Patent Publication Number: US-9428847-B2

Title: Apparatus, methods, and fluid compositions for electrostatically-driven solvent ejection or particle formation

Description:
BENEFIT CLAIMS TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional App. No. 61/349,832 entitled “Apparatus, methods, and fluid compositions for electrostatically-driven solvent ejection or particle formation” filed May 29, 2010 in the names of Ashley S. Scott, Evan E. Koslow, Andrew L. Washington, Jr., John A. Robertson, Adria F. Lotus, Jocelyn J. Tindale, Tatiana Lazareva, and Michael J. Bishop, said provisional application being hereby incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND 
     The field of the present invention relates to electrostatically-driven solvent ejection or particle formation. In particular, apparatus, methods, and reduced-conductivity fluid compositions are disclosed herein for electrostatically-driven (ESD) solvent ejection (e.g., spraying or atomization) or particle formation (e.g., formation of particles or fibers, including nanoparticles or nanofibers). 
     Nanostatics Corporation and GABAE Industries Corporation are parties to a joint research agreement that was in effect before the date the invention claimed herein was made. The invention claimed herein was made on behalf of Nanostatics Corporation as a result of activities undertaken within the scope of the joint research agreement. 
     The subject matter disclosed herein may be related to subject matter disclosed in co-owned: (i) U.S. non-provisional App. No. 11/634,012 entitled “Electrospraying/electrospinning array utilizing a replacement array of individual tip flow restriction” filed Dec. 5, 2006 (now Pat. No. 7,629,030); (ii) U.S. provisional App. No. 61/161,498 entitled “Electrospinning Cationic Polymers and Method” filed Mar. 19, 2009; (iii) U.S. provisional App. No. 61/256,873 entitled “Electrospinning with reduced current or using fluid of reduced conductivity” filed Oct. 30, 2009; and (iv) U.S. non-provisional App. No. 12/728,070 entitled “Fluid formulations for electric-field-driven spinning of fibers” filed Mar. 19, 2010 (now Pat. No. 8,518,319). Each of said provisional and non-provisional applications is hereby incorporated by reference as if fully set forth herein. Each of said applications was made on behalf of, and is owned by, Nanostatics Corporation. 
     “Electrospinning” and “electrospraying” conventionally refer to the production of, respectively, fibers or droplets, which may be “spun” as fibers or “sprayed” as droplets by applying high electrostatic fields to one or more fluid-filled spraying or spinning tips (i.e., emitters or spinnerets). Under suitable conditions and with suitable fluids, so-called nanofibers or nanodroplets can be formed from a Taylor cone that forms at each tip (although the terms are also applied to production of larger droplets or fibers). The high electrostatic field typically (at least when using a conventional, relatively conductive fluid) produces the Taylor cone at each tip opening from which fibers or droplets are emitted, the cone having a characteristic full angle of about 98.6°. The sprayed droplets or spun fibers are typically collected on a target substrate typically positioned several tens of centimeters away; solvent evaporation from the droplets or fibers during transit to the target typically plays a significant role in the formation of the droplets or fibers by conventional electrospinning and electrospraying. A high voltage supply provides an electrostatic potential difference (and hence the electrostatic field) between the spinning tip (usually at high voltage, either positive or negative) and the target substrate (usually grounded). A number of reviews of electrospinning have been published, including (i) Huang et al, “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,”  Composites Science and Technology , Vol. 63, pp. 2223-2253 (2003), (ii) Li et al, “Electrospinning of nanofibers: reinventing the wheel?”,  Advanced Materials , Vol. 16, pp. 1151-1170 (2004), (iii) Subbiath et al, “Electrospinning of nanofibers,”  Journal of Applied Polymer Science , Vol. 96, pp. 557-569 (2005), and (iv) Bailey,  Electrostatic Spraying of Liquids  (John Wiley &amp; Sons, New York, 1988). Details of conventional electrospinning materials and methods can be found in the preceding references and various other works cited therein, and need not be repeated here. 
     Conventional fluids for electrospinning (melts, solutions, colloids, suspensions, or mixtures, including many listed in the preceding references) typically possess significant fluid conductivity (e.g., ionic conductivity in a polar solvent, or a conducting polymer). Fluids conventionally deemed suitable for electrospinning have conductivity typically between 100 μS/cm and about 1 S/cm (Filatov et al;  Electrospinning of Micro - and Nanofibers ; Begell House, Inc; New York; 2007; p 6). It has been observed that electrospinning of nanometer-scale fibers using conventional fluids typically requires conductivity of about 1 mS/cm or more; lower conductivity typically yields micron-scale fibers. In addition, conventional methods of electrospinning typically include a syringe pump or other driver/controller of the flow of fluid to the spinning tip or emitter, and a conduction path between one pole of the high voltage supply (typically the high voltage pole) and the fluid to be spun. Such arrangements are shown, for example, in U.S. Pat. Pub. No. 2005/0224998 (hereafter, the &#39;998 publication), which is incorporated by reference as if fully set forth herein. In FIG. 1 of the &#39;998 publication is shown an electrospinning arrangement in which high voltage is applied directly to a conductive emitter (e.g., a spinning tip or nozzle), thereby establishing a conduction path between the high voltage supply and the fluid being spun. In FIGS. 2, 5, 6A, and 6B of the &#39;998 publication are shown various electrospinning arrangements in which an electrode is placed within a chamber containing the fluid to be spun, thereby establishing a conduction path between one pole of the high voltage supply and the fluid. The chamber communicates with a plurality of spinning tips. In any of those arrangements, significant current (typically greater than 0.3 μA per spinning tip, often greater than 1 μA/tip) flows along with the spun polymer material. Conventional electrospinning fluids are deposited on metal target substrates so that current carried by the deposited material can flow out of the substrate (either to a common ground or back to the other pole of the high voltage supply), thereby “completing the circuit” and avoiding charge buildup on the target substrate. Even so, flow rates for electrospinning of conventional fluids are typically limited to a few μL/min/nozzle, particularly if nanofibers are desired (increasing the flow rate tends to increase the average diameter of fibers spun from conventional electrospinning fluids). Electrospinning onto nonconductive or insulating substrates has proven problematic due to charge buildup on the insulating substrate that eventually suppresses the electrospinning process. Application of electric fields greater than a few kV/cm to conventional fluids or to metal spinning tips often leads to arcing between the tip and the target substrate, typically precluding useful electrospinning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically an exemplary apparatus for electrostatically-driven (ESD) solvent ejection or particle formation. 
         FIGS. 2A and 2B  illustrate schematically an exemplary multi-nozzle head for ESD solvent ejection or particle formation. 
         FIG. 3  illustrates schematically multiple fluid jets ejected during ESD solvent ejection and particle formation. 
         FIG. 4  illustrates schematically a single fluid jet ejected during conventional Taylor cone electrospinning. 
         FIG. 5A  illustrates schematically another exemplary apparatus for ESD solvent ejection or particle formation. 
         FIG. 5B  illustrates schematically another exemplary apparatus for ESD solvent ejection or particle formation. 
         FIG. 6  illustrates schematically another exemplary apparatus for ESD solvent ejection or particle formation. 
         FIG. 7  illustrates schematically another exemplary apparatus for ESD solvent ejection or particle formation. 
         FIG. 8  illustrates schematically another exemplary apparatus for ESD solvent ejection or particle formation. 
         FIG. 9  illustrates schematically an exemplary external electrode for ESD solvent ejection or particle formation. 
         FIG. 10  illustrates schematically multiple fluid jets and solvent droplets ejected during ESD solvent ejection without particle formation. 
     
    
    
     The embodiments shown in the Figures are exemplary, and should not be construed as limiting the scope of the present disclosure or appended claims. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Conventional electrospinning of polymer-containing fibers or nanofibers, or electrospraying of small droplets, can be employed to produce a variety of useful materials. However, scaling up (beyond the laboratory or prototype level) an electrospinning process that employs conventional, relatively conductive fluid compositions has proven to be problematic. To achieve production-type quantities, multiple electrospinning tips are often employed, usually in an arrayed arrangement. However, the conductive fluids used and the significant current (often greater than 1 μA per tip) carried by fibers emerging from each tip lead to impractically large overall current and to undesirable electrostatic interactions among the electrospinning tips and fibers; these limit the number and density of electrospinning tips that can be successfully employed. Similar difficulties are typically encountered when electrospinning from a porous membrane emitter. Electrospinning onto non-conductive target surfaces is also problematic, as noted above. 
     Apparatus, methods, and fluid compositions are disclosed herein for electrostatically-driven (ESD) solvent ejection (e.g., spraying or atomization) or particle formation (e.g., formation of particles or fibers, including nanoparticles or nanofibers) by physical mechanism(s) distinct from conventional, evaporative electrospraying or electrospinning of conductive fluids from a single Taylor cone formed at an emitter orifice. The methods disclosed or claimed herein can be readily scaled up to production-scale quantities of material produced. The fluid compositions are emitted from electrically-insulating emitters (e.g., nozzles, capillaries, or tips) toward a target surface that is nonconductive or electrically isolated, and which need not be connected to a ground or voltage supply or positioned near any electrical ground (although the presence of an electrical ground plane behind or beneath an insulating target can help to direct particles toward the target once they form). Voltage can be, but need not be, applied directly to the fluid. Some of the fluid compositions disclosed herein exhibit substantially reduced conductivity (less than about 1 mS/cm, preferably less than about 100 μS/cm; some compositions less than about 50 μS/cm, less than about 30 μS/cm, or less than about 20 μS/cm) relative to conventional electrospinning fluid compositions (greater than about 100 μS/cm; typically greater than about 1 mS/cm for producing polymer nanofibers). 
     Some of the disclosed compositions comprise a first material having a dielectric constant greater than about 25 mixed into a liquid solvent having a dielectric constant less than about 15; in some disclosed examples the dielectric constant of the liquid solvent is less than about 10, or less than about 5. Some of the disclosed compositions include a salt, a surfactant (ionic or nonionic), or a dissolved ionic liquid. The nonconductive emitters, nonconductive or isolated target surface, and/or the reduced conductivity of some of the fluid compositions disclosed herein can at least partly mitigate the undesirable electrostatic interactions described above, can enable flow rates greater than about 100 μL/min/emitter, can enable use of multiple emitters spaced within, e.g., one centimeter or less of one another, can enable deposition of particles or fibers onto an electrically insulating or electrically isolated collection surface, or can enable formation and deposition of particles in the absence of a counter-electrode near the collection surface that is grounded or connected to the voltage supply driving the deposition. 
     Those reduced conductivity fluid compositions, and use of electrically insulating emitters and collection surface, can also enable use of higher voltages and/or smaller emitter-to-target distances (e.g., from just a few centimeters down to about 5 millimeters), which typically would result in arcing in a conventional electrospinning arrangement using conventional fluids. Emitter-to-target distances of about 5-20 cm are typically required in conventional electrospinning arrangements: close enough to enable application of sufficiently large electric fields without applying voltage high enough to cause arcing, but far enough to enable adequate evaporation of solvent from the spun fibers before they reach the target. Seemingly paradoxically, the compositions disclosed herein can also be employed in an arrangement wherein the target or collection surface is more than about 30 cm, or even 40 or 50 cm or more, from the emitter. Emission of the fluid composition into such an large, unimpeded volume appears to enhance the flow rate of the fluid and production rate of spun fibers (described further below). 
     Under conditions disclosed herein, and using fluid formulations disclosed herein, conventional Taylor cone formation, and conventional electrospinning or electrospraying from that Taylor cone, appear to be suppressed in favor of a different, non-evaporative mechanism for solvent ejection and particle formation from the fluid composition after it exits the emitter (fibers and nanofibers being considered elongated particles). Therefore, the term “electrostatically-driven (ESD) solvent ejection and particle formation,” or simply “ESD solvent ejection,” shall be employed to describe the observed phenomena disclosed herein and shall be considered distinct from conventional electrospinning or electrospraying. 
     Exemplary apparatus are illustrated schematically in the drawings, each comprising a nozzle  102  (the emitter) with an orifice  104  at its distal end, into which is introduced a fluid composition (described further below). Although nozzles  102  are shown and described in the exemplary embodiments, any suitable emitter can be equivalently employed. The nozzle  102  is supported by an insulating stand  106  or other suitable structure that electrically isolates the nozzle from its surroundings, and the nozzle  102  itself comprises one or more electrically insulating materials such as glass, plastic, polytetrafluoroethylene (PTFE), nylon, or other suitable insulating material that is also chemically compatible with the fluid composition. The nozzle  102  can act as a reservoir for the fluid composition (e.g., as in  FIG. 1 ), or can communicate with a fluid reservoir. Multiple nozzles  102  can be employed, and can each communicate with a common fluid reservoir  108 , if desired (as in  FIGS. 2A / 2 B, for example). Flow of the fluid through the nozzle  102  can be driven by gravity by arranging for a suitable fluid head above the nozzle orifice  104 , or can be driven by a pump (e.g., a syringe pump) or other flow-regulating device. The orifice  104  can be arranged to provide a suitable level of hydrodynamic resistance to flow of the fluid. In one suitable arrangement, a capillary tube (comprising, e.g., PTFE) can be inserted into the distal end of the nozzle  102  so that the distal end of the capillary tube acts as the orifice  104  and the proximal end of the capillary tube communicates with the interior of the nozzle  102  or with a fluid reservoir. In another suitable arrangement, a capillary tube acts as the entire emitter with its distal end acting as the orifice  104  (as in FIGS.  2 A/ 2 B, for example) and with its proximal end in communication with a fluid reservoir  108 . An example of a suitable capillary tube has an inner diameter of about 0.5 mm and a length of about 2 to 20 cm or more; other suitable lengths or diameters can be employed to yield desired fluid flow characteristics. Suitable length and diameter of a capillary tube can be at least partly determined by the viscosity of the fluid composition, for example, with a longer or narrower capillary typically being employed for a less viscous fluid composition. Although nozzles  102  are shown and described in the exemplary embodiments, any suitable emitter can be equivalently employed, including but not limited to fritted glass, porous ceramic, a porous polymer membrane, one or more micromachined channels in an insulating plate, or interstitial channels among a bundle of fibers, filaments, or rods. If a porous or fritted material is employed as an emitter, the corresponding orifices are formed by individual pores of the material where they reach an edge or surface of the material. 
     A wide range of fluid compositions can be employed. A first group of suitable fluid compositions include compositions comprising a first material having a dielectric constant greater than about 25 mixed into a liquid solvent having a dielectric constant less than about 15. Many examples of suitable fluid compositions are described below that exhibit at least that degree of dielectric contrast. Most of the disclosed examples of high dielectric contrast fluid compositions also include a polymer dissolved, emulsified, or otherwise dispersed in the liquid solvent. In some exemplary fluid compositions of the first group, the first material has a dielectric constant greater than about 30, or the liquid solvent has a dielectric constant less than about 10 or less than about 5; other exemplary fluid compositions having still greater dielectric contrast are disclosed and can be employed. One or more additional materials can be included in the composition, each having a dielectric constant between those of the low-dielectric liquid solvent and the high-dielectric material, forming a so-called “dielectric ladder.” A second group of exemplary fluid compositions comprise a salt, a surfactant (ionic or nonionic), or an ionic liquid dissolved or mixed into a liquid solvent, along with a dissolved, emulsified, or dispersed polymer. There can be some overlap between those first two groups of suitable fluid compositions, e.g., a salt, surfactant, or ionic liquid can act as a high dielectric material in a high contrast fluid composition, often as the “top rung” in a dielectric ladder. A third group of examples of suitable fluid compositions can comprise a polymer dissolved, emulsified, or dispersed in a liquid solvent, wherein the liquid solvent has a dielectric constant greater than about 8 and the primary dielectric contrast is between the solvent and the polymer, which has a dielectric constant less than about 4. In the third group of exemplary fluid compositions, there appears to be a positive correlation between solvent dielectric constant and maximum viscosity that permits ESD solvent ejection. Specific examples from all three groups of fluid composition types are described below. Exemplary compositions in all three groups exhibit conductivity less than about 1 mS/cm, preferably less than about 100 μS/cm. Conductivity less than about 50 μS/cm, less than about 30 μS/cm, or less than about 20 μS/cm can be advantageously employed. 
     A power supply  110  applies a voltage to the fluid composition, in the examples of  FIGS. 1, 2A / 2 B,  5 A,  5 B, and  6  through an insulated or shielded cable  112  and an electrode  114  that is immersed in the fluid composition (within the emitter  102  or within a fluid reservoir  108 ). When a suitable fluid composition is employed (e.g., having sufficiently large dielectric contrast and/or sufficiently low conductivity), applying sufficient voltage causes non-evaporative ejection of the solvent from the fluid composition after the fluid exits the emitter  102  through the orifice  104  (i.e., ESD solvent ejection). High-speed photography reveals that, upon application of sufficient voltage via immersed electrode  114 , the fluid composition that exits the emitter  102  through orifice  104  forms one or more discrete fluid jets  342 . Each of those jets rapidly becomes unstable and breaks up within about 2 to 3 mm from its corresponding point of formation (illustrated schematically in  FIG. 3 ). Those jets  342  emerge from a portion of the meniscus  344  of the fluid that does not appear to form a typical Taylor cone (at least not one that is visibly protruding from the nozzle orifice  104 ), in contrast with a fluid jet emerging from a conventional, conductive electrospinning fluid (illustrated schematically in  FIG. 4 , with jet  442  emerging from a Taylor cone  444  formed at and visibly protruding from the orifice  404  of an emitter  402 ). While it may be possible for both types of fluid jets (ESD ejection and conventional Taylor cone electrospinning) to emerge from the fluid composition when voltage is applied, use of a fluid composition of one of the types disclosed herein, in an apparatus arranged and operated as disclosed herein, appears to favor production of fluid jets  342  that behave substantially as shown in  FIG. 3 , and to suppress production of a fluid jet  442  that emerges from a corresponding Taylor cone and behaves substantially as shown in  FIG. 4 . 
     As illustrated schematically in  FIG. 3 , in ESD solvent ejection each of the fluid jets  342  typically (but not always) emerges at an angle with respect to the emitter  102 . The jets  342  can vary, somewhat stochastically, in number and direction, sometimes forming an arrangement that resembles the ribs of an open umbrella. High-speed photography reveals that each fluid jet  342  abruptly breaks up and ejects solvent within about 2 to 3 mm of its corresponding point of formation. The solvent appears to be ejected in a direction substantially transverse to the emitter, and the ejection appears to be non-evaporative. The ejected solvent can subsequently evaporate, but appears to be ejected from the jet  342  initially as droplets  346 . 
     The jet behavior depicted schematically in  FIG. 3  has been observed previously (Eda et al; “Solvent effects on jet evolution during electrospinning of semi-dilute polystyrene solutions”; European Polymer Journal, Vol 43 p 1154 (2007)). However, previous workers failed to recognize the potential utility of that observed jet behavior. Applied electric fields were limited in previous work to less than about 4-5 kV/cm (most employed conducting emitters). By employing insulating emitters, an insulating or insulated collection surface, and relatively low-conductivity fluid compositions, larger electric fields can be employed that appears to enhance the jet behavior depicted in  FIG. 3  and to suppress the jet behavior depicted in  FIG. 4 . This preferential behavior is advantageous because of the substantially larger fluid flow rates that can be achieved, e.g., greater than about 100 μL/min/emitter for the jets of  FIG. 3 . Rates as high as 2 mL/min/emitter have been observed with fluid compositions that include polymer, and up to 10 mL/min/emitter has been observed with fluid compositions that do not include polymer. 
     If the fluid composition includes a polymer, ESD ejection of the solvent causes formation of polymer particles or fibers  348  and separation of those particles or fibers  348  from the ejected solvent. Fibers can be considered as elongated particles, and the terms “particle” and “fiber” may be used somewhat interchangeably in the subsequent discussion to encompass both fibers as well as non-elongated particles. The methods and fluid compositions disclosed herein for ESD solvent ejection and particle formation can be advantageously employed for forming polymer fibers (including polymer nanofibers, e.g., fibers having an average diameter less than about 500 nm) in larger quantities at faster rates than conventional electrospinning. In conventional electrospinning ( FIG. 4 ), the jet  442  typically remains intact over ten or more centimeters after emerging from the Taylor cone  444 . After the first several centimeters, the jet  442  begins to elongate and whip due to electrostatic interactions before being deposited on a collecting surface; however, the jet  442  typically remains intact until it is deposited. Solvent evaporates from the jet  442 , and the collecting surface typically must be located about 10 to 20 centimeters from the emitter  402  to allow sufficient solvent evaporation to leave the deposited fibers substantially devoid of solvent. 
     In contrast, in ESD solvent ejection ( FIG. 3 ) the polymer particles  348  appear in the high-speed photography to be ejected from the jets  342  in a direction substantially transverse to the emitter (e.g., substantially transverse with respect to nozzle  102 ) within about 2 to 3 mm of their corresponding points of formation, i.e., where the jets  342  break up and eject solvent. The polymer fibers  348  appear to be ejected at a substantially lower velocity than the ejected solvent droplets  346 , thereby effecting a separation. The polymer particles  348  are deposited on a collection surface  130 , as described further below. In addition to high-speed photographic evidence of an ESD solvent ejection mechanism that is non-evaporative, further evidence for such a mechanism includes the observation that polymer fibers  348 , substantially devoid of the liquid solvent, can be deposited on a collection surface  130  that is less than about 1 cm away from the emitter orifice  104  (i.e., distance d in  FIG. 1  less than about 1 cm; d≈0.5 cm has been employed), using a solvent such as, e.g., d-limonene that has a relatively high boiling point (176° C.) and a relatively low vapor pressure (2 mm Hg at 20° C.). Calculations indicate that an evaporative solvent removal mechanism could not remove such a high-boiling solvent over such a small distance. Therefore, a non-evaporative ESD solvent ejection mechanism can be inferred from the deposition of essentially solvent-free fibers with the emitter orifice  104  less than a centimeter from the collection surface  130 . 
     In the example of  FIG. 1 , polymer fibers  348  are deposited on a collection surface  130  that is positioned between the emitter orifice  104  and an electrically grounded surface  120  (typically conductive and in the example of  FIG. 1  connected via wire  122  to a common ground with power supply  110 ; can be referred to as a “counter electrode” or “ground plane”). Electrostatic interactions arising from the presence of grounded surface  120  tend to propel the polymer fibers  348  toward the collection surface  130 . However, the collection surface  130  itself need not be conductive, and preferably is insulating or only slightly conductive, to reduce the likelihood of arcing at higher applied voltage. The arrangement of  FIG. 1  can be employed to deposit polymer fibers onto a wide variety of slightly conductive or electrically insulating collection surfaces  130 , including but not limited to paper or other cellulosic material, fibrous or textile materials, polymer films such as Mylar (i.e., biaxially-oriented polyethylene terephthalate or boPET), Saran (i.e., polyvinylidene chloride), or polytetrafluoroethylene, or composite materials such as fiberglass. Although the grounded surface  120  is shown in  FIG. 1  as being larger in transverse extent than the collection surface  130 , this need not be the case. In fact, it can be advantageous to arrange the collection surface  130  to effectively block any potential charge transfer between the fluid jet and the grounded surface  120 , in effect “breaking the circuit” that would be formed by the high voltage supply  110 , the fluid, the grounded surface  120 , and common ground connection  122  (e.g., as in conventional electrospinning). When collecting polymer fibers on a slightly conductive material (e.g., cellulosic paper), fiber collection rates can be increased by interposing an impermeable, insulating layer (e.g., a Mylar sheet) between the grounded surface  120  and the collection surface  130 . The presence of grounded surface  120  preferably serves only to define the electrostatic field lines, but is not intended to carry any substantial current. 
     In the arrangement of  FIG. 1  (with a grounded surface  120  connected to a common ground  122  with the power supply  110 ), the distance d between the nozzle orifice  104  and the collection surface can be as small as about 0.5 cm or about 1 cm or can be as large as about 10-15 cm or more (provided the applied voltage is sufficiently large, e.g., greater than about 5 kV per centimeter of separation between the nozzle orifice  104  and the grounded surface  120 ). Solvent is ejected from the jets  342  within about 2-3 mm, enabling deposition of polymer fibers  348  onto collection surface  130  substantially devoid of solvent even at a distance of less than 1 cm for a single nozzle. It has been observed in a multiple nozzle arrangement, however, that solvent ejected from the jets of adjacent nozzles can be deposited along with the fibers of those nozzles, for example, when the nozzles are about 3 cm apart and the collection surface is closer than about 10 cm. Larger nozzle-to-surface distance d or higher applied voltage, optionally coupled with gas-flow-based solvent recovery (if needed or desired), can be employed to yield deposited fibers substantially devoid of solvent in a multiple nozzle arrangement. 
     In another exemplary arrangement for ESD solvent ejection, illustrated schematically in  FIG. 5A , the collection surface  130  is positioned on an electrically isolated surface  124  that acts merely as a mechanical support, with no adjacent or juxtaposed ground plane or counter electrode. The high voltage supply  110  remains grounded through ground connection  118 . The general surroundings (e.g., furnishings, other nearby equipment, walls, floor, ceiling, or the earth&#39;s surface) will typically provide some effective “ground,” typically distant enough to only negligibly affect behavior of the fluid jets  342  or polymer fibers  348 . Support surface  124  can be omitted if the collection surface  130  is sufficiently rigid to be self-supporting. When the arrangement of  FIG. 5A  is employed, the ejected polymer fibers tend to be ejected transversely from the jets  342  over a transverse distance up to about 10 or more cm in all directions and then tend to drift somewhat aimlessly. To effect deposition of the polymer fibers  348  onto the collection surface  130 , gas flow (positive or negative pressure, e.g., provided by a blower, vacuum belt, or similar device) or other standard means can be employed to propel the polymer fibers onto the collection surface  130 . Instead or in addition, gas flow can be employed to collect or recover the ejected solvent, as droplets or as vapor (as noted above). Any suitable gas can be employed, including ambient air; ionized gas can be employed and in some circumstances has been observed to enhance ESD solvent ejection by stabilizing the jets  342  and/or suppressing corona discharge from the nozzle. In the exemplary arrangement of  FIG. 6 , the collection surface comprises living tissue  132  and no adjacent or juxtaposed ground plane or counter electrode is employed. 
     The exemplary arrangement illustrated schematically in  FIG. 5B  includes a surface  126  that is grounded through a ground connection  128  that is not connected directly to ground connection  118  of the high voltage supply  110 . Such a ground connection shall be referred to as “indirect,” as opposed to the “direct” ground connection  122  shown in  FIG. 1 . At smaller nozzle-surface separations (e.g., separation less than about 10 cm with greater than about 5 kV per cm of separation), the arrangements of  FIGS. 1 and 5B  behave similarly. However, the arrangement of  FIG. 5B  (that includes only an indirect ground connection  128  to surface  126 ) is observed to exhibit, at larger separations between the nozzle orifice and grounded surface  120 , behavior distinct from that exhibited by the arrangement of  FIG. 1  (that includes a direct ground connection  122  to surface  120 ). In either arrangement, for example, an applied voltage of about 15 kV and a nozzle-surface separation of about 3 cm results in ESD solvent ejection. However, movement of the grounded surface  120  away from the nozzle orifice  104  eventually quenches the ESD solvent ejection in the arrangement of  FIG. 1  (e.g., at a separation greater than about 5 cm). Such quenching of ESD solvent ejection is not observed in the arrangement of  FIG. 5B ; in some instances, the flow rate per nozzle has been observed to increase at substantially larger separations. 
     At such substantially larger nozzle-surface separations (e.g., up to 30 cm, 40 cm, 50 cm, or more), the behavior of the arrangement of  FIG. 5B  resembles the behavior of the arrangement of  FIG. 5A  (with an isolated collection surface and no ground surface). The observed difference in behavior of the arrangements of  FIGS. 1 and 5B  can be exploited to achieve greater flow rates or polymer fiber deposition rates by eliminating a direct ground connection between the high voltage supply  110  and a collection surface  130  or ground surface  126 . For example, in a manufacturing environment with nozzles arranged so that the deposited polymer fibers are collected on a substrate moving along a conveyor, various metal components of the conveyor can act as surface  126  that has an indirect ground connection  128 , i.e., separate from the ground connection  118  of the high voltage supply  110 . Enhanced polymer fiber collection rates can be thereby achieved, relative to those obtained if the high voltage supply and conveyor shared a direct, common ground connection. An indirect ground connection can be realized in a variety of ways, e.g., by connection to separate electrical outlets, by connection to separate, distinct circuits of a building&#39;s electrical wiring, or by connection of the surface  126  to literal earth ground while high voltage supply is grounded through building wiring; other indirect ground connections can be employed. 
     It has been observed that emitting the fluid jets  342  and fibers  348  into a larger, unimpeded volume of space appears to enhance the flow rate of the fluid composition through the emitter. A collection surface  130  positioned 30 cm, 40 cm, or 50 cm from the nozzle  102 , or even farther, appears to result in increased flow rates of the fluid composition through the nozzle orifice  104  (in the arrangements of  FIGS. 5A and 5B , for example). The larger volume available may at least partly account for the enhanced flow rates exhibited by  FIGS. 5A and 5B  (at large separations) relative to  FIG. 1  (at smaller separation). Enhancement of flow rate of up to about 50% or more has been observed relative to flow rates with the collection surface less than about 5 cm from the nozzle  102 . At such large distances, the presence or absence of an indirectly grounded surface  126  only minimally affects the behavior of jets  342  or polymer fibers  348 . The combined effect of a relatively large transverse “cloud” of polymer fibers produced by each nozzle at an enhanced flow rate can be advantageously employed for depositing large amounts of polymer fibers over a relatively wide area. 
     The exemplary arrangements of  FIGS. 7 and 8  correspond to those of  FIGS. 1 and 5A , respectively, except that the immersed electrode  114  is replaced by an external electrode  116  positioned outside and adjacent the emitter  102 . The external electrode  116  is positioned upstream from the emitter orifice  104 , i.e., the external electrode  116  is positioned so that the emitter  102  points substantially away from the electrode  116 . The distances D (electrode  116  to collection surface  130 ) and d (emitter orifice  104  to collection surface  130 ) can be varied independently. The arrangement of  FIG. 7  is analogous to that of  FIG. 1 , in that the collection surface  130  is positioned between the emitter orifice  104  and a grounded surface  120 . The arrangement of  FIG. 8  is analogous to that of  FIG. 5A , in that the collection surface  130  is electrically isolated, i.e., there is no counter electrode. The arrangement of  FIG. 8  can also be used to deposit polymer fibers on living tissue, in a manner analogous to that shown in  FIG. 6 , or can include an indirect ground connection for a surface  126 , as in  FIG. 5B . In the arrangements of  FIGS. 7 and 8 , there is no direct conduction path between the fluid composition in the emitters  102  and the external electrode  116 . In other words, there is no possibility of establishing a “circuit” comprising the high voltage supply  110 , the fluid composition, and the collection surface  130 . 
     Any suitable external electrode  116  can be employed.  FIG. 9  illustrates details of a particular type of electrode  116  that can be used. The exemplary electrode  116  depicted in  FIG. 9  is a so-called ionization bar or “pinner” bar, and includes a plurality of ionization pins  117 . Alternatively, the nozzles  102  can extend through one or more openings in a conductive plate electrode, as shown and described in App No. 61/256,873 (incorporated above). 
     Sufficiently large voltage (positive or negative) must be applied to the fluid composition via the electrode  114  or  116  to form polymer fibers by ESD solvent ejection from the emitted fluid composition. The precise voltage threshold can vary somewhat depending on the particular fluid composition being employed and the arrangement of the emitter  102  and collecting surface  130 . 
     In the arrangements of  FIGS. 1 and 7  (that include a grounded counter electrode surface  120 ), a voltage threshold for forming fluid jets depends on the distance between the emitter orifice  104  and the grounded surface  120 , as well as the fluid composition and properties. Because the emitter  102  is non-conductive, quantifying the electric field strength or the electric field gradient near the emitter orifice  104  is problematic. However, the behavior of the fluid exiting the emitter orifice  104  can be correlated with the applied voltage divided by the distance d between the emitter orifice  104  and the grounded surface  120 . That quantity (voltage-distance quotient; readily measured) should be distinguished from the electric field strength (not readily measured), despite the similarity of the units employed (i.e., kV/cm). 
     For the arrangements of  FIGS. 1 and 7  (employing electrically insulating nozzles or emitters), with d less than about 10 cm or less than about 5 cm, the following progression of general fluid behaviors is often observed. The voltage ranges are approximate and can vary substantially among differing fluid compositions. Up to a voltage-distance quotient of about 3 kV/cm, conventional electrospinning from a single Taylor cone per emitter is typically observed, particularly when employing conventional, conductive electrospinning fluids. Flow rates are typically less than about 5 μL/min/emitter. With a voltage-distance quotient between about 3 kV/cm and about 5-6 kV/cm, conventional electrospinning is observed from multiple Taylor cones per emitter, with flow rates between about 5 and about 15 μL/min/emitter. Arcing between the fluid and the ground surface  120  (or any nearby grounded surface or object) may begin to occur, depending on the conductivity of the fluid, and may limit the voltage that can be applied to a particular fluid composition. With a voltage-distance quotient between about 5-6 kV/cm and about 10 kV/cm, a mixture of conventional electrospinning from multiple Taylor cones per emitter and non-evaporative, ESD solvent ejection is observed. The relative weight of those parallel processes shifts away from conventional electrospinning and toward non-evaporative, ESD solvent ejection as voltage is increased, as dielectric contrast of the fluid is increased, or as fluid conductivity is decreased. Flow rates between about 20 and about 300 μL/min/emitter are often observed, and tend to increase with applied voltage. Arcing tends to occur unless fluid conductivity is kept below about 1 mS/cm, preferably less than about 100 μS/cm, more preferably less than about 30 μS/cm or less than about 20 μS/cm. For voltage-distance quotients above 10 kV/cm, conventional Taylor cone electrospinning is substantially eliminated and non-evaporative, ESD solvent ejection predominates. Conventional electrospinning solutions typically cannot be employed due to arcing. Using fluid compositions and electrode/emitter/target arrangements disclosed herein, flow rates from several hundred μL/min/nozzle up to and over 1 mL/min/nozzle have been observed, enabling polymer fiber deposition rates greater than about 0.5 g/hr/nozzle, often up to several g/hr/nozzle. 
     In the arrangement of  FIGS. 5A, 6, and 8  (no counter electrode), there is no well-defined distance that correlates with the behavior of the fluid exiting the emitter orifice  104 ; the only measured parameter that correlates with that fluid behavior is the applied voltage relative to earth ground. A voltage threshold is observed between about 10 kV and about 15 kV, and appears to vary with the composition and properties of the fluid (e.g., dielectric constant, conductivity, and/or viscosity). Above the threshold voltage, the presently disclosed, non-evaporative, ESD solvent ejection with concomitant particle formation is observed. At lower applied voltages (still above the threshold voltage), conventional electrospinning from a visible Taylor cone can sometimes also be observed. As the voltage increases further beyond the threshold, conventional Taylor cone electrospinning tends to be suppressed or eliminated, while non-evaporative, ESD solvent ejection is enhanced. As noted above, the arrangement of  FIG. 5B  (including an indirect ground connection  128  for surface  126 ) exhibits both types of behavior (i.e., similar to  FIG. 1  or similar to  FIG. 5A ), depending on the nozzle-surface distance and the applied voltage. 
     Another characteristic that distinguishes the methods and fluid compositions disclosed herein from conventional electrospinning with conventional fluids becomes apparent when the applied voltage is turned off. Conventional Taylor cone electrospinning ceases almost immediately upon turning off the voltage supply. In contrast, when using a low conductivity, high dielectric contrast fluid in any of the arrangements of  FIG. 1, 5A, 5B, 6, 7 , or  8 , the non-evaporative, ESD solvent ejection and polymer fiber formation continues, often for several minutes. A progression of behaviors of the fluid exiting the nozzle orifice  104  is typically observed. Just after the voltage is turned off, there is little change in the behavior fluid jets  342  exiting the emitter orifice  104 . Over the course of several minutes, (1) some multiple Taylor cone electrospinning begins to occur along with the ESD solvent ejection, (2) the ESD solvent ejection stops, (3) the Taylor cone electrospinning is reduced to a single cone and jet, and (4) the last jet stops. During the progression, dripping sometimes occurs, and as each drop separates from the fluid in the emitter a brief spurt of multiple fluid jets occurs, which diminish in intensity and duration with each successive drop. 
     The continuation of fluid jets exiting the nozzle orifice  104  after the applied voltage is turned off is indicative of at least one characteristic relaxation time of the system, and that characteristic relaxation time can be exploited to enhance the ESD solvent ejection process and formation of polymer fibers (and to reduce any parallel Taylor cone electrospinning by the duty cycle of the voltage cycling). By cycling the applied voltage on and off at a frequency on the order of the reciprocal of the relevant relaxation time, enhancement of non-evaporative, ESD solvent ejection can be achieved. Rather than attempting to measure or characterize the relevant relaxation time, it can be more expedient to vary the frequency at which the applied voltage is cycled and note which frequency (or range of frequencies) appear to enhance the desired ESD solvent ejection process. For non-evaporative, ESD solvent ejection, suitable frequencies for enhancement have been observed between about 0.1 Hz and about 100 Hz. 
     Polymer fibers formed by the methods disclosed herein using fluid compositions having high dielectric contrast and low conductivity can be advantageously employed for a wide variety of purposes, particularly when the fibers formed are nanofibers, i.e., have diameters less than about 1 μm, or typically less than about 500 nm. Such purposes can include but are not limited to filtration, protective gear, biomedical applications, or materials engineering. For example, a mesh of polymer nanofibers can form at least a portion of a filtration medium that transmits only particles smaller than about 1 μm. In another example, a matrix of polymer nanofibers can be employed to retain small particles (e.g., less than 0.1 μm) of other materials (e.g., super absorbent polymers, zeolites, activated charcoal, or carbon black) to yield a material having various desired properties. A full discussion of the many uses of the fibers thus formed is beyond the scope of this disclosure. A wide array of polymers, liquid solvents, low-dielectric liquid solvents (e.g., dielectric constant less than about 15), high-dielectric materials (e.g., dielectric constant greater than about 25), salts, surfactants, and/or ionic liquids can be employed, depending on the desired properties of the nanofibers produced, and many examples are given below. For a given polymer to be deposited on a given collection surface, some optimization of parameters typically will be required to produce suitable or optimal fibers or nanofibers. Those parameters can include: identity, dielectric constant, and weight percent of the low-dielectric solvent; presence, identity, and weight percent of the high-dielectric material, salt, surfactant, or ionic liquid; presence, identity, and weight percent of any additional high dielectric material(s); conductivity and viscosity of the fluid composition; nature of the emitter (e.g., nozzle(s), channel(s), or permeable membrane), emitter orifice diameter; emitter hydrodynamic resistance; applied voltage; presence of a grounded surface and its distance from the emitter orifice; distance between the emitter orifice and the collection surface. The principles and examples disclosed herein will enable those skilled in the art to identify and optimize many other combinations of polymer, low-dielectric solvent, and high-dielectric material that are not explicitly disclosed herein that yield desirable polymer fibers or nanofibers; those other combinations, and the fiber or nanofibers thus produced, shall fall within the scope of the present disclosure or the appended claims. 
     Many combinations of chemically compatible and sufficiently soluble polymers, high-dielectric materials, salts, surfactants, or ionic liquids can be employed with a given solvent to produce a fluid composition that exhibits ESD solvent ejection. Table 1 is a list of examples of fluid compositions that exhibit ESD solvent ejection; those that include a polymer have been employed according to the methods disclosed herein to produce polymer fibers or nanofibers by ESD solvent ejection. The listed formulations are exemplary, are intended to illustrate general principles guiding selection of fluid components, and are not intended to limit the overall scope of the present disclosure or appended claims. However, specific disclosed exemplary formulations, or ranges of formulations, can be considered preferred embodiments and may therefore be further distinguished from the prior art on that basis. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 fluid compositions yielding polymer nanofibers 
               
               
                 by ESD solvent ejection 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 high- 
                   
                   
               
               
                   
                   
                 dielectric, 
               
               
                   
                   
                 ionic liquid, 
                 intermediate 
                 intermediate 
               
               
                 polymer 
                 solvent 
                 or salt 
                 dielectric 
                 dielectric 
               
               
                   
               
               
                 polystyrene 
                 d-limonene 
                 [P66614] 
                 acetone 
                   
               
               
                 23.4% 
                 62.3% 
                 [R2PO2] 
                 13.7% 
               
               
                   
                   
                 0.68% 
               
               
                 polystyrene 
                 d-limonene 
                 DMSO 
                 acetone 
               
               
                 17.2% 
                 40.1% 
                 10.0% 
                 32.7% 
               
               
                 polystyrene 
                 d-limonene 
                 [P66614] 
                 DMSO 
                 MEK 
               
               
                 17.2% 
                 40.0% 
                 [R2PO2] 
                 10.0% 
                 32.7% 
               
               
                   
                   
                 0.05% 
               
               
                 polystyrene 
                 d-limonene 
                 DMSO 
                 MEK 
               
               
                 17.2% 
                 40.1% 
                 10.0% 
                 32.8% 
               
               
                 polystyrene 
                 d-limonene 
                 [P66614] 
                 DMSO 
                 acetone 
               
               
                 17.2% 
                 40.1% 
                 [R2PO2] 
                 10.0% 
                 32.7% 
               
               
                   
                   
                 0.05% 
               
               
                 polystyrene 
                 d-limonene 
                 [P66614] 
                 DMSO 
                 MEK 
               
               
                 17.2% 
                 40.1% 
                 [Dec] 
                 10.0% 
                 32.7% 
               
               
                   
                   
                 0.05% 
               
               
                 polystyrene 
                 d-limonene 
                 PC 
                 MEK 
               
               
                 15.6% 
                 36.5% 
                 18.1% 
                 29.7% 
               
               
                 polystyrene 
                 d-limonene 
                 [P66614] 
                 PC 
                 MEK 
               
               
                 17.2% 
                 40.2% 
                 [Dec] 
                 10.0% 
                 32.6% 
               
               
                   
                   
                 0.05% 
               
               
                 polystyrene 
                 d-limonene 
                 BaTiO 3   
               
               
                 29.4% 
                 68.6% 
                 2.0% 
               
               
                 polystyrene 
                 d-limonene 
                 BaTiO 3   
                 [P66614][Dec] 
                 MEK 
               
               
                 18.7% 
                 43.7% 
                 1.3% 
                 0.05% 
                 36.2% 
               
               
                 polystyrene 
                 d-limonene 
                 TiO 2   
                 [P66614][Dec] 
                 MEK 
               
               
                 20.0% 
                 56.5% 
                 0.1% 
                 0.05% 
                 23.3% 
               
               
                 polystyrene 
                 d-limonene 
                 [bmim][PF6] 
                 MEK 
               
               
                 21.0% 
                 50.0% 
                 0.5% 
                 28.0% 
               
               
                 PVP 
                 EtOH 
               
               
                 25.4% 
                 74.6% 
               
               
                 PVP 
                 MeOH 
               
               
                 25.0% 
                 75.0% 
               
               
                 PVAc 
                 MeOH 
               
               
                 15.0% 
                 85.0% 
               
               
                 PVAc 
                 DCM 
               
               
                 15.1% 
                 84.9% 
               
               
                 PVAc 
                 DCM 
               
               
                 8.3% 
                 91.7% 
               
               
                 PVP 
                 DCM 
               
               
                 15.0% 
                 85.0% 
               
               
                 polystyrene 
                 d-limonene 
                 [bmim][PF6] 
                 DMF 
               
               
                 22.37% 
                 67.12% 
                 0.056% 
                 10.45% 
               
               
                 polystyrene 
                 d-limonene 
                 TiO 2   
                 MEK 
                 [bmim][PF6] 
               
               
                 26.86% 
                 61.76% 
                 0.90% 
                 10.43% 
                 0.05% 
               
               
                 polystyrene 
                 d-limonene 
                 TiO 2   
                 MEK 
                 [bmim][PF6] 
               
               
                 28.21% 
                 65.25% 
                 0.94% 
                 5.55% 
                 0.05 
               
               
                 polystyrene 
                 d-limonene 
                 TiO 2   
                 DMF 
                 [bmim][PF6] 
               
               
                 26.85% 
                 61.69% 
                 0.89% 
                 10.5% 
                 0.06% 
               
               
                 polystyrene 
                 d-limonene 
                 TiO 2   
                 DMF 
                 [bmim][PF6] 
               
               
                 28.3% 
                 65.13% 
                 0.94% 
                 5.57% 
                 0.05% 
               
               
                 polystyrene 
                 d-limonene 
                 tap water 
                 DeMULS 
               
               
                 19.67% 
                 62.3% 
                 16.39% 
                 DLN-532CE 
               
               
                   
                   
                   
                 1.64% 
               
               
                 polysulfone 
                 d-limonene 
                 [bmim][PF6] 
                 NMP 
                 DMF 
               
               
                 21.41% 
                 26.1% 
                 2.55% 
                 9.99% 
                 39.96% 
               
               
                 polystyrene 
                 d-limonene 
                 [bmim][PF6] 
                 DMF 
               
               
                 17.48% 
                 40.79% 
                 0.091% 
                 22.72% 
               
               
                 PCMS 
               
               
                 18.92% 
               
               
                 polystyrene 
                 d-limonene 
                 [bmim][PF6] 
                 DMF 
               
               
                 17.94% 
                 53.83% 
                 0.053% 
                 8.52% 
               
               
                 PCMS 
               
               
                 19.64% 
               
               
                 polystyrene 
                 d-limonene 
                 [bmim][PF6] 
                 DMF 
               
               
                 19.9% 
                 46.44% 
                 0.096% 
                 25.86% 
               
               
                 PCMS 
               
               
                 7.69% 
               
               
                 PEI 
                 d-limonene 
                 KCI 
                 NMP 
                 DMF 
               
               
                 15.9% 
                 53.83% 
                 0.9% 
                 49.18% 
                 13.62% 
               
               
                   
               
            
           
         
       
     
     In some exemplary compositions, ESD solvent ejection and formation of polymer fibers or nanofibers has been demonstrated with fluid compositions based on polystyrene dissolved in d-limonene, in combination with a variety of high-dielectric materials and/or other materials. Other aromatic polymers and/or other terpene, terpenoid, or aromatic solvents have been observed to exhibit similar behavior. D-limonene is attractive for use as the liquid solvent because it is considered “green” (e.g., it is available from natural, renewable sources, lacks significant toxicity, and does not raise significant environmental or disposal issues). In one group of exemplary fluid compositions, polystyrene typically comprises between about 10% and about 25% of the composition by weight, preferably between about 15% and about 20%. D-limonene typically comprises between about 30% and about 70% of the composition by weight, preferably between about 35% and about 45%. A variety of high-dielectric materials can be employed with polystyrene/d-limonene that result in ESD ejection of the d-limonene solvent and production of polystyrene fibers or nanofibers. Propylene carbonate (PC), dimethyl sulfoxide (DMSO), and dimethyl formamide (DMF) have been employed as a high-dielectric material, alone or in combination with methyl ethyl ketone (MEK) or acetone used as an intermediate dielectric material. Intermediate dielectric materials can often be employed to increase the solubility of the high-dielectric material in the polystyrene/limonene (or other polymer/low-dielectric) solution, forming a so-called “dielectric ladder.” In another exemplary fluid composition, water is employed as the high dielectric material in a polystyrene/d-limonene solution, with DeMULS DLN-532CE surfactant (DeForest Enterprises, Inc) acting as an emulsifier to enable mixing of the water into the d-limonene solution. Polyvinyl alcohol, a soap, a detergent, or other emulsifying agent can be employed. 
     Ionic liquids (e.g., trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate aka [P66614][R2PO2], trihexyltetradecylphosphonium decanoate aka [P66614][Dec], or 1-butyl-3-methylimidazolium hexafluorophosphate aka [bmim][PF6]) have been employed as high-dielectric components, with various combinations of PC, DMSO, MEK, and acetone employed as intermediate steps in the dielectric ladder. Various inorganic salts (e.g., LiCl, AgNO 3 , CuCl 2 , or FeCl 3 ) have been employed, in combination with DMF, MEK, or N-methyl-2-pyrrolidone (NMP), as disclosed in application Ser. No. 12/728,070, already incorporated by reference. It has been observed that as the dielectric ladder is ascended, progressively lower material concentrations are required for the fluid to exhibit ESD solvent ejection. Note for example the relative concentrations of the various materials in the exemplary compositions listed in Table 1. Solid particles suspended in the fluid can act as the high-dielectric material in a high dielectric contrast composition, with or without intermediate “dielectric ladder” components. Barium titanate (BaTiO 3 ) and titanium oxide (TiO 2 ) have been employed and can give rise to ESD solvent ejection, alone in a polystyrene/d-limonene solution, or in combination with other fluid components mentioned here or listed in Table 1. 
     In some other exemplary compositions, ESD solvent ejection and formation of polymer fibers or nanofibers has been demonstrated with fluid compositions based on polysulfone dissolved in d-limonene, in combination with DMF, NMP, and an ionic liquid. In some typical examples, polysulfone comprises between about 15% and about 30% of the composition by weight, d-limonene comprises between about 20% and about 30% of the composition by weight, NMP comprises between about 5% and about 20% by weight, DMF comprises between about 20% and about 40% by weight, and the ionic liquid comprises between about 1.5% and about 3% by weight. 
     In some other exemplary compositions, ESD solvent ejection and formation of polymer fibers or nanofibers has been demonstrated with fluid compositions based on mixtures of polystyrene and polycarbomethylsilane (PCMS) dissolved in d-limonene, in combination with DMF and an ionic liquid. In some typical examples, polystyrene comprises between about 15% and about 25% of the composition by weight, PCMS comprises between about 5% and about 20% by weight, d-limonene comprises between about 40% and about 55% of the composition by weight, DMF comprises between about 5% and about 30% by weight, and the ionic liquid comprises between about 0.05% and about 0.2% by weight. 
     The use of PCMS in combination with polystyrene, and UV curing of the resulting deposited polymer material, can be employed to form nanofibers to increase the heat resistance of the of those nanofibers. For example, nanofibers formed from polystyrene alone are observed to melt at about 127° C. That temperature may in some instances be too low for the nanofibers to withstand subsequent processing of the material on which they are deposited. In one example of a filtration medium, the medium is heated to about 190° C. for at least 30 seconds, resulting in melting of the deposited polystyrene nanofibers. It has been observed, however, the use of PCMS in combination with polystyrene, and UV curing of the resulting nanofibers, enables the cured nanofibers to survive intact after being heated to about 190° C. for several minutes. A mercury lamp (maximum output at a wavelength of 254 nm) can be employed for curing the polystyrene/PCMS nanofibers, and using a lamp producing about 50 W at 254 nm for a curing time on the order of an hour provides adequate curing. That curing time can be reduced by using a higher wattage lamp or by increasing the fraction of the lamp output that impinges on the fibers (e.g., using focusing or collecting optics). 
     In still other exemplary compositions, ESD solvent ejection and formation of polymer fibers or nanofibers has been demonstrated with fluid compositions based on polyetherimide (PEI) dissolved in d-limonene, in combination with DMF, NMP, and a salt. In some typical examples, PEI comprises between about 10% and about 25% of the composition by weight, d-limonene comprises between about 15% and about 25% of the composition by weight, NMP comprises between about 20% and about 60% by weight, DMF comprises between about 5% and about 25% by weight, and the salt comprises between about 0.25% and about 4% by weight. 
     Low conductivity polymer solutions (less than about 100 μS/cm), without substantial material components in addition to the polymer and solvent, have also been demonstrated to exhibit ESD solvent ejection and polymer fiber formation. Examples include solutions of polyvinylpyrrolidone (PVP) and polyvinylacetate (PVAc) dissolved in ethanol (EtOH), methanol (MeOH), or dichloromethane (DCM) and observed to exhibit ESD solvent ejection. For high dielectric solvents, such solutions can be regarded as exhibiting high dielectric contrast, between polymer (typically having a dielectric constant less than about 5) and solvent. This is the case for the MeOH and EtOH formulations. However, the DCM formulations do not exhibit a similar degree of dielectric contrast with the polymers, but nevertheless exhibit ESD solvent ejection under certain conditions. For PVP and PVAc solutions in DCM, ESD solvent ejection is appears to be inhibited by the viscosity of the polymer solution. For example, for PVP in DCM, a 25% PVP solution (viscosity about 67 cps) was observed not to exhibit ESD solvent ejection, while a 15% PVP solution in DCM (viscosity about 20 cps) did exhibit ESD solvent ejection. A similar trend was noted for solutions of PVAc in DCM. The apparent quenching of ESD solvent ejection by high viscosity is more readily apparent in solvents having a dielectric constant less than about 10 than in higher dielectric solvents. Other polymer/solvent combinations can be employed, but a minimum threshold dielectric constant of the solvent between about 6 and about 8 seems to be required for the solvent to exhibit ESD solvent ejection. 
     In addition to forming polymer fibers or nanofibers, additional particles can be deposited on the collection surface during collection of the polymer fibers, thereby retaining the additional particles in a matrix formed by the collected polymer fibers. Any suitable deposition method can be employed for depositing the additional particles that is compatible with formation of the polymer fibers. In one example, if air flow (e.g., from a vacuum belt) is employed to propel the polymer fibers to the collection surface as they are formed, that air flow can also entrain the additional particles and propel them to the collection surface as well. Whatever means are employed, simultaneous collection of the polymer fibers and deposition of the additional particles results in the additional particles being incorporated into a matrix formed by the collected fibers. If polymer nanofibers are formed, they can readily enable retention and immobilizations of additional particles that are as small as about 0.1 μm. The additional particles can comprise any suitable, desired material. In one example, super absorbent polymer particles (e.g., sodium polyacrylate) can be incorporated into a polymer nanofibers matrix in an absorbent product such as a diaper. In another example, zeolite or activated charcoal particles can be incorporated into a polymer nanofiber matrix in a filtration medium, resulting in both particulate and vapor interception capabilities. Additional examples abound. 
     In addition to producing polymer particles or fibers, methods disclosed herein can be employed for atomizing a low-dielectric solvent using a fluid composition comprising the low-dielectric liquid solvent and a high-dielectric constant additive, but no polymer. As illustrated schematically in  FIG. 10 , one or more fluid jets emerge from the fluid surface  344  at the emitter orifice  104 . Within about 2 or 3 millimeters, the jets  342  eject solvent droplets  346  and break up. With no polymer present in the fluid, no particles or fibers are produced. The droplets produced under typical conditions (see above) appear to be less than about 2 μm in average diameter; other droplet diameters can be produced. The production of small solvent droplets can be advantageously employed in a variety of applications, e.g., for fuel injection into an engine cylinder or for spray treatment of a surface. Without any polymer in the fluid composition, fluid viscosity is likely to be quite low, which can be compensated by suitable adaptation of the emitter  102  and emitter orifice  104 , e.g., to increase hydrodynamic resistance. 
     It is intended that equivalents of the disclosed exemplary embodiments and methods shall fall within the scope of the present disclosure or appended claims. It is intended that the disclosed exemplary embodiments and methods, and equivalents thereof, may be modified while remaining within the scope of the present disclosure or appended claims. 
     In the foregoing Detailed Description, various features may be grouped together in several exemplary embodiments to streamline the disclosure or to disclose preferred embodiments. This method of disclosure is not to be interpreted as reflecting an intention that any claimed embodiment requires more features than are expressly recited in the corresponding claim. Rather, as the appended claims reflect, inventive subject matter may lie in less than all features of a single disclosed exemplary embodiment, or in combinations of features that do not appear in combination in any single disclosed embodiment. Thus, the appended claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate disclosed embodiment. However, the present disclosure and appended claims shall also be construed as implicitly disclosing any embodiment having any suitable combination of disclosed or claimed features (i.e., combinations of features that are not incompatible or mutually exclusive), including those combinations of features that are not explicitly disclosed herein. In particular, any suitable combination of parameters or features for performing the disclosed or claimed methods (e.g., any one or more of applied voltage, emitted-collector distance, emitter geometry, and so forth) can be combined with any suitable fluid composition (e.g., any suitable combination of one or more of specific polymer(s), solvent(s), dielectric material(s), and so forth). It should be further noted that the scope of the appended claims do not necessarily encompass the whole of the subject matter disclosed herein. 
     For purposes of the present disclosure and appended claims, the conjunction “or” is to be construed inclusively (e.g., “a dog or a cat” would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat, or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or any two, or all three”), unless: (i) it is explicitly stated otherwise, e.g., by use of “either . . . or”, “only one of . . . ”, or similar language; or (ii) two or more of the listed alternatives are mutually exclusive within the particular context, in which case “or” would encompass only those combinations involving non-mutually-exclusive alternatives. For purposes of the present disclosure or appended claims, the words “comprising,” “including,” “having,” and variants thereof shall be construed as open ended terminology, with the same meaning as if the phrase “at least” were appended after each instance thereof. 
     In the appended claims, if the provisions of 35 USC §112 ¶ 6 are desired to be invoked in an apparatus claim, then the word “means” will appear in that apparatus claim. If those provisions are desired to be invoked in a method claim, the words “a step for” will appear in that method claim. Conversely, if the words “means” or “a step for” do not appear in a claim, then the provisions of 35 USC §112 ¶ 6 are not intended to be invoked for that claim.