of the dispersing apparatus may be generally similar to the corresponding components of the apparatus called the SPRAY TRIODE® atomizer, disclosed in certain embodiments of U.S. Pat. No. 4,255,777, the disclosure of which is hereby incorporated by reference herein. 
     The solidifiable fluid may comprise any solidifiable polymer in a liquid form, such as a liquid polymer or a liquid solution including a polymeric material. In certain preferred embodiments, the fluid comprises a molten polymer such as polyethylene terephthalate (“PET”). The molten PET is supplied from fluid source  37 , which may comprise a feed system, such as the feed system  37  shown in FIG.  2 . The feed system of FIG. 2 is a laboratory apparatus. For commercial applications, a commercially available extruder for melting the PET and supplying the same under pressure is used. For example, a screw type extruder, which melts polymeric material at least in part under the influence of friction within the extruder, may be used. These extruders are well known in the art. 
     The feed system  37  includes a reservoir  41  in which PET in a granular form is placed. The reservoir has a first end  45  and a second end  46  opposite from the first end  45 . The dispersing apparatus  10  is attached to the reservoir  41  at the first end  45  through a coupling  48 . Preferably, a plurality of heaters  51  is used to melt the granular PET. As shown in FIG. 2, a band heater  51   a  is located at the coupling  48  between the reservoir  41  and dispersing apparatus  10 . Heaters are also preferably located on the dispersing apparatus  10 . A band heater  51   b  is located on the apparatus  10 , at the first end  13  of the apparatus  10 . A band heater  51   c  is also located on the dispersing apparatus  10 , at the second end  15  of the apparatus  10 . 
     The reservoir  41  is preferably heated interiorly and exteriorly. The reservoir  41  includes a rope heater  51   d  located closer to the second end  46  than band heaters  51   e,  which are located closer to the first end  45  of the reservoir  41 . A heater is also preferably disposed within the reservoir, such as rod heater  51   f,  which is mounted on the second end  46  by a thermocouple  54 . 
     The heaters heat the granulated PET contained in the reservoir  41  to the operating temperature for melting the PET. The temperature for melting the PET is between about 290° C. and 295° C. 
     For example, the particular feed system  37  shown in FIG. 2 has a maximum operating temperature of 310° C. The reservoir  41  shown in FIG. 2 is a 1 liter reservoir. The heaters, by way of example, may be: a 150 Watt band heater for band heater  51   a;  a 100 Watt band heater for band heater  51   b;  a 100 Watt band heater for band heater  51   c;  a 500 Watt rope heater for rope heater  51   d;  650 Watt band heaters for band heaters  51   e;  and a 600 Watt rod heater for rod heater  51   f.  A sufficient amount of heat must be generated to melt enough PET for operation. For example, heaters  51   a-f  generate enough heat to melt several hundred grams of PET. The reservoir must have sufficient capacity for storing the molten PET. Preferably, the temperature is monitored at several points manually or, more preferably, automatically monitored. For example, the temperature may be monitored at points Ta, Tb, Td, Ti, To, Tm, Tr, Tt, and Tu shown in FIG. 2 to ensure that the temperature at those points does not exceed the maximum temperature for the components of the system. 
     The feed system  37  includes an assembly  60  for supplying pressure to the reservoir  41 . The assembly  60  is attached to thermocouple  54  and supplies a pressurized gas, such as air to the reservoir  41 . The pressure supplied to the reservoir provides a flow of molten PET through the apparatus  10 . The assembly  60  has a first end  58  attached to a supply of pressurized gas and a second end  56  which leads to a vacuum or vent. The actual pressure required to supply a flow of molten PET depends upon the viscosity of the particular PET material utilized. 
     In embodiments using molten polymer, apparatus  10  is preferably designed to accommodate the heat of the molten polymer. By way of example, the atomizer disclosed in certain embodiments of U.S. Pat. No. 4,255,777, the disclosure of which is hereby incorporated by reference herein, may be mounted in a stainless steel ½″ tee, modified to accommodate the atomizer. Such a device withstands pressures over 40 bar while being exposed to temperatures of 325° C. and up. 
     In operation, the molten PET is supplied through supply line  19  of the apparatus  10 , flowing through the radial channels  44  and axial grooves  49  within the body  11 . The PET flows to chamber  12  through the grooves  49  on either side of the electrode  25 . As the PET flows towards orifice  22  in a stream, the PET flows past the tip  40  of the electrode  25 . The voltage source  50  is operated to develop a charge on the tip  40  of the electrode  25 . An electric field is developed between the electrode  25  and the body  11 . The PET flows through the electric field between the electrode  25  and body  11  prior to exiting through the orifice  22 . As the PET flows through the electric field, a charge is injected into the PET stream, providing the stream with a net charge. 
     Various portions of the stream of charged fluid repel each other under the influence of the net charge. The stream is disrupted under the influence of the net charge and begins to disperse, as shown in FIG.  3 . At the same time, the molten PET cools and begins to solidify. Although the invention is not limited to any theory of operation, it is believed that the stream  62  issuing from the orifice  22  in a longitudinal direction  64  begins to disperse into elongated filaments  66  extending outwardly from the stream  62 . Filaments  66  are developed at intervals along the stream  62 . It is believed that these elongated filaments  66  of PET solidify into fibers as the molten PET cools. The fibers collect in the space outside the orifice  22  and may be directed toward electrode  52 , in circumstances where the fibers retain a charge. 
     A throughput orders of magnitude greater than the known electrostatic methods discussed above is achieved for liquid polymers. By utilizing orifices having different sizes and varying the pressure of the solidifiable fluid, the throughput flow rate can be increased. The improved throughput is surprising in that prior art electrostatic methods of generating fibers have been unsuccessful in producing fibers on a large scale. Utilizing the above-disclosed method, fibers of PET have been produced at flow rates on the order of 1 gram per second through a single orifice. 
     Embodiments in accordance with the invention have achieved throughputs great enough for industrial production of fibers for use in non-woven materials, fabrics, filtration materials, agricultural applications and materials used in medical fields. 
     The solidifiable fluid may comprise virtually any solidifiable fluid with a conductivity and/or charge mobility low enough that the charge injection process does not short out. In other words, if the charge travels though the fluid to the body of the apparatus prior to exiting the orifice of the apparatus, the stream of fluid will not receive a net charge and will not disrupt into filaments  66  (see FIG. 3) under the influence of net charge. If the fluid conductivity exceeds a conductivity on the order of 10 4  cu and/or charge mobility exceeds 10 −6  m 2 /V • sec, the fluid is inappropriate for using with the apparatus  10 . When textile grade standard IV 0.640 molten PET was utilized in a device as shown in FIGS. 1 and 2, the current from the electrode to the body of the device, plotted against the input voltage is as shown in FIG.  14 . 
     Fibers may be formed from any solidifiable material. For example, a ceramic and binder material may be used to form fibers in methods according to embodiments of the present invention. Metals, for example, may also be used to form fibers in other methods according to embodiments of the present invention. Another example is forming fibers from liquid flowing glass. Solidifiable fluids for forming fibers in methods according to the present invention include molten polymers and polymeric materials in a liquid solution. For example, the following solutions may be used: Tetra Hydrofurane and Urethane and LEXAN® and methylene chloride. Methods in accordance with embodiments of the invention may be used to form rigid or semi-rigid fibers. Fibers may be formed by solidifying the stream of solidifiable liquid into fibers of a solid or semi-solid material. 
     Fibers may be formed from any polymeric material. Just by way of example, fibers may be formed from polyesters, including: the polytetra fluoroethylene material known as TEFLON®; polyethylene terephthalate (PET); polybutylene terephthalate; polycarbonates such as LEXAN®; thermoplastic polyurethanes such as the materials known as PELLETHANE® or ESTANE®, Nylon, and a number of others. By manipulating the properties of the liquid polymer, or selection of the type of liquid polymer, fibers can be produced having virtually any strength and can be used as reinforcement of materials. 
     Direct charge injection for producing fibers may be achieved utilizing the charge injection devices described in certain embodiments of U.S. Pat. Nos. 4,255,777, 4,991,774, 5,093,602, 5,378,957, 5,391,958, and 5,478,266, the disclosures of which are hereby incorporated by reference herein. Certain preferred embodiments of the present invention include charge injection devices having features disclosed in certain embodiments of U.S. Pat. Nos. 6,161,785, 6,206,307, 6,227,465 and 6,474,573, the disclosures of which are all hereby incorporated by reference herein. 
     In electrostatic atomizers, corona induced breakdown in the vicinity of the exiting charged stream has been experienced. When a critical level of charge is reached, corona-induced breakdown occurs and the plume of atomized fluid collapses. Should it be necessary or desirable to reduce the occurrence of this phenomenon in the dispersing apparatus, the dispersing apparatus  110  may be provided with a control-feedback system as shown in FIGS. 4-6 and as disclosed in certain embodiments of U.S. patent application Ser. No. 09/430,633, filed Oct. 29, 1999, the disclosure of which is hereby incorporated by reference herein. Alternatively, the pulsing atomizer of certain embodiments of U.S. patent application Ser. No. 09/430,632, filed Oct. 29, 1999, the disclosure of which is hereby incorporated by reference herein, may be used to address corona induced breakdown. 
     The embodiment of the invention shown in FIGS. 4-6 has a dispersing apparatus  110  with a body  111  defining an orifice  122 . A voltage potential source  150  is connected to a central electrode  125  and a fluid source  137  supplies a fluid to the passages within the body  111 . These elements are substantially as discussed above in connection with FIGS. 1 and 2 and similar elements in FIGS. 1 and 4 have similar reference numerals. 
     The dispersing apparatus  110  includes a sensor comprising a loop antenna  170 . The antenna, for example, may be comprised of a 0.5-millimeter diameter insulated wire in the shape of an open loop curving around the orifice  122  of the apparatus  110 . Power source  150  comprises a high voltage power source including a controller  180  and DC—DC converter  162  shown in FIG.  5 . The controller  180  comprises a circuit having a central processing unit (“CPU”)  163  connected to a dual digital resistor  164 . Resistor  164  is connected to an analog switch  181 , which is in turn connected to an amplifier  182 . Amplifier  182  is connected to the DC—DC converter. A transistor  185  is connected to the switch  181  and CPU  163 . The circuit includes another amplifier  183 , to which the antenna  170  is connected. Amplifiers  182  and  183  may be included in one component, in other embodiments. There are many components known to those of ordinary skill in the art that can be utilized in the circuit shown in FIG.  5 . The controller  180  is operated to vary the operating voltage for the dispersing apparatus  110 , supplied by the voltage source  150 . The antenna  170  detects signals and the components of the controller  180  control the operating voltage of the voltage source  150  to avoid corona-induced breakdown as disclosed in U.S. Pat. No. 6,206,307. 
     The orifice may, in certain preferred embodiments, be provided with a fixture  200  for varying the size of the orifice. As shown in FIG. 7, an apparatus  210 , which is generally similar to the apparatus  10  as shown in FIGS. 1 and 2, includes the fixture  200  mounted on first end  213 . Fixture  200  comprises a generally cylindrical sleeve  220  having a wall  221  which partially covers forward wall  216  of the apparatus  210 . The wall  221  has a curvilinear edge  223  which joins with the sleeve  220 . Wall  221  ends in the substantially linear edge  224  interrupted by a circular cutout  225 . The cutout  225  is positioned along the wall  224  so that orifice  222  of the apparatus  210  may be exposed and unobstructed by the wall  221 . The fixture  200  has an initial position, as shown in FIG. 8, in which the orifice  222  is exposed in cutout  225 . Fixture  200  is rotatably mounted on the apparatus  210  and rotatable in a direction  226  so as to move wall  221  over the orifice  222 , as shown in FIG.  9 . In this manner, the orifice  222  may be partially obstructed by wall  221 , thereby diminishing the effective size of the orifice  222 . If the size of the orifice  222  should be changed to change the flow rate of the fluid during operation, the fixture  200  is rotated to vary the size of the orifice  222 . In addition, it may be desirable to change the size of the orifice between operations of the apparatus  210 . For example, the apparatus  210  may be operated with a solidifiable fluid having a first viscosity. The size of the orifice may be changed to operate the same apparatus  210  with a solidifiable fluid having a second viscosity to achieve the same throughput as achieved for the fluid having the first viscosity. In another example, the apparatus  210  may be operated with the orifice  222  partially obstructed by wall  221  of the fixture  200 . In order to flush the orifice  222  of any debris or clogs, the fixture  200  may be rotated in a direction opposite to direction  226  to fully expose the orifice  222  and power to the central electrode may be turned off so that uncharged fluid issues from orifice  222 . In this manner, the orifice  222  may be flushed of debris. The variable orifice disclosed in certain embodiments of U.S. Pat. No. 6,161,785, the disclosure of which is hereby incorporated by reference herein, may also utilized with a dispersing apparatus as discussed in the embodiments above. 
     Certain embodiments disclosed in U.S. Pat. No. 6,474,573, the disclosure of which is hereby incorporated by reference herein, provide multiple orifices in a single nozzle referred to as the SPRITZ CHIP device. Similar structures can be used to provide multiple fluid streams for fibers formation. For example, such an embodiment is shown in FIGS. 10 and 11. A dispersing apparatus includes a body  320  having a first wall  324  and a second wall  325  generally parallel to the first wall but spaced therefrom. The first wall  324  defines a plurality of discharge orifices  326 . The first wall  324  may be formed from a conductive material or from a dielectric material such as silicon dioxide. Where the first wall  324  comprises a dielectric material, an external electrode  350  common to all the orifices  326  is formed on an exterior surface  328  of the first wall  324  by depositing a coating of an electrically conductive material such as a metal on this surface. First wall  324  and second wall  325  are held apart from one another by an insulating internal structure  321 , which may comprise a plurality of walls subdividing the space between the walls into a large number of hexagonal chambers or internal spaces  322 . Hexagonal spaces  322  are disposed on center with orifices  326 , so that each orifice is aligned with the center of one hexagonal space. Emitter electrodes  344  are mounted to second wall  325  and are in alignment with orifices  326 . Second wall  325  may be comprised of an insulative material, or incorporates a dielectric layer  327  and a conductive layer  323  electrically connected to all of the emitter electrodes  344 . The second wall  325  has a large number of fluid passages  330  extending through it. These orifices form a filter for filtering the solidifiable fluid to be utilized in forming fibers. The relative size of the passages  330  depends upon the particular solidifiable fluid utilized and the fluid viscosity. The size of the passages  330  is exaggerated in FIGS. 10 and 11 for quality of illustration. 
     Dispersing apparatus according to this embodiment of the invention can be fabricated using micro-mechanical fabrication techniques, similar to the techniques used for forming semiconductor chips and related devices. Photo-etching techniques, plating, vacuum deposition or other conventional techniques used in semiconductor fabrication may be used. The emitter electrodes can be formed by etching and/or deposition on the same mass of material used to form the second wall  325 . For example, tungsten emitters can be formed by sputtering, by vapor deposition or by chemical vapor deposition. In a variant of this technique, the internal structure  321  can be fabricated together with the second wall  325  so that the internal structure is integral with the second wall. Also, although the internal structure is shown as completely dividing the space between walls  324  and wall  325  into entirely separate spaces  322 , these spaces may communicate with one another. 
     In another embodiment of the invention, the spaces  422  are open to the passages for delivery of the solidifiable fluid. (See FIGS.  12  and  13 ). Thus, second wall  425  does not include holes for filtering the solidifiable fluid. The remaining features of this embodiment are generally similar to those of FIGS. 10 and 11 and similar features of FIGS. 12 and 13 have reference numerals similar to FIGS. 10 and 11. 
     The devices shown in FIGS. 10 through 13 are used in a manner similar to the device discussed above with reference to FIGS. 1 and 2. For example, the electrode  344  is connected to a high voltage terminal of a power supply, whereas the second electrode  350  is connected to a lower potential, preferably by connecting the second electrode to ground. A third, grounded electrode (not shown), is provided remote from the device. The solidifiable fluid is delivered to the hexagonal space  322  through the fluid entry holes  330  and passes out through discharge orifices  326 . Here again, the electric field between electrode  344  and the external electrode  350  causes injection of electrical charge into the fluid passing downstream into discharge orifices  326 . The injected electrical charge causes dispersement of the fluid and the formation of fibers. 
     The devices shown in FIGS. 10 through 13 can be fabricated in any size, and the size of the orifices, hexagonal spaces and the distance between first wall and second wall depend upon the solidifiable fluid utilized. 
     The use of multiple orifices in the device provides several significant advantages. First, plugging or other problems affecting one orifice will not cause complete failure of the device. Also, any number of orifices can be used to provide a device with greater or lesser flow capability without altering the other operating characteristics of the device. A multi-orifice device can be utilized to produce fibers on a large, industrial or commercial scale. 
     Charged injection to form fibers in accordance with the invention can also be accomplished using an electron beam in proximity to an orifice so that electrons in the beam impinge on the fluid, either as it issues from the orifice, or just before the stream passes through the orifice. Electron beam devices previously used for atomization of liquids are disclosed in U.S. Pat. Nos. 5,378,957, 5,093,602, 5,391,958, the disclosures of which are hereby incorporated by reference herein and copies of which are annexed hereto. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the invention as defined by the appended claims. 
     EXAMPLES 
     In the following examples, two types of PET were utilized in the apparatus of FIGS. 1 and 2. The diameter of the orifice was 406 micrometers and standard IV 0.640 PET was fed through the apparatus. The size of the orifice affects the operating pressure required for a given throughput of the fluid, as well as the attainable charged density. Larger orifice diameters reduce the operating pressure and achieve a lower charge density. In the 406 micrometer diameter apparatus, the maximum charge density achieved in the stream of PET is about 62% of the maximum charge density achieved in a 250-micrometer diameter orifice apparatus. The reservoir of the apparatus was pressurized to 19 bar (275 PSI). The flow rate of the molten PET through the 406 micrometer diameter orifice was 0.8 grams per second. The volumetric flow rate of the PET was about 0.57 milliliters per second. 
     The charge density of the fluid issuing from the orifice of the dispersing apparatus varies across the diameter of the orifice. The outer portions of the stream of fluid are highly charged, as compared with the central portion of the stream. The mean charge density for the 406 micrometer apparatus was 0.88 coulombs/m 3 . 
     An operating voltage of only 2.7 kilovolts was required to charge the molten PET sufficiently to develop fibers. This is a surprising feature of the fiber development process. When the apparatus is utilized with Mil-C-7024 type II calibrating fluid, 5-6 kilovolts is required to disrupt the stream of calibrating fluid. 
     The fibers generated in the 406 micrometer diameter apparatus were generally smooth and tapered. A small fraction of fibers were branched and included junction points between fibers. Many of the fibers were hollow. It is believed that the hollow fibers resulted from bubbles trapped in the molten PET extend during the fiber generation process. Many of the textile grade PET fibers had diameters of 100 micrometers or more. 
     The 406 micrometer diameter apparatus was utilized with standard IV 0.589 PET. This PET is less viscous than the textile grade PET discussed above. The textile grade PET has a viscosity of 1845 poise at 295° C. and the less viscous PET has a viscosity of 1180 poise at 295° C. 
     The feed system was operated at the same pressure. The fibers produced had diameters below 100 micrometers and many had diameters of 10 micrometers or less. Relatively large droplets of about 700 micrometers in diameter were attached to the fibers. It is believed that the textile grade PET did not produce such droplets because the textile grade PET cooled before droplets were formed. As seen in FIG. 3, for example, the stream of fluid issuing from the orifice disrupts into elongated filaments which may eventually form droplets. On the other hand, the branching produced in the textile grade PET indicates that the stream of PET cooled prior to formation of independent fibers. Thus, controlled heating of the zone outside the orifice, in which fiber generation occurs, may be utilized to enhance the production of fibers. The PET fibers retained a charge after being formed. The finer fibers retained a higher charge and were attracted the ground electrode spaced from the orifice. 
     Methods according to embodiments of the present invention inject a net charge into the solidifiable fluid and the charge is trapped within the fiber after the fluid solidifies. The charged fibers can be later used as, for example, material for an electrostatic filter. 
     The PET fibers had diameters of 10 micrometers or less. Much smaller fibers may be produced utilizing methods and apparatus in accordance with embodiments of the invention. In another example, an apparatus as shown in FIGS. 1 and 2 was utilized to form fibers from a thermoplastic polyurethane known as PELLETHANE®, provided in a solution with tetra hydrofurane. The fibers produced ranged in diameter from about 20 nanometers to about 500 nanometers.