Patent Publication Number: US-11383234-B2

Title: Electronic control of fluidic species

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/829,371, filed Dec. 1, 2017, which is a continuation of U.S. patent application Ser. No. 15/695,184, filed Sept. 5, 2017, which is a continuation of U.S. patent application Ser. No. 14/255,101, filed Apr. 17, 2014, which is a continuation of U.S. patent application Ser. No. 11/360,845, filed Feb. 23, 2006, which is a continuation of International Patent Application Ser. No. PCT/US2004/027912, filed Aug. 27, 2004, which claims priority to U.S. Provisional Patent Application Ser. No. 60/498,091, filed Aug. 27, 2003, entitled “Electronic Control of Fluidic Species,” by Link, et al., all incorporated herein by reference. 
    
    
     GOVERNMENT FUNDING 
     This invention was made with government support under Grant Nos. 0213805 and 0243716 awarded by the National Science Foundation (NSF). The Government has certain rights to this invention. 
    
    
     FIELD OF INVENTION 
     The present invention generally relates to systems and methods for the control of fluidic species and, in particular, to systems and methods for the electronic control of fluidic species. 
     BACKGROUND 
     The manipulation of fluids to form fluid streams of desired configuration, discontinuous fluid streams, droplets, particles, dispersions, etc., for purposes of fluid delivery, product manufacture, analysis, and the like, is a relatively well-studied art. For example, highly monodisperse gas bubbles, less than 100 microns in diameter, have been produced using a technique referred to as capillary flow focusing. In this technique, gas is forced out of a capillary tube into a bath of liquid, the tube is positioned above a small orifice, and the contraction flow of the external liquid through this orifice focuses the gas into a thin jet which subsequently breaks into roughly equal-sized bubbles via capillary instability. In a related technique, a similar arrangement can be used to produce liquid droplets in air. 
     An article entitled “Generation of Steady Liquid Microthreads and Micron-Sized Monodisperse Sprays and Gas Streams,” Phys. Rev. Lett., 80:2, Jan. 12, 1998 (Ganan-Calvo) describes formation of a microscopic liquid thread by a laminar accelerating gas stream, giving rise to a fine spray. An articled entitled “Dynamic Pattern Formation in a Vesicle-Generating Microfluidic Device,” Phys. Rev. Lett., 86:18, Apr. 30, 2001 (Thorsen, et al.) describes formation of a discontinuous water phase in a continuous oil phase via microfluidic cross-flow by introducing water, at a “T” junction between two microfluidic channels, into flowing oil. 
     U.S. Pat. No. 6,120,666, issued Sep. 19, 2000, describes a microfabricated device having a fluid focusing chamber for spatially confining first and second sample fluid streams for analyzing microscopic particles in a fluid medium, for example, in biological fluid analysis. U.S. Pat. No. 6,116,516, issued Sep. 12, 2000, describes formation of a capillary microjet, and formation of a monodisperse aerosol via disassociation of the microjet. U.S. Pat. No. 6,187,214, issued Feb. 13, 2001, describes atomized particles in a size range of from about 1 to about 5 microns, produced by the interaction of two immiscible fluids. U.S. Pat. No. 6,248,378, issued Jun. 19, 2001, describes production of particles for introduction into food using a microjet and a monodisperse aerosol formed when the microjet dissociates. 
     Microfluidic systems have been described in a variety of contexts, typically in the context of miniaturized laboratory (e.g., clinical) analysis. Other uses have been described as well. For example, International Patent Publication No. WO 01/89789, published Nov. 29, 2001 by Anderson, et al., describes multi-level microfluidic systems that can be used to provide patterns of materials, such as biological materials and cells, on surfaces. Other publications describe microfluidic systems including valves, switches, and other components. 
     While significant advances have been made in dynamics at the macro- or microfluidic scale, improved techniques and the results of these techniques are needed. 
     SUMMARY OF THE INVENTION 
     The present invention relates to systems and methods for the electronic control of fluidic species. The subject matter of this invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles. 
     In one aspect, the invention provides a method. In one set of embodiments, the method is a method of combining at least two species in a controlled manner in a microfluidic system. The method includes acts of providing a series of droplets flowing in a microfluidic system; selecting a first droplet from the series of droplets and separating the first droplet from at least some other droplets in the series of droplets (where the first droplet has a maximum cross-sectional dimension of less than about 100 microns and contains a first chemical, biological, or biochemical species), providing a second droplet separate from the series of droplets (where the second droplet has a maximum cross-sectional dimension of less than about 100 microns and contains a second chemical, biological, or biochemical species), selectively urging the first droplet and/or the second droplet toward a location where coalescence can occur and allowing the first droplet and the second droplet to coalesce into one combined droplet, and determining a reaction involving at least the first species in the first droplet and the second species in the second droplet. 
     The method, according to another set of embodiments, is a method of sorting droplets in a controlled manner in a microfluidic system. The method includes acts of providing a series of droplets flowing in a microfluidic system, and selecting a first droplet from the series of droplets and separating the first droplet from at least some other droplets in the series of droplets. In some cases, the first droplet has a maximum cross-sectional dimension of less than about 100 microns. 
     In yet another set of embodiments, the method is a method of imparting charge to one or more droplets in a microfluidic system. The method can include acts of providing a droplet in a microfluidic system, imparting a dipole moment to the droplet, and dividing the droplet, while the dipole moment is present, into at least two subdroplets, at least one of the subdroplets carrying a charge resulting from the dipole moment imparted to the primary droplet. 
     In still another set of embodiments, the method is a method of combining at least two droplets in a microfluidic system. The method includes acts of providing at least two droplets in a microfluidic system, exposing the droplets to an electric field thereby inducing, in the droplets, dipole moments, and coalescing the at least two droplets into a single droplet at least in part via droplet-droplet attraction due to the induced dipole moments. 
     The method, according to another set of embodiments, includes a step of producing a charge of at least about 10-14 C on a first fluid surrounded by a second, liquid fluid. The method, according to still another set of embodiments, includes a step of applying an electric force of at least about 10-9 N on a first fluid surrounded by a second, liquid fluid. 
     In yet another set of embodiments, in a first fluid comprising first droplets and second droplets, the first fluid surrounded by a second, liquid fluid, the method comprises sorting the first and second droplets at a rate of at least about 10 or at least about 100 droplets/s. In still another set of embodiments, the method includes steps of providing droplets of a first fluid surrounded by a second, liquid fluid, where the droplets have a ratio of droplets containing a first species to droplets free of the first species, and sorting the droplets to increase the ratio of droplets containing the first species to droplets free of the first species by at least a factor of about 2. 
     In another set of embodiments, in a first fluid comprising first droplets and second droplets where the first fluid is surrounded by a second, liquid fluid, the method comprises sorting the first and second droplets without substantially altering a flowrate of the second, liquid fluid. In yet another set of embodiments, the method includes a step of dividing a first fluidic droplet, surrounded by a second, liquid fluid, into two droplets using an electric field. 
     In another aspect, the invention is an article. The article, in one set of embodiments, includes a first fluidic droplet having a charge of at least about 10-14 C, surrounded by a second, liquid fluid. The article, in another set of embodiments, includes droplets comprising a first fluid surrounded by a second, liquid fluid, where at least about 90% of the droplets each consists of the same number of entities of a species. In yet another aspect, the invention is an apparatus. According to one set of embodiments, the apparatus includes a microfluidic channel, and an electric field generator constructed and arranged to generate an electric field of at least about 1 V/micrometer within the microfluidic channel. 
     In still another aspect, the present invention is directed to a method of making one or more of the embodiments described herein. In yet another aspect, the present invention is directed to a method of using one or more of the embodiments described herein. In still another aspect, the present invention is directed to a method of promoting one or more of the embodiments described herein. 
     Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control. If two or more applications incorporated by reference include conflicting and/or inconsistent disclosure with respect to each other, then the later-filed application shall control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For the purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures: 
         FIGS. 1A and 1B  illustrate the splitting of droplets in accordance with one embodiment of the invention; 
         FIGS. 2A and 2B  illustrate an apparatus in accordance with an embodiment of the invention, before the application of an electric field thereto; 
         FIGS. 3A and 3B  illustrate the apparatus of  FIGS. 2A and 2B  after the application of an electric field thereto; 
         FIGS. 4A and 4B  illustrate the apparatus of  FIGS. 2A and 2B  after the application of a reversed electric field thereto; 
         FIG. 5  is a schematic diagram of droplet splitting, in accordance with one embodiment of the invention; 
         FIGS. 6A and 6B  are schematic diagrams of additional embodiments of the invention; 
         FIGS. 7A and 7B  illustrate the formation of droplets in accordance with an embodiment of the invention; 
         FIGS. 8A-8F  illustrate the sorting and/or splitting of droplets in accordance with one embodiment of the invention; 
         FIGS. 9A-9D  illustrate the sorting and/or splitting of droplets in accordance with another embodiment of the invention; 
         FIGS. 10A-10D  illustrate the sorting of fluidic droplets according to another embodiment of the invention; 
         FIGS. 11A-11C  illustrate the sorting of fluidic droplets according to yet another embodiment of the invention; 
         FIGS. 12A-12J  illustrate fluidic mixing in droplets having two or more fluid regions, according to one embodiment of the invention; 
         FIGS. 13A-13D  illustrate uncharged and charged droplets in channels, according to certain embodiments of the invention; 
         FIGS. 14A-14C  illustrate various embodiments of the invention comprising alternating droplets of a first fluid and of a second fluid; and 
         FIGS. 15A-15E  illustrate an example of a device featuring various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention generally relates to the control and manipulation of fluidic species, typically surrounded by a liquid (e.g., suspended). Various aspects of the invention relate to forming fluidic droplets, splitting droplets into multiple droplets, creating charges on droplets, inducing dipoles in droplets, causing droplets to fuse or coalesce, causing mixing to occur within droplets, sorting and/or separating droplets, and/or sensing and/or determining droplets and/or components within the droplets. Combinations of these and/or other systems and methods of controlling and manipulating of fluidic species are also envisioned, for example, systems and methods as disclosed in U.S. Provisional Patent Application Ser. No. 60/498,091, filed Aug. 27, 2003, by Link, et. al.; U.S. Provisional Patent Application Ser. No. 60/392,195, filed Jun. 28, 2002, by Stone, et. al.; U.S. Provisional Patent Application Ser. No. 60/424,042, filed Nov. 5, 2002, by Link, et al.; U.S. Pat. No. 5,512,131, issued Apr. 30, 1996 to Kumar, et al.; International Patent Publication WO 96/29629, published Jun. 26, 1996 by Whitesides, et al.; U.S. Pat. No. 6,355,198, issued Mar. 12, 2002 to Kim, et al.; International Patent Application Serial No.: PCT/US01/16973, filed May 25, 2001 by Anderson, et al., published as WO 01/89787 on Nov. 29, 2001; International Patent Application Ser. No. PCT/US03/20542, filed Jun. 30, 2003 by Stone, et al., published as WO 2004/002627 on Jan. 8, 2004; International Patent Application Ser. No. PCT/US2004/010903, filed Apr. 9, 2004 by Link, et al.; and U.S. Provisional Patent Application Ser. No. 60/461,954, filed Apr. 10, 2003, by Link, et al.; each of which is incorporated herein by reference. 
     In various aspects of the invention, a fluidic system as disclosed herein may include a droplet formation system, a sensing system, a controller, and/or a droplet sorting and/or separation system, or any combination of these systems. Such systems and methods may be positioned in any suitable order, depending on a particular application, and in some cases, multiple systems of a given type may be used, for example, two or more droplet formation systems, two or more droplet separation systems, etc. As examples of arrangements, systems of the invention can be arranged to form droplets, to dilute fluids, to control the concentration of species within droplets, to sort droplets to select those with a desired concentration of species or entities (e.g., droplets each containing one molecule of reactant), to fuse individual droplets to cause reaction between species contained in the individual droplets, to determine reaction(s) and/or rates of reaction(s) in one or more droplets, etc. Many other arrangements can be practiced in accordance with the invention. 
     Droplet Production/Formation 
     One aspect of the invention relates to systems and methods for producing droplets of fluid surrounded by a liquid. The fluid and the liquid may be essentially immiscible in many cases, i.e., immiscible on a time scale of interest (e.g., the time it takes a fluidic droplet to be transported through a particular system or device). In certain cases, the droplets may each be substantially the same shape or size, as further described below. The fluid may also contain other species, for example, certain molecular species (e.g., as further discussed below), cells, particles, etc. 
     In one set of embodiments, electric charge may be created on a fluid surrounded by a liquid, which may cause the fluid to separate into individual droplets within the liquid. In some embodiments, the fluid and the liquid may be present in a channel, e.g., a microfluidic channel, or other constricted space that facilitates application of an electric field to the fluid (which may be “AC” or alternating current, “DC” or direct current etc.), for example, by limiting movement of the fluid with respect to the liquid. Thus, the fluid can be present as a series of individual charged and/or electrically inducible droplets within the liquid. In one embodiment, the electric force exerted on the fluidic droplet may be large enough to cause the droplet to move within the liquid. In some cases, the electric force exerted on the fluidic droplet may be used to direct a desired motion of the droplet within the liquid, for example, to or within a channel or a microfluidic channel (e.g., as further described herein), etc. As one example, in apparatus  5  in  FIG. 3A , droplets  15  created by fluid source  10  can be electrically charged using an electric filed created by electric field generator  20 . 
     Electric charge may be created in the fluid within the liquid using any suitable technique, for example, by placing the fluid within an electric field (which may be AC, DC, etc.), and/or causing a reaction to occur that causes the fluid to have an electric charge, for example, a chemical reaction, an ionic reaction, a photocatalyzed reaction, etc. In one embodiment, the fluid is an electrical conductor. As used herein, a “conductor” is a material having a conductivity of at least about the conductivity of 18 megohm (MOhm or) water. The liquid surrounding the fluid may have a conductivity less than that of the fluid. For instance, the liquid may be an insulator, relative to the fluid, or at least a “leaky insulator,” i.e., the liquid is able to at least partially electrically insulate the fluid for at least a short period of time. Those of ordinary skill in the art will be able to identify the conductivity of fluids. In one non-limiting embodiment, the fluid may be substantially hydrophilic, and the liquid surrounding the fluid may be substantially hydrophobic. 
     In some embodiments, the charge created on the fluid (for example, on a series of fluidic droplets) may be at least about 10-22 C/micrometer3. In certain cases, the charge may be at least about 10-21 C/micrometer3, and in other cases, the charge may be at least about 10-20 C/micrometer3, at least about 10-19 C/micrometer3, at least about 10-18 C/micrometer3, at least about 10-17 C/micrometer3, at least about 10-16 C/micrometer3, at least about 10-15 C/micrometer3, at least about 10-14 C/micrometer3, at least about 10-13 C/micrometer3, at least about 10-12 C/micrometer3, at least about 10-11 C/micrometer3, at least about 10-10 C/micrometer3, or at least about 10-9 C/micrometer3 or more. In certain embodiments, the charge created on the fluid may be at least about 10-21 C/micrometer2, and in some cases, the charge may be at least about 10-20 C/micrometer2, at least about 10-19 C/micrometer2, at least about 10-18 C/micrometer2, at least about 10-17 C/micrometer2, at least about 10-16 C/micrometer2, at least about 10-15 C/micrometer2, at least about 10-14 C/micrometer2, or at least about 10-13 C/micrometer2 or more. In other embodiments, the charge may be at least about 10-14 C/droplet, and, in some cases, at least about 10-13 C/droplet, in other cases at least about 10-12 C/droplet, in other cases at least about 10-11 C/droplet, in other cases at least about 10-10 C/droplet, or in still other cases at least about 10-9 C/droplet. 
     The electric field, in some embodiments, is generated from an electric field generator, i.e., a device or system able to create an electric field that can be applied to the fluid. The electric field generator may produce an AC field (i.e., one that varies periodically with respect to time, for example, sinusoidally, sawtooth, square, etc.), a DC field (i.e., one that is constant with respect to time), a pulsed field, etc. The electric field generator may be constructed and arranged to create an electric field within a fluid contained within a channel or a microfluidic channel. The electric field generator may be integral to or separate from the fluidic system containing the channel or microfluidic channel, according to some embodiments. As used herein, “integral” means that portions of the components integral to each other are joined in such a way that the components cannot be manually separated from each other without cutting or breaking at least one of the components. 
     Techniques for producing a suitable electric field (which may be AC, DC, etc.) are known to those of ordinary skill in the art. For example, in one embodiment, an electric field is produced by applying voltage across a pair of electrodes, which may be positioned on or embedded within the fluidic system (for example, within a substrate defining the channel or microfluidic channel), and/or positioned proximate the fluid such that at least a portion of the electric field interacts with the fluid. The electrodes can be fashioned from any suitable electrode material or materials known to those of ordinary skill in the art, including, but not limited to, silver, gold, copper, carbon, platinum, copper, tungsten, tin, cadmium, nickel, indium tin oxide (“ITO”), etc., as well as combinations thereof. In some cases, transparent or substantially transparent electrodes can be used. In certain embodiments, the electric field generator can be constructed and arranged (e.g., positioned) to create an electric field applicable to the fluid of at least about 0.01 V/micrometer, and, in some cases, at least about 0.03 V/micrometer, at least about 0.05 V/micrometer, at least about 0.08 V/micrometer, at least about 0.1 V/micrometer, at least about 0.3 V/micrometer, at least about 0.5 V/micrometer, at least about 0.7 V/micrometer, at least about 1 V/micrometer, at least about 1.2 V/micrometer, at least about 1.4 V/micrometer, at least about 1.6 V/micrometer, or at least about 2 V/micrometer. In some embodiments, even higher electric field intensities may be used, for example, at least about 2 V/micrometer, at least about 3 V/micrometer, at least about 5 V/micrometer, at least about 7 V/micrometer, or at least about 10 V/micrometer or more. 
     In some embodiments, an electric field may be applied to fluidic droplets to cause the droplets to experience an electric force. The electric force exerted on the fluidic droplets may be, in some cases, at least about 10-16 N/micrometer3. In certain cases, the electric force exerted on the fluidic droplets may be greater, e.g., at least about 10-15 N/micrometer3, at least about 10-14 N/micrometer3, at least about 10-13 N/micrometer3, at least about 10-12 N/micrometer3, at least about 10-11 N/micrometer3, at least about 10-10 N/micrometer3, at least about 10-9 N/micrometer3, at least about 10-8 N/micrometer3, or at least about 10-7 N/micrometer3 or more. In other embodiments, the electric force exerted on the fluidic droplets, relative to the surface area of the fluid, may be at least about 10-15 N/micrometer2, and in some cases, at least about 10-14 N/micrometer2, at least about 10-13 N/micrometer2, at least about 10-12 N/micrometer2, at least about 10-11 N/micrometer2, at least about 10-10 N/micrometer2, at least about 10-9 N/micrometer2, at least about 10-8 N/micrometer2, at least about 10-7 N/micrometer2, or at least about 10-6 N/micrometer2 or more. In yet other embodiments, the electric force exerted on the fluidic droplets may be at least about 10-9 N, at least about 10-8 N, at least about 10-7 N, at least about 10-6 N, at least about 10-5 N, or at least about 10-4 N or more in some cases. 
     In some embodiments of the invention, systems and methods are provided for at least partially neutralizing an electric charge present on a fluidic droplet, for example, a fluidic droplet having an electric charge, as described above. For example, to at least partially neutralize the electric charge, the fluidic droplet may be passed through an electric field and/or brought near an electrode, e.g., using techniques such as those described herein. Upon exiting of the fluidic droplet from the electric field (i.e., such that the electric field no longer has a strength able to substantially affect the fluidic droplet), and/or other elimination of the electric field, the fluidic droplet may become electrically neutralized, and/or have a reduced electric charge. 
     In another set of embodiments, droplets of fluid can be created from a fluid surrounded by a liquid within a channel by altering the channel dimensions in a manner that is able to induce the fluid to form individual droplets. The channel may, for example, be a channel that expands relative to the direction of flow, e.g., such that the fluid does not adhere to the channel walls and forms individual droplets instead, or a channel that narrows relative to the direction of flow, e.g., such that the fluid is forced to coalesce into individual droplets. One example is shown in  FIG. 7A , where channel  510  includes a flowing fluid  500  (flowing downwards), surrounded by liquid  505 . Channel  510  narrows at location  501 , causing fluid  500  to form a series of individual fluidic droplets  515 . In other embodiments, internal obstructions may also be used to cause droplet formation to occur. For instance, baffles, ridges, posts, or the like may be used to disrupt liquid flow in a manner that causes the fluid to coalesce into fluidic droplets. 
     In some cases, the channel dimensions may be altered with respect to time (for example, mechanically or electromechanically, pneumatically, etc.) in such a manner as to cause the formation of individual fluidic droplets to occur. For example, the channel may be mechanically contracted (“squeezed”) to cause droplet formation, or a fluid stream may be mechanically disrupted to cause droplet formation, for example, through the use of moving baffles, rotating blades, or the like. As a non-limiting example, in  FIG. 7B , fluid  500  flows through channel  510  in a downward direction. Fluid  500  is surrounded by liquid  505 . Piezoelectric devices  520  positioned near or integral to channel  510  may then mechanically constrict or “squeeze” channel  510 , causing fluid  500  to break up into individual fluidic droplets  515 . 
     In yet another set of embodiments, individual fluidic droplets can be created and maintained in a system comprising three essentially mutually immiscible fluids (i.e., immiscible on a time scale of interest), where one fluid is a liquid carrier, and the second fluid and the third fluid alternate as individual fluidic droplets within the liquid carrier. In such a system, surfactants are not necessarily required to ensure separation of the fluidic droplets of the second and third fluids. As an example, with reference to  FIG. 14A , within channel  700 , a first fluid  701  and a second fluid  702  are each carried within liquid carrier  705 . First fluid  701  and second fluid  702  alternate as a series of alternating, individual droplets, each carried by liquid carrier  705  within channel  700 . As the first fluid, the second fluid, and the liquid carrier are all essentially mutually immiscible, any two of the fluids (or all three fluids) can come into contact without causing droplet coalescence to occur. A photomicrograph of an example of such a system is shown in  FIG. 14B , illustrating first fluid  701  and second fluid  702 , present as individual, alternating droplets, each contained within liquid carrier  705 . 
     One example of a system involving three essentially mutually immiscible fluids is a silicone oil, a mineral oil, and an aqueous solution (i.e., water, or water containing one or more other species that are dissolved and/or suspended therein, for example, a salt solution, a saline solution, a suspension of water containing particles or cells, or the like). Another example of a system is a silicone oil, a fluorocarbon oil, and an aqueous solution. Yet another example of a system is a hydrocarbon oil (e.g., hexadecane), a fluorocarbon oil, and an aqueous solution. In these examples, any of these fluids may be used as the liquid carrier. Non-limiting examples of suitable fluorocarbon oils include octadecafluorodecahydronaphthalene: 
     or 1-(1,2,2,3,3,4,4,5,5,6,6-undecafluorocyclohexyl)ethanol: 
     A non-limiting example of such a system is illustrated in  FIG. 14B . In this figure, fluidic network  710  includes a channel containing liquid carrier  705 , and first fluid  701  and second fluid  702 . Liquid carrier  705  is introduced into fluidic network  710  through inlet  725 , while first fluid  701  is introduced through inlet  721 , and second fluid  702  is introduced through inlet  722 . Channel  716  within fluidic network  710  contains liquid carrier  715  introduced from inlet  725 . Initially, first fluid  701  is introduced into liquid carrier  705 , forming fluidic droplets therein. Next, second fluid  702  is introduced into liquid  705 , forming fluidic droplets therein that are interspersed with the fluidic droplets containing first fluid  701 . Thus, upon reaching channel  717 , liquid carrier  705  contains a first set of fluidic droplets containing first fluid  701 , interspersed with a second set of fluidic droplets containing second fluid  702 . In the embodiment illustrated, channel  706  optionally comprises a series of bends, which may allow mixing to occur within each of the fluidic droplets, as further discussed below. However, it should be noted that in this embodiment, since first fluid  701  and second fluid  702  are essentially immiscible, significant fusion and/or mixing of the droplets containing first fluid  701  with the droplets containing second fluid  702  is not generally expected. 
     Other examples of the production of droplets of fluid surrounded by a liquid are described in International Patent Application Ser. No. PCT/US2004/010903, filed Apr. 9, 2004 by Link, et al. and International Patent Application Ser. No. PCT/US03/20542, filed Jun. 30, 2003 by Stone, et al., published as WO 2004/002627 on Jan. 8, 2004, each incorporated herein by reference. 
     In some embodiments, the fluidic droplets may each be substantially the same shape and/or size. The shape and/or size can be determined, for example, by measuring the average diameter or other characteristic dimension of the droplets. The term “determining,” as used herein, generally refers to the analysis or measurement of a species, for example, quantitatively or qualitatively, and/or the detection of the presence or absence of the species. “Determining” may also refer to the analysis or measurement of an interaction between two or more species, for example, quantitatively or qualitatively, or by detecting the presence or absence of the interaction. Examples of suitable techniques include, but are not limited to, spectroscopy such as infrared, absorption, fluorescence, UV/visible, FTIR (“Fourier Transform Infrared Spectroscopy”), or Raman; gravimetric techniques; ellipsometry; piezoelectric measurements; immunoassays; electrochemical measurements; optical measurements such as optical density measurements; circular dichroism; light scattering measurements such as quasielectric light scattering; polarimetry; refractometry; or turbidity measurements. 
     The “average diameter” of a plurality or series of droplets is the arithmetic average of the average diameters of each of the droplets. Those of ordinary skill in the art will be able to determine the average diameter (or other characteristic dimension) of a plurality or series of droplets, for example, using laser light scattering, microscopic examination, or other known techniques. The diameter of a droplet, in a non-spherical droplet, is the mathematically-defined average diameter of the droplet, integrated across the entire surface. The average diameter of a droplet (and/or of a plurality or series of droplets) may be, for example, less than about 1 mm, less than about 500 micrometers, less than about 200 micrometers, less than about 100 micrometers, less than about 75 micrometers, less than about 50 micrometers, less than about 25 micrometers, less than about 10 micrometers, or less than about 5 micrometers in some cases. The average diameter may also be at least about 1 micrometer, at least about 2 micrometers, at least about 3 micrometers, at least about 5 micrometers, at least about 10 micrometers, at least about 15 micrometers, or at least about 20 micrometers in certain cases. 
     In certain embodiments of the invention, the fluidic droplets may contain additional entities, for example, other chemical, biochemical, or biological entities (e.g., dissolved or suspended in the fluid), cells, particles, gases, molecules, or the like. In some cases, the droplets may each be substantially the same shape or size, as discussed above. In certain instances, the invention provides for the production of droplets consisting essentially of a substantially uniform number of entities of a species therein (i.e., molecules, cells, particles, etc.). For example, about 90%, about 93%, about 95%, about 97%, about 98%, or about 99%, or more of a plurality or series of droplets may each contain the same number of entities of a particular species. For instance, a substantial number of fluidic droplets produced, e.g., as described above, may each contain 1 entity, 2 entities, 3 entities, 4 entities, 5 entities, 7 entities, 10 entities, 15 entities, 20 entities, 25 entities, 30 entities, 40 entities, 50 entities, 60 entities, 70 entities, 80 entities, 90 entities, 100 entities, etc., where the entities are molecules or macromolecules, cells, particles, etc. In some cases, the droplets may each independently contain a range of entities, for example, less than 20 entities, less than 15 entities, less than 10 entities, less than 7 entities, less than 5 entities, or less than 3 entities in some cases. In one set of embodiments, in a liquid containing droplets of fluid, some of which contain a species of interest and some of which do not contain the species of interest, the droplets of fluid may be screened or sorted for those droplets of fluid containing the species as further described below (e.g., using fluorescence or other techniques such as those described above), and in some cases, the droplets may be screened or sorted for those droplets of fluid containing a particular number or range of entities of the species of interest, e.g., as previously described. Thus, in some cases, a plurality or series of fluidic droplets, some of which contain the species and some of which do not, may be enriched (or depleted) in the ratio of droplets that do contain the species, for example, by a factor of at least about 2, at least about 3, at least about 5, at least about 10, at least about 15, at least about 20, at least about 50, at least about 100, at least about 125, at least about 150, at least about 200, at least about 250, at least about 500, at least about 750, at least about 1000, at least about 2000, or at least about 5000 or more in some cases. In other cases, the enrichment (or depletion) may be in a ratio of at least about 104, at least about 105, at least about 106, at least about 107, at least about 108, at least about 109, at least about 1010, at least about 1011, at least about 1012, at least about 1013, at least about 1014, at least about 1015, or more. For example, a fluidic droplet containing a particular species may be selected from a library of fluidic droplets containing various species, where the library may have about 105, about 106, about 107, about 108, about 109, about 1010, about 1011, about 1012, about 1013, about 1014, about 1015, or more items, for example, a DNA library, an RNA library, a protein library, a combinatorial chemistry library, etc. In certain embodiments, the droplets carrying the species may then be fused, reacted, or otherwise used or processed, etc., as further described below, for example, to initiate or determine a reaction. 
     Splitting Droplets 
     In another aspect, the invention relates to systems and methods for splitting a fluidic droplet into two or more droplets. The fluidic droplet may be surrounded by a liquid, e.g., as previously described, and the fluid and the liquid are essentially immiscible in some cases. The two or more droplets created by splitting the original fluidic droplet may each be substantially the same shape and/or size, or the two or more droplets may have different shapes and/or sizes, depending on the conditions used to split the original fluidic droplet. In many cases, the conditions used to split the original fluidic droplet can be controlled in some fashion, for example, manually or automatically (e.g., with a processor, as discussed below). In some cases, each droplet in a plurality or series of fluidic droplets may be independently controlled. For example, some droplets may be split into equal parts or unequal parts, while other droplets are not split. 
     According to one set of embodiments, a fluidic droplet can be split using an applied electric field. The electric field may be an AC field, a DC field, etc. The fluidic droplet, in this embodiment, may have a greater electrical conductivity than the surrounding liquid, and, in some cases, the fluidic droplet may be neutrally charged. In some embodiments, the droplets produced from the original fluidic droplet are of approximately equal shape and/or size. In certain embodiments, in an applied electric field, electric charge may be urged to migrate from the interior of the fluidic droplet to the surface to be distributed thereon, which may thereby cancel the electric field experienced in the interior of the droplet. In some embodiments, the electric charge on the surface of the fluidic droplet may also experience a force due to the applied electric field, which causes charges having opposite polarities to migrate in opposite directions. The charge migration may, in some cases, cause the drop to be pulled apart into two separate fluidic droplets. The electric field applied to the fluidic droplets may be created, for example, using the techniques described above, such as with a reaction an electric field generator, etc. 
     As a non-limiting example, in  FIG. 1A , where no electric field is applied, fluidic droplets  215  contained in channel  230  are carried by a surrounding liquid, which flows towards intersection  240 , leading to channels  250  and  255 . In this example, the surrounding liquid flows through channels  250  and  255  at equal flowrates. Thus, at intersection  240 , fluidic droplets  215  do not have a preferred orientation or direction, and move into exit channels  250  and  255  with equal probability due to the surrounding liquid flow. In contrast, in  FIG. 1B , while the surrounding liquid flows in the same fashion as  FIG. 1A , under the influence of an applied electric field of 1.4 V/micrometers, fluidic droplets  215  are split into two droplets at intersection  240 , forming new droplets  216  and  217 . Droplet  216  moves to the left in channel  250 , while droplet  217  moves to the right in channel  255 . 
     A schematic of this process can be seen in  FIG. 5 , where a neutral fluidic droplet  530 , surrounded by a liquid  535  in channel  540 , is subjected to applied electric field  525 , created by electrodes  526  and  527 . Electrode  526  is positioned near channel  542 , while electrode  527  is positioned near channel  544 . Under the influence of electric field  525 , charge separation is induced within fluidic droplet  530 , i.e., such that a positive charge is induced at one end of the droplet, while a negative charge is induced at the other end of the droplet. The droplet may then split into a negatively charged droplet  545  and a positively charged droplet  546 , which then may travel in channels  542  and  544 , respectively. In some cases, one or both of the electric charges on the resulting charged droplets may also be neutralized, as previously described. 
     Other examples of splitting a fluidic droplet into two droplets are described in International Patent Application Ser. No. PCT/US2004/010903, filed Apr. 9, 2004 by Link, et al.; U.S. Provisional Patent Application Ser. No. 60/498,091, filed Aug. 27, 2003, by Link, et. al.; and International Patent Application Ser. No. PCT/US03/20542, filed Jun. 30, 2003 by Stone, et al., published as WO 2004/002627 on Jan. 8, 2004, each incorporated herein by reference. 
     Fusing Droplets 
     The invention, in yet another aspect, relates to systems and methods for fusing or coalescing two or more fluidic droplets into one droplet. For example, in one set of embodiments, systems and methods are provided that are able to cause two or more droplets (e.g., arising from discontinuous streams of fluid) to fuse or coalesce into one droplet in cases where the two or more droplets ordinarily are unable to fuse or coalesce, for example, due to composition, surface tension, droplet size, the presence or absence of surfactants, etc. In certain microfluidic systems, the surface tension of the droplets, relative to the size of the droplets, may also prevent fusion or coalescence of the droplets from occurring in some cases. 
     In one embodiment, two fluidic droplets may be given opposite electric charges (i.e., positive and negative charges, not necessarily of the same magnitude), which may increase the electrical interaction of the two droplets such that fusion or coalescence of the droplets can occur due to their opposite electric charges, e.g., using the techniques described herein. For instance, an electric field may be applied to the droplets, the droplets may be passed through a capacitor, a chemical reaction may cause the droplets to become charged, etc. As an example, as is shown schematically in  FIG. 13A , uncharged droplets  651  and  652 , carried by a liquid  654  contained within a microfluidic channel  653 , are brought into contact with each other, but the droplets are not able to fuse or coalesce, for instance, due to their size and/or surface tension. The droplets, in some cases, may not be able to fuse even if a surfactant is applied to lower the surface tension of the droplets. However, if the fluidic droplets are electrically charged with opposite charges (which can be, but are not necessarily of, the same magnitude), the droplets may be able to fuse or coalesce. For instance, in  FIG. 13B , positively charged droplets  655  and negatively charged droplets  656  are directed generally towards each other such that the electrical interaction of the oppositely charged droplets causes the droplets to fuse into fused droplets  657 . 
     In another embodiment, the fluidic droplets may not necessarily be given opposite electric charges (and, in some cases, may not be given any electric charge), and are fused through the use of dipoles induced in the fluidic droplets that causes the fluidic droplets to coalesce. In the example illustrated in  FIG. 13C , droplets  660  and  661  (which may each independently be electrically charged or neutral), surrounded by liquid  665  in channel  670 , move through the channel such that they are the affected by an applied electric field  675 . Electric field  675  may be an AC field, a DC field, etc., and may be created, for instance, using electrodes  676  and  677 , as shown here. The induced dipoles in each of the fluidic droplets, as shown in  FIG. 13C , may cause the fluidic droplets to become electrically attracted towards each other due to their local opposite charges, thus causing droplets  660  and  661  to fuse to produce droplet  663 . In  FIG. 13D , droplets  660  and  661 . 
     It should be noted that, in various embodiments, the two or more droplets allowed to coalesce are not necessarily required to meet “head-on.” Any angle of contact, so long as at least some fusion of the droplets initially occurs, is sufficient. As an example, in  FIG. 12H , droplets  73  and  74  each are traveling in substantially the same direction (e.g., at different velocities), and are able to meet and fuse. As another example, in  FIG. 12I , droplets  73  and  74  meet at an angle and fuse. In  FIG. 12J , three fluidic droplets  73 ,  74  and  68  meet and fuse to produce droplet  79 . 
     Other examples of fusing or coalescing fluidic droplets are described in International Patent Application Ser. No. PCT/US2004/010903, filed Apr. 9, 2004 by Link, et al., incorporated herein by reference. 
     Mixing within Droplets 
     In a related aspect, the invention relates to systems and methods for allowing the mixing of more than one fluid to occur within a fluidic droplet. For example, in various embodiments of the invention, two or more fluidic droplets may be allowed to fuse or coalesce, as described above, and then, within the fused droplet, the two or more fluids from the two or more original fluidic droplets may then be allowed to mix. It should be noted that when two droplets fuse or coalesce, perfect mixing within the droplet does not instantaneously occur. Instead, for example, as is shown in  FIG. 12B , the coalesced droplet may initially be formed of a first fluid region  71  (from first droplet  73 ) and a second fluid region  72  (from second droplet  74 ). Thus, in some cases, the fluid regions may remain as separate regions, for example, due to internal “counter-revolutionary” flow within the fluidic droplet (shown in  FIG. 12G  with droplet  968 , direction indicated by arrow  977 ), thus resulting in a non-uniform fluidic droplet  75 , as is shown in  FIG. 12A . 
     However, in other cases, the fluid regions within the fluidic droplet may be allowed to mix, react, or otherwise interact with each other, as is shown in  FIG. 7B , resulting in mixed or partially mixed fluidic droplet  78 . The mixing may occur through natural means, for example, through diffusion (e.g., through the interface between the regions), through reaction of the fluids with each other, through fluid flow within the droplet (i.e., convection), etc. However, in some cases, mixing of the regions  71  and  72  may be enhanced through certain systems external of the fluidic droplet. For example, the fluidic droplet may be passed through one or more channels or other systems which cause the droplet to change its velocity and/or direction of movement. The change of direction may alter convection patterns within the droplet, causing the fluids to be at least partially mixed. As an example, in  FIG. 12C , droplet  76  may be passed through one or more bends within a channel, causing the fluids within droplet  76  to be at least partially mixed, resulting in droplet  79 ; or droplet  76  may pass by one or more obstructions within the channel, etc. As another example, in  FIG. 12D , droplet  76  passes through one or more expansion regions  77  within a channel, causing the fluids within droplet  76  to be at least partially mixed, resulting in droplet  79 . In  FIG. 12E , droplet  76  passes through one or more constriction regions  69 , causing the fluids within droplet  76  to be at least partially mixed, resulting in droplet  79 . Combinations are also possible. For example, in  FIG. 12F , droplet  76  passes through bend  70 , expansion region  77 , and constriction region  69 , causing at least partial mixing of the fluid regions within the droplet to occur. As yet another example, channel  706  in  FIG. 14B  contains a series of bends, which may allow mixing of the fluids within the droplets within channel  706  to occur. 
     In one set of embodiments, a fluid may be injected into a fluidic droplet, which may cause mixing of the injected fluid with the other fluids within the fluidic droplet to occur. The fluid may be microinjected into the fluidic droplet in some cases, e.g., using a microneedle or other such device. In other cases, the fluid may be injected directly into a fluidic droplet using a fluidic channel as the fluidic droplet comes into contact with the fluidic channel. For instance, referring now to  FIG. 14C , channel  750  contains a carrier fluid  755  containing a series of fluidic droplets  760 . Droplet  761  is in contact with fluidic channel  752 . A fluid can then be introduced into fluidic droplet  761  through fluidic channel  752 , which fluid may be the same or different than the fluid in fluidic droplet  761 . 
     Other examples of fluidic mixing in droplets are described in International Patent Application Ser. No. PCT/US2004/010903, filed Apr. 9, 2004 by Link, et al., incorporated herein by reference. 
     Screening/Sorting Droplets 
     In still another aspect, the invention provides systems and methods for screening or sorting fluidic droplets in a liquid, and in some cases, at relatively high rates. For example, a characteristic of a droplet may be sensed and/or determined in some fashion (e.g., as further described below), then the droplet may be directed towards a particular region of the device, for example, for sorting or screening purposes. 
     In some embodiments, a characteristic of a fluidic droplet may be sensed and/or determined in some fashion, for example, as described herein (e.g., fluorescence of the fluidic droplet may be determined), and, in response, an electric field may be applied or removed from the fluidic droplet to direct the fluidic droplet to a particular region (e.g. a channel). In some cases, high sorting speeds may be achievable using certain systems and methods of the invention. For instance, at least about 10 droplets per second may be determined and/or sorted in some cases, and in other cases, at least about 20 droplets per second, at least about 30 droplets per second, at least about 100 droplets per second, at least about 200 droplets per second, at least about 300 droplets per second, at least about 500 droplets per second, at least about 750 droplets per second, at least about 1000 droplets per second, at least about 1500 droplets per second, at least about 2000 droplets per second, at least about 3000 droplets per second, at least about 5000 droplets per second, at least about 7500 droplets per second, at least about 10,000 droplets per second, at least about 15,000 droplets per second, at least about 20,000 droplets per second, at least about 30,000 droplets per second, at least about 50,000 droplets per second, at least about 75,000 droplets per second, at least about 100,000 droplets per second, at least about 150,000 droplets per second, at least about 200,000 droplets per second, at least about 300,000 droplets per second, at least about 500,000 droplets per second, at least about 750,000 droplets per second, at least about 1,000,000 droplets per second, at least about 1,500,000 droplets per second, at least about 2,000,000 or more droplets per second, or at least about 3,000,000 or more droplets per second may be determined and/or sorted in such a fashion. 
     In one set of embodiments, a fluidic droplet may be directed by creating an electric charge (e.g., as previously described) on the droplet, and steering the droplet using an applied electric field, which may be an AC field, a DC field, etc. As an example, in reference to  FIGS. 2-4 , an electric field may be selectively applied and removed (or a different electric field may be applied, e.g., a reversed electric field as shown in  FIG. 4A ) as needed to direct the fluidic droplet to a particular region. The electric field may be selectively applied and removed as needed, in some embodiments, without substantially altering the flow of the liquid containing the fluidic droplet. For example, a liquid may flow on a substantially steady-state basis (i.e., the average flowrate of the liquid containing the fluidic droplet deviates by less than 20% or less than 15% of the steady-state flow or the expected value of the flow of liquid with respect to time, and in some cases, the average flowrate may deviate less than 10% or less than 5%) or other predetermined basis through a fluidic system of the invention (e.g., through a channel or a microchannel), and fluidic droplets contained within the liquid may be directed to various regions, e.g., using an electric field, without substantially altering the flow of the liquid through the fluidic system. As a particular example, in  FIGS. 2A, 3A and 4A , a liquid containing fluidic droplets  15  flows from fluid source  10 , through channel  30  to intersection  40 , and exits through channels  50  and  55 . In  FIG. 2A , fluidic droplets  15  are directed through both channels  50  and  55 , while in  FIG. 3A , fluidic droplets  15  are directed to only channel  55  and, in  FIG. 4A , fluidic droplets  15  are directed to only channel  50 . 
     In another set of embodiments, a fluidic droplet may be sorted or steered by inducing a dipole in the fluidic droplet (which may be initially charged or uncharged), and sorting or steering the droplet using an applied electric field. The electric field may be an AC field, a DC field, etc. For example, with reference to  FIG. 9A , a channel  540 , containing fluidic droplet  530  and liquid  535 , divides into channel  542  and  544 . Fluidic droplet  530  may have an electric charge, or it may be uncharged. Electrode  526  is positioned near channel  542 , while electrode  527  is positioned near channel  544 . Electrode  528  is positioned near the junction of channels  540 ,  542 , and  544 . In  FIGS. 9C and 9D , a dipole is induced in the fluidic droplet using electrodes  526 ,  527 , and/or  528 . In  FIG. 9C , a dipole is induced in droplet  530  by applying an electric field  525  to the droplet using electrodes  527  and  528 . Due to the strength of the electric field, the droplet is strongly attracted to the right, into channel  544 . Similarly, in  FIG. 9D , a dipole is induced in droplet  530  by applying an electric field  525  to the droplet using electrodes  526  and  528 , causing the droplet to be attracted into channel  542 . Thus, by applying the proper electric field, droplet  530  can be directed to either channel  542  or  544  as desired. 
     In other embodiments, however, the fluidic droplets may be screened or sorted within a fluidic system of the invention by altering the flow of the liquid containing the droplets. For instance, in one set of embodiments, a fluidic droplet may be steered or sorted by directing the liquid surrounding the fluidic droplet into a first channel, a second channel, etc. As a non-limiting example, with reference to  FIG. 10A , fluidic droplet  570  is surrounded by a liquid  575  in channel  580 . Channel  580  divides into three channels  581 ,  582 , and  583 . The flow of liquid  575  can be directed into any of channels  581 ,  582 , and  583  as desired, for example, using flow-controlling devices known to those of ordinary skill in the art, for example, valves, pumps, pistons, etc. Thus, in  FIG. 10B , fluidic droplet  570  is directed into channel  581  by directing liquid  575  to flow into channel  581  (indicated by arrows  574 ); in  FIG. 10C , fluidic droplet  570  is directed into channel  582  by directing liquid  575  to flow into channel  582  (indicated by arrows  574 ); and in  FIG. 10D , fluidic droplet  570  is directed into channel  583  by directing liquid  575  to flow into channel  583  (indicated by arrows  574 ). 
     In another set of embodiments, pressure within a fluidic system, for example, within different channels or within different portions of a channel, can be controlled to direct the flow of fluidic droplets. For example, a droplet can be directed toward a channel junction including multiple options for further direction of flow (e.g., directed toward a branch, or fork, in a channel defining optional downstream flow channels). Pressure within one or more of the optional downstream flow channels can be controlled to direct the droplet selectively into one of the channels, and changes in pressure can be effected on the order of the time required for successive droplets to reach the junction, such that the downstream flow path of each successive droplet can be independently controlled. In one arrangement, the expansion and/or contraction of liquid reservoirs may be used to steer or sort a fluidic droplet into a channel, e.g., by causing directed movement of the liquid containing the fluidic droplet. The liquid reservoirs may be positioned such that, when activated, the movement of liquid caused by the activated reservoirs causes the liquid to flow in a preferred direction, carrying the fluidic droplet in that preferred direction. For instance, the expansion of a liquid reservoir may cause a flow of liquid towards the reservoir, while the contraction of a liquid reservoir may cause a flow of liquid away from the reservoir. In some cases, the expansion and/or contraction of the liquid reservoir may be combined with other flow-controlling devices and methods, e.g., as described herein. Non-limiting examples of devices able to cause the expansion and/or contraction of a liquid reservoir include pistons and piezoelectric components. In some cases, piezoelectric components may be particularly useful due to their relatively rapid response times, e.g., in response to an electrical signal. 
     As a non-limiting example, in  FIG. 11A , fluidic droplet  600  is surrounded by a liquid  605  in channel  610 . Channel  610  divides into channels  611 ,  612 . Positioned in fluidic communication with channels  611  and  612  are liquid reservoirs  617  and  618 , which may be expanded and/or contracted, for instance, by piezoelectric components  615  and  616 , by a piston (not shown), etc. In  FIG. 11B , liquid reservoir  617  has been expanded, while liquid reservoir  618  has been contracted. The effect of the expansion/contractions of the reservoirs is to cause a net flow of liquid towards channel  611 , as indicated by arrows  603 . Thus, fluidic droplet  600 , upon reaching the junction between the channels, is directed to channel  611  by the movement of liquid  605 . The reverse situation is shown in  FIG. 11C , where liquid reservoir  617  has contracted while liquid reservoir  618  has been expanded. A net flow of liquid occurs towards channel  612  (indicated by arrows  603 ), causing fluidic droplet  600  to move into channel  612 . It should be noted, however, that reservoirs  617  and  618  do not both need to be activated to direct fluidic droplet  600  into channels  611  or  612 . For example, in one embodiment, fluidic droplet  600  may be directed to channel  611  by the expansion of liquid reservoir  617  (without any alteration of reservoir  618 ), while in another embodiment, fluidic droplet  600  may be directed to channel  611  by the contraction of liquid reservoir  618  (without any alteration of reservoir  617 ). In some cases, more than two liquid reservoirs may be used. 
     In some embodiments, the fluidic droplets may be sorted into more than two channels. Non-limiting examples of embodiments of the invention having multiple regions within a fluidic system for the delivery of droplets are shown in  FIGS. 6A and 6B . Other arrangements are shown in  FIGS. 10A-10D . In  FIG. 6A , charged droplets  315  in channel  330  may be directed as desired to any one of exit channels  350 ,  352 ,  354 , or  356 , by applying electric fields to control the movement of the droplets at intersections  340 ,  341 , and  342 , using electrodes  321 / 322 ,  323 / 324 , and  325 / 326 , respectively. In  FIG. 6A , droplets  315  are directed to channel  354  using applied electric fields  300  and  301 , using principles similar to those discussed above. Similarly, in  FIG. 6B , charged droplets  415  in channel  430  can be directed to any one of exit channels  450 ,  452 ,  454 ,  456 , or  458 , by applying electric fields to control the movement of the droplets at intersections  440 ,  441 ,  442 , and  443 , using electrodes  421 / 422 ,  423 / 424 ,  425 / 426 , and  427 / 428 , respectively. In this figure, droplets  415  are directed to channel  454 ; of course, the charged droplets may be directed to any other exit channel as desired. 
     In another example, in apparatus  5 , as schematically illustrated in  FIG. 2A , fluidic droplets  15  created by fluid source  10  are positively charged due to an applied electric field created using electric field generator  20 , which comprises two electrodes  22 ,  24 . Fluidic droplets  15  are directed through channel  30  by a liquid containing the droplets, and are directed towards intersection  40 . At intersection  40 , the fluidic droplets do not have a preferred orientation or direction, and move into exit channels  50  and  55  with equal probability (in this embodiment, liquid drains through both exit channels  50  and  55  at substantially equal rates). Similarly, fluidic droplets  115  created by fluid source  110  are negatively charged due to an applied electric field created using electric field generator  120 , which comprises electrodes  122  and  124 . After traveling through channel  130  towards intersection  140 , the fluidic droplets do not have a preferred orientation or direction, and move into exit channels  150  and  155  with equal probability, as the liquid exits through exit channels  150  and  155  at substantially equal rates. A representative photomicrograph of intersection  140  is shown in  FIG. 2B . 
     In the schematic diagram of  FIG. 3A , an electric field  100  of 1.4 V/micrometer has been applied to apparatus  5  of  FIG. 2A , in a direction towards the right of apparatus  5 . Positively-charged fluidic droplets  15  in channel  30 , upon reaching intersection  40 , are directed to the right in channel  55  due to the applied electric field  100 , while the liquid containing the droplets continues to exit through exit channels  50  and  55  at substantially equal rates. Similarly, negatively-charged fluidic droplets  115  in channel  130 , upon reaching intersection  140 , are directed to the left in channel  150  due to the applied electric field  100 , while the liquid fluid continues to exit the device through exit channels  150  and  155  at substantially equal rates. Thus, electric field  100  can be used to direct fluidic droplets into particular channels as desired. A representative photomicrograph of intersection  140  is shown in  FIG. 3B . 
       FIG. 4A  is a schematic diagram of apparatus  5  of  FIG. 2A , also with an applied electric field  100  of 1.4 V/micrometer, but in the opposite direction (i.e., −1.4 V/micrometer). In this figure, positively-charged fluidic droplets  15  in channel  30 , upon reaching intersection  40 , are directed to the left into channel  50  due to the applied electric field  100 , while negatively-charged fluidic droplets  115  in channel  130 , upon reaching intersection  140 , are directed to the right into channel  155  due to applied electric field  100 . The liquid containing the droplets exits through exit channels  50  and  55 , and  150  and  155 , at substantially equal rates. A representative photomicrograph of intersection  140  is shown in  FIG. 4B . 
     In some embodiments of the invention, a fluidic droplet may be sorted and/or split into two or more separate droplets, for example, depending on the particular application. Any of the above-described techniques may be used to spilt and/or sort droplets. As a non-limiting example, by applying (or removing) a first electric field to a device (or a portion thereof), a fluidic droplet may be directed to a first region or channel; by applying (or removing) a second electric field to the device (or a portion thereof), the droplet may be directed to a second region or channel; by applying a third electric field to the device (or a portion thereof), the droplet may be directed to a third region or channel; etc., where the electric fields may differ in some way, for example, in intensity, direction, frequency, duration, etc. In a series of droplets, each droplet may be independently sorted and/or split; for example, some droplets may be directed to one location or another, while other droplets may be split into multiple droplets directed to two or more locations. 
     As one particular example, in  FIG. 8A , fluidic droplet  550 , surrounding liquid  555  in channel  560  may be directed to channel  556 , channel  557 , or be split in some fashion between channels  562  and  564 . In  FIG. 8B , by directing surrounding liquid  555  towards channel  562 , fluidic droplet  550  may be directed towards the left into channel  562 ; in  FIG. 8C , by directing surrounding liquid  555  towards channel  564 , fluidic droplet  550  may be directed towards the right into channel  564 , In  FIG. 8D , an electric field may be applied, in combination with control of the flow of liquid  555  surrounding fluidic droplet  550 , that causes the droplet to impact junction  561 , which may cause the droplet to split into two separate fluidic droplets  565 ,  566 . Fluidic droplet  565  is directed to channel  562 , while fluidic droplet  566  is directed to channel  566 . A high degree of control of the applied electric field may be achieved to control droplet formation; thus, for example, after fluidic droplet  565  has been split into droplets  565  and  566 , droplets  565  and  566  may be of substantially equal size, or either of droplets  565  and  566  may be larger, e.g., as is shown in  FIGS. 8E and 8F , respectively. 
     As another example, in  FIG. 9A , channel  540 , carrying fluidic droplet  530  and liquid  535 , divides into channel  542  and  544 . Fluidic droplet  530  may be electrically charged, or it may uncharged. Electrode  526  is positioned near channel  542 , while electrode  527  is positioned near channel  544 . Electrode  528  is positioned near the junction of channels  540 ,  542 , and  544 . When fluidic droplet  530  reaches the junction, it may be subjected to an electric field, and/or directed to a channel or other region, for example, by directing the surrounding liquid into the channel. As shown in  FIG. 9B , fluidic droplet  530  may be split into two separate droplets  565  and  566  by applying an electric field  525  to the droplet using electrodes  526  and  527 . In  FIG. 9C , a dipole can be induced in droplet  530  by applying an electric field  525  to the droplet using electrodes  527  and  528 . Due to the strength of the applied electric field, the droplet may be strongly attracted to the right, into channel  544 . Similarly, in  FIG. 9D , a dipole may be induced in droplet  530  by applying an electric field  525  to the droplet using electrodes  526  and  528 , causing the droplet to be attracted into channel  542 . By controlling which electrodes are used to induce an electric field across droplet  530 , and/or the strength of the applied electric field, one or more fluidic droplets within channel  540  may be sorted and/or split into two droplets, and each droplet may independently be sorted and/or split. 
     Sensing Droplets; Sensing the Content of Droplets 
     In certain aspects of the invention, sensors are provided that can sense and/or determine one or more characteristics of the fluidic droplets, and/or a characteristic of a portion of the fluidic system containing the fluidic droplet (e.g., the liquid surrounding the fluidic droplet) in such a manner as to allow the determination of one or more characteristics of the fluidic droplets. Characteristics determinable with respect to the droplet and usable in the invention can be identified by those of ordinary skill in the art. Non-limiting examples of such characteristics include fluorescence, spectroscopy (e.g., optical, infrared, ultraviolet, etc.), radioactivity, mass, volume, density, temperature, viscosity, pH, concentration of a substance, such as a biological substance (e.g., a protein, a nucleic acid, etc.), or the like. 
     In some cases, the sensor may be connected to a processor, which in turn, cause an operation to be performed on the fluidic droplet, for example, by sorting the droplet, adding or removing electric charge from the droplet, fusing the droplet with another droplet, splitting the droplet, causing mixing to occur within the droplet, etc., for example, as previously described. For instance, in response to a sensor measurement of a fluidic droplet, a processor may cause the fluidic droplet to be split, merged with a second fluidic droplet, etc. 
     One or more sensors and/or processors may be positioned to be in sensing communication with the fluidic droplet. “Sensing communication,” as used herein, means that the sensor may be positioned anywhere such that the fluidic droplet within the fluidic system (e.g., within a channel), and/or a portion of the fluidic system containing the fluidic droplet may be sensed and/or determined in some fashion. For example, the sensor may be in sensing communication with the fluidic droplet and/or the portion of the fluidic system containing the fluidic droplet fluidly, optically or visually, thermally, pneumatically, electronically, or the like. The sensor can be positioned proximate the fluidic system, for example, embedded within or integrally connected to a wall of a channel, or positioned separately from the fluidic system but with physical, electrical, and/or optical communication with the fluidic system so as to be able to sense and/or determine the fluidic droplet and/or a portion of the fluidic system containing the fluidic droplet (e.g., a channel or a microchannel, a liquid containing the fluidic droplet, etc.). For example, a sensor may be free of any physical connection with a channel containing a droplet, but may be positioned so as to detect electromagnetic radiation arising from the droplet or the fluidic system, such as infrared, ultraviolet, or visible light. The electromagnetic radiation may be produced by the droplet, and/or may arise from other portions of the fluidic system (or externally of the fluidic system) and interact with the fluidic droplet and/or the portion of the fluidic system containing the fluidic droplet in such as a manner as to indicate one or more characteristics of the fluidic droplet, for example, through absorption, reflection, diffraction, refraction, fluorescence, phosphorescence, changes in polarity, phase changes, changes with respect to time, etc. As an example, a laser may be directed towards the fluidic droplet and/or the liquid surrounding the fluidic droplet, and the fluorescence of the fluidic droplet and/or the surrounding liquid may be determined. “Sensing communication,” as used herein may also be direct or indirect. As an example, light from the fluidic droplet may be directed to a sensor, or directed first through a fiber optic system, a waveguide, etc., before being directed to a sensor. 
     Non-limiting examples of sensors useful in the invention include optical or electromagnetically-based systems. For example, the sensor may be a fluorescence sensor (e.g., stimulated by a laser), a microscopy system (which may include a camera or other recording device), or the like. As another example, the sensor may be an electronic sensor, e.g., a sensor able to determine an electric field or other electrical characteristic. For example, the sensor may detect capacitance, inductance, etc., of a fluidic droplet and/or the portion of the fluidic system containing the fluidic droplet. 
     As used herein, a “processor” or a “microprocessor” is any component or device able to receive a signal from one or more sensors, store the signal, and/or direct one or more responses (e.g., as described above), for example, by using a mathematical formula or an electronic or computational circuit. The signal may be any suitable signal indicative of the environmental factor determined by the sensor, for example a pneumatic signal, an electronic signal, an optical signal, a mechanical signal, etc. 
     As a particular non-limiting example, a device of the invention may contain fluidic droplets containing one or more cells. The cells may be exposed to a fluorescent signal marker that binds if a certain condition is present, for example, the marker may bind to a first cell type but not a second cell type, the marker may bind to an expressed protein, the marker may indicate viability of the cell (i.e., if the cell is alive or dead), the marker may be indicative of the state of development or differentiation of the cell, etc., and the cells may be directed through a fluidic system of the invention based on the presence/absence, and/or magnitude of the fluorescent signal marker. For instance, determination of the fluorescent signal marker may cause the cells to be directed to one region of the device (e.g., a collection chamber), while the absence of the fluorescent signal marker may cause the cells to be directed to another region of the device (e.g., a waste chamber). Thus, in this example, a population of cells may be screened and/or sorted on the basis of one or more determinable or targetable characteristics of the cells, for example, to select live cells, cells expressing a certain protein, a certain cell type, etc. 
     Definitions 
     A variety of definitions are now provided which will aid in understanding various aspects of the invention. Following, and interspersed with these definitions, is further disclosure that will more fully describe the invention. As noted, various aspects of the present invention relate to droplets of fluid surrounded by a liquid (e.g., suspended). The droplets may be of substantially the same shape and/or size, or of different shapes and/or sizes, depending on the particular application. As used herein, the term “fluid” generally refers to a substance that tends to flow and to conform to the outline of its container, i.e., a liquid, a gas, a viscoelastic fluid, etc. Typically, fluids are materials that are unable to withstand a static shear stress, and when a shear stress is applied, the fluid experiences a continuing and permanent distortion. The fluid may have any suitable viscosity that permits flow. If two or more fluids are present, each fluid may be independently selected among essentially any fluids (liquids, gases, and the like) by those of ordinary skill in the art, by considering the relationship between the fluids. The fluids may each be miscible or immiscible. For example, two fluids can be selected to be essentially immiscible within the time frame of formation of a stream of fluids, or within the time frame of reaction or interaction. Where the portions remain liquid for a significant period of time, then the fluids should be essentially immiscible. Where, after contact and/or formation, the dispersed portions are quickly hardened by polymerization or the like, the fluids need not be as immiscible. Those of ordinary skill in the art can select suitable miscible or immiscible fluids, using contact angle measurements or the like, to carry out the techniques of the invention. 
     As used herein, a first entity is “surrounded” by a second entity if a closed planar loop can be drawn around the first entity through only the second entity. A first entity is “completely surrounded” if closed loops going through only the second entity can be drawn around the first entity regardless of direction (orientation of the loop). In one embodiment, the first entity is a cell, for example, a cell suspended in media is surrounded by the media. In another embodiment, the first entity is a particle. In yet another embodiment, the first entity is a fluid. The second entity may also be a fluid in some cases (e.g., as in a suspension, an emulsion, etc.), for example, a hydrophilic liquid may be suspended in a hydrophobic liquid, a hydrophobic liquid may be suspended in a hydrophilic liquid, a gas bubble may be suspended in a liquid, etc. Typically, a hydrophobic liquid and a hydrophilic liquid are essentially immiscible with respect to each other, where the hydrophilic liquid has a greater affinity to water than does the hydrophobic liquid. Examples of hydrophilic liquids include, but are not limited to, water and other aqueous solutions comprising water, such as cell or biological media, salt solutions, etc., as well as other hydrophilic liquids such as ethanol. Examples of hydrophobic liquids include, but are not limited to, oils such as hydrocarbons, silicone oils, mineral oils, fluorocarbon oils, organic solvents etc. Other examples of suitable fluids have been previously described. 
     Similarly, a “droplet,” as used herein, is an isolated portion of a first fluid that is completely surrounded by a second fluid. It is to be noted that a droplet is not necessarily spherical, but may assume other shapes as well, for example, depending on the external environment. In one embodiment, the droplet has a minimum cross-sectional dimension that is substantially equal to the largest dimension of the channel perpendicular to fluid flow in which the droplet is located. 
     As mentioned, in some, but not all embodiments, the systems and methods described herein may include one or more microfluidic components, for example, one or more microfluidic channels. “Microfluidic,” as used herein, refers to a device, apparatus or system including at least one fluid channel having a cross-sectional dimension of less than 1 mm, and a ratio of length to largest cross-sectional dimension of at least 3:1. A “microfluidic channel,” as used herein, is a channel meeting these criteria. The “cross-sectional dimension” of the channel is measured perpendicular to the direction of fluid flow within the channel. Thus, some or all of the fluid channels in microfluidic embodiments of the invention may have maximum cross-sectional dimensions less than 2 mm, and in certain cases, less than 1 mm. In one set of embodiments, all fluid channels containing embodiments of the invention are microfluidic or have a largest cross sectional dimension of no more than 2 mm or 1 mm. In certain embodiments, the fluid channels may be formed in part by a single component (e.g. an etched substrate or molded unit). Of course, larger channels, tubes, chambers, reservoirs, etc. can be used to store fluids and/or deliver fluids to various components or systems of the invention. In one set of embodiments, the maximum cross-sectional dimension of the channel(s) containing embodiments of the invention is less than 500 microns, less than 200 microns, less than 100 microns, less than 50 microns, or less than 25 microns. 
     A “channel,” as used herein, means a feature on or in an article (substrate) that at least partially directs flow of a fluid. The channel can have any cross-sectional shape (circular, oval, triangular, irregular, square or rectangular, or the like) and can be covered or uncovered. In embodiments where it is completely covered, at least one portion of the channel can have a cross-section that is completely enclosed, or the entire channel may be completely enclosed along its entire length with the exception of its inlet(s) and/or outlet(s). A channel may also have an aspect ratio (length to average cross sectional dimension) of at least 2:1, more typically at least 3:1, 5:1, 10:1, 15:1, 20:1, or more. An open channel generally will include characteristics that facilitate control over fluid transport, e.g., structural characteristics (an elongated indentation) and/or physical or chemical characteristics (hydrophobicity vs. hydrophilicity) or other characteristics that can exert a force (e.g., a containing force) on a fluid. The fluid within the channel may partially or completely fill the channel. In some cases where an open channel is used, the fluid may be held within the channel, for example, using surface tension (i.e., a concave or convex meniscus). 
     The channel may be of any size, for example, having a largest dimension perpendicular to fluid flow of less than about 5 mm or 2 mm, or less than about 1 mm, or less than about 500 microns, less than about 200 microns, less than about 100 microns, less than about 60 microns, less than about 50 microns, less than about 40 microns, less than about 30 microns, less than about 25 microns, less than about 10 microns, less than about 3 microns, less than about 1 micron, less than about 300 nm, less than about 100 nm, less than about 30 nm, or less than about 10 nm. In some cases the dimensions of the channel may be chosen such that fluid is able to freely flow through the article or substrate. The dimensions of the channel may also be chosen, for example, to allow a certain volumetric or linear flowrate of fluid in the channel. Of course, the number of channels and the shape of the channels can be varied by any method known to those of ordinary skill in the art. In some cases, more than one channel or capillary may be used. For example, two or more channels may be used, where they are positioned inside each other, positioned adjacent to each other, positioned to intersect with each other, etc. 
     In one set of embodiments, the fluidic droplets may contain cells or other entities, such as proteins, viruses, macromolecules, particles, etc. As used herein, a “cell” is given its ordinary meaning as used in biology. The cell may be any cell or cell type. For example, the cell may be a bacterium or other single-cell organism, a plant cell, or an animal cell. If the cell is a single-cell organism, then the cell may be, for example, a protozoan, a trypanosome, an amoeba, a yeast cell, algae, etc. If the cell is an animal cell, the cell may be, for example, an invertebrate cell (e.g., a cell from a fruit fly), a fish cell (e.g., a zebrafish cell), an amphibian cell (e.g., a frog cell), a reptile cell, a bird cell, or a mammalian cell such as a primate cell, a bovine cell, a horse cell, a porcine cell, a goat cell, a dog cell, a cat cell, or a cell from a rodent such as a rat or a mouse. If the cell is from a multicellular organism, the cell may be from any part of the organism. For instance, if the cell is from an animal, the cell may be a cardiac cell, a fibroblast, a keratinocyte, a heptaocyte, a chondracyte, a neural cell, a osteocyte, a muscle cell, a blood cell, an endothelial cell, an immune cell (e.g., a T-cell, a B-cell, a macrophage, a neutrophil, a basophil, a mast cell, an eosinophil), a stem cell, etc. In some cases, the cell may be a genetically engineered cell. In certain embodiments, the cell may be a Chinese hamster ovarian (“CHO”) cell or a 3T3 cell. 
     Materials 
     A variety of materials and methods, according to certain aspects of the invention, can be used to form any of the above-described components of the systems and devices of the invention. In some cases, the various materials selected lend themselves to various methods. For example, various components of the invention can be formed from solid materials, in which the channels can be formed via micromachining, film deposition processes such as spin coating and chemical vapor deposition, laser fabrication, photolithographic techniques, etching methods including wet chemical or plasma processes, and the like. See, for example, Scientific American, 248:44-55, 1983 (Angell, et al). In one embodiment, at least a portion of the fluidic system is formed of silicon by etching features in a silicon chip. Technologies for precise and efficient fabrication of various fluidic systems and devices of the invention from silicon are known. In another embodiment, various components of the systems and devices of the invention can be formed of a polymer, for example, an elastomeric polymer such as polydimethylsiloxane (“PDMS”), polytetrafluoroethylene (“PTFE” or Teflon®), or the like. 
     Different components can be fabricated of different materials. For example, a base portion including a bottom wall and side walls can be fabricated from an opaque material such as silicon or PDMS, and a top portion can be fabricated from a transparent or at least partially transparent material, such as glass or a transparent polymer, for observation and/or control of the fluidic process. Components can be coated so as to expose a desired chemical functionality to fluids that contact interior channel walls, where the base supporting material does not have a precise, desired functionality. For example, components can be fabricated as illustrated, with interior channel walls coated with another material. Material used to fabricate various components of the systems and devices of the invention, e.g., materials used to coat interior walls of fluid channels, may desirably be selected from among those materials that will not adversely affect or be affected by fluid flowing through the fluidic system, e.g., material(s) that is chemically inert in the presence of fluids to be used within the device. 
     In one embodiment, various components of the invention are fabricated from polymeric and/or flexible and/or elastomeric materials, and can be conveniently formed of a hardenable fluid, facilitating fabrication via molding (e.g. replica molding, injection molding, cast molding, etc.). The hardenable fluid can be essentially any fluid that can be induced to solidify, or that spontaneously solidifies, into a solid capable of containing and/or transporting fluids contemplated for use in and with the fluidic network. In one embodiment, the hardenable fluid comprises a polymeric liquid or a liquid polymeric precursor (i.e. a “prepolymer”). Suitable polymeric liquids can include, for example, thermoplastic polymers, thermoset polymers, or mixture of such polymers heated above their melting point. As another example, a suitable polymeric liquid may include a solution of one or more polymers in a suitable solvent, which solution forms a solid polymeric material upon removal of the solvent, for example, by evaporation. Such polymeric materials, which can be solidified from, for example, a melt state or by solvent evaporation, are well known to those of ordinary skill in the art. A variety of polymeric materials, many of which are elastomeric, are suitable, and are also suitable for forming molds or mold masters, for embodiments where one or both of the mold masters is composed of an elastomeric material. A non-limiting list of examples of such polymers includes polymers of the general classes of silicone polymers, epoxy polymers, and acrylate polymers. Epoxy polymers are characterized by the presence of a three-membered cyclic ether group commonly referred to as an epoxy group, 1,2-epoxide, or oxirane. For example, diglycidyl ethers of bisphenol A can be used, in addition to compounds based on aromatic amine, triazine, and cycloaliphatic backbones. Another example includes the well-known Novolac polymers. Non-limiting examples of silicone elastomers suitable for use according to the invention include those formed from precursors including the chlorosilanes such as methylchlorosilanes, ethylchlorosilanes, phenylchlorosilanes, etc. 
     Silicone polymers are preferred in one set of embodiments, for example, the silicone elastomer polydimethylsiloxane. Non-limiting examples of PDMS polymers include those sold under the trademark Sylgard by Dow Chemical Co., Midland, Mich., and particularly Sylgard 182, Sylgard 184, and Sylgard 186. Silicone polymers including PDMS have several beneficial properties simplifying fabrication of the microfluidic structures of the invention. For instance, such materials are inexpensive, readily available, and can be solidified from a prepolymeric liquid via curing with heat. For example, PDMSs are typically curable by exposure of the prepolymeric liquid to temperatures of about, for example, about 65° C. to about 75° C. for exposure times of, for example, about an hour. Also, silicone polymers, such as PDMS, can be elastomeric and thus may be useful for forming very small features with relatively high aspect ratios, necessary in certain embodiments of the invention. Flexible (e.g., elastomeric) molds or masters can be advantageous in this regard. 
     One advantage of forming structures such as microfluidic structures of the invention from silicone polymers, such as PDMS, is the ability of such polymers to be oxidized, for example by exposure to an oxygen-containing plasma such as an air plasma, so that the oxidized structures contain, at their surface, chemical groups capable of cross-linking to other oxidized silicone polymer surfaces or to the oxidized surfaces of a variety of other polymeric and non-polymeric materials. Thus, components can be fabricated and then oxidized and essentially irreversibly sealed to other silicone polymer surfaces, or to the surfaces of other substrates reactive with the oxidized silicone polymer surfaces, without the need for separate adhesives or other sealing means. In most cases, sealing can be completed simply by contacting an oxidized silicone surface to another surface without the need to apply auxiliary pressure to form the seal. That is, the pre-oxidized silicone surface acts as a contact adhesive against suitable mating surfaces. Specifically, in addition to being irreversibly sealable to itself, oxidized silicone such as oxidized PDMS can also be sealed irreversibly to a range of oxidized materials other than itself including, for example, glass, silicon, silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, glassy carbon, and epoxy polymers, which have been oxidized in a similar fashion to the PDMS surface (for example, via exposure to an oxygen-containing plasma). Oxidation and sealing methods useful in the context of the present invention, as well as overall molding techniques, are described in the art, for example, in an article entitled “Rapid Prototyping of Microfluidic Systems and Polydimethylsiloxane,” Anal. Chem., 70:474-480, 1998 (Duffy et al.), incorporated herein by reference. 
     Another advantage to forming microfluidic structures of the invention (or interior, fluid-contacting surfaces) from oxidized silicone polymers is that these surfaces can be much more hydrophilic than the surfaces of typical elastomeric polymers (where a hydrophilic interior surface is desired). Such hydrophilic channel surfaces can thus be more easily filled and wetted with aqueous solutions than can structures comprised of typical, unoxidized elastomeric polymers or other hydrophobic materials. 
     In one embodiment, a bottom wall is formed of a material different from one or more side walls or a top wall, or other components. For example, the interior surface of a bottom wall can comprise the surface of a silicon wafer or microchip, or other substrate. Other components can, as described above, be sealed to such alternative substrates. Where it is desired to seal a component comprising a silicone polymer (e.g. PDMS) to a substrate (bottom wall) of different material, the substrate may be selected from the group of materials to which oxidized silicone polymer is able to irreversibly seal (e.g., glass, silicon, silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, epoxy polymers, and glassy carbon surfaces which have been oxidized). Alternatively, other sealing techniques can be used, as would be apparent to those of ordinary skill in the art, including, but not limited to, the use of separate adhesives, thermal bonding, solvent bonding, ultrasonic welding, etc. 
     The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention. 
     Example 1 
       FIG. 15A  illustrates an example of a fluidic system having some of the features described above. In this example, fluidic droplets are introduced into fluidic system  800  (shown schematically in  FIG. 15A . Fluidic system  800  comprises an inlet region  801  (labeled “a”), a bifurcation region  802  (“b”), a region comprising posts  803  (“c”), and a collection region  804  (“d”). Inlet region  801  produces a series of droplets  810  contained within liquid  815 . The droplets have an average diameter of about 20 micrometers. The liquid, in this example, is water, and the fluidic droplets comprise hexadecane with about 3 wt % SPAN80 (a surfactant). The series of droplets are illustrated in  FIG. 15B , which is an enlargement of inlet region  801  shown schematically in  FIG. 15A . 
     In  FIG. 15C , a series of bifurcations (shown to the left of the photomicrograph) divide the fluidic droplets into a series of channels. Since the droplets are not purposefully directed towards any particular channel, the droplets are randomly dispersed within the channels. The droplets are then carried to a region comprising a series of posts  818 . An enlargement of this region is illustrated in  FIG. 15D . As can be seen, the individual droplets maintain their individual identities and do not fuse, due to the presence of the surfactant. In  FIG. 15E , the droplets are collected in collection region  804 . 
     Example 2 
     In this example, precision manipulation of streams of fluids with a microfluidic device is demonstrated. This technology enables high throughput reactors that use minute quantities of reagents. As the scale of these reactors shrinks, contamination effects due to surface adsorption and diffusion may restrict the smallest quantities that can be used. Confinement of reagents in droplets in an immiscible carrier fluid may overcome these limitations, but demands new fluid-handling technology. An example platform technology is presented based on charged droplets and electric fields that enables electrically addressable droplet generation, highly efficient droplet coalescence, precision droplet breaking and recharging, and controllable droplet sorting. 
     In this example, a generic and robust platform technology is presented for manipulating and controlling individual droplets in a microfluidic device. By combining electrostatic charge on the droplets and electric fields on the devices modules that create, recombine, split, and sort droplets individually are illustrated, providing exquisite control over individual microreactors while retaining high purity and enabling very high throughput. By incorporating the forces that result from charging the aqueous fluid in an electric field, E, smaller droplets are produced with more precise control of their individual timing than is feasible with other strategies that rely solely on viscous forces to overcome surface tension; this provides a robust droplet generation module that allows the production of microreactors with volumes as small as femtoliters. Incorporating charge of opposite signs on different droplets allows droplets to be controllably and reliably merged, overcoming the stabilizing forces due to surface tension and lubrication; this provides a robust droplet coalescence module that allows the precise mixing of aliquots of reactants. By incorporating the extensional force induced by an electric field large droplets may be controllably split into smaller aliquots for further analysis, and in some cases, it may simultaneously recharge neutral droplets for further processing; this provides a robust splitting or charging module that allows multiple assays to be performed on the same materials. By incorporating the forces produced by electric fields on charged droplets, individual droplets may be steered into selected channels; this provides a robust droplet sorting module that allows desired reaction products to be selected. These modules are useful for high speed manipulation and control of individual droplets, and can serve as the technology for droplet-based microfluidic devices. Moreover, because all control is achieved by switching electric fields, there are no moving parts and frequencies as high as 106 Hz are feasible; this facilitates very high throughput combinatorial technology. 
     Soft lithography was used to pattern channels in polydimethylsiloxane (PDMS), a transparent polymer material. A glass slide forms the top of the channel. Electric fields were incorporated by patterning indium-tin-oxide (ITO) electrodes on the surface of the glass slide adjacent to the channels and seal the slide to the PDMS using an oxygen plasma. Devices fabricated in PDMS have the advantage of being strongly hydrophobic ensuring that the oil carrier phase wets their surfaces and that the water droplets do not contact the walls of the channel walls, facilitating the isolation of biomolecules and eliminating cross contamination due to surface interactions. 
     A flow-focusing geometry was used to form the droplets. A water stream was infused from one channel through a narrow constriction; counter propagating oil streams hydrodynamically focus the water stream reducing its size as it passes through the constriction. This droplet generator can be operated in a flow regime that produces a steady stream of uniform droplets of water in oil. The size of the water droplets was controlled by the relative flow rates of the oil and the water; the viscous forces overcome surface tension to create uniform drops. If the flow rate of the water was too high a longer jet of fluid passes through the orifice and breaks up into droplets further downstream; these droplets were less uniform in size. If the flow rate of the water was too low, the droplet breakup in the orifice becomes irregular, producing a wider range of droplet sizes. 
     Electric fields were then incorporated to create an electrically addressable emulsification system. To achieve this, high voltage was applied to the aqueous stream and charge the oil water interface. The water stream behaved as a conductor while the oil was an insulator; electrochemical reactions charged the fluid interface like a capacitor. At snap-off, the charge on the interface remains on the droplet. In addition, the droplet volume, Vd, and frequency, f, could be tailored over at least three orders of magnitude without changing the infusion rate of the oil or water. Droplet size and frequency were not independent in this example; instead their product is determined by the infusion rate of the dispersed phase Qd=fVd. The droplet size decreased with increasing field strength. At low field strength, the droplet size was determined by the flowrate of the continuous phase. However, at high field strength, droplet size was determined by the electric field and decreased rapidly with E. 
     The dependence of the droplet size on applied voltage for three different flow rates is as follows. At low applied voltages the electric field had a negligible effect, and droplet formation was driven by the competition between surface tension and viscous flow. By contrast, at high electric field strengths, there was a significant additional force on the growing drop, F=qE, where q is the charge on the droplet. Since the droplet interface behaved as a capacitor, q is proportional to the applied voltage, V. This led to a V 2 dependence of the force, which accounted for the decrease in droplet size with increasing applied field. For even higher electric fields, the charged interface of the water stream was repelled by the charged drops. 
     In one embodiment, oil and water streams converge at 30 micron orifice. A voltage V applied to indium-tin-oxide (ITO) electrodes on the glass produced an electric field E to capacitively charge the aqueous-oil interface. Droplet size was found to be independent of charge at low field strengths but decreased at higher fields. Droplet size is a function of voltage, showing the crossover between flow-dominated and field-dominated snap-off for three different flow rates of the continuous phase oil (Qc=80 nL/s, 110 nL/s, and 140 nL/s). The infusion rate of the water was constant at Qd=20 nL/s. 
     The electronic control afforded by the field-induced droplet formation provides an additional benefit in this example: it allowed the phase of the droplet break-off to be adjusted within the production cycle. This was accomplished by increasing the field above the critical break-off field only at the instant the droplet is required. This provided a convenient means to precisely synchronize the production and arrival of individual droplets in specific locations. 
     An important component in some droplet-based reaction confinement system is a mixer which combines two or more reagents to initiate a chemical reaction. An example of a mixer uses electrostatic charges; placing charges of opposite sign on each droplet and applying an electric field causes them to coalesce. As an example, a device is illustrated having two separate nozzles that generate droplets with different compositions and opposite charges. Droplets were brought together at the confluence of the two streams. The electrodes used to charge the droplets upon formation also provide the electric field to force the droplets across the stream lines, leading to coalesce. Slight variations in the structure of the two nozzles resulted in slight differences in the frequency and phase of their droplet generation in the absence of a field. Thus the droplets differed in size, even though the infusion rates were identical. Moreover, the droplets did not arrive at the point of confluence at exactly the same time. As a result the droplets did not coalesce. By contrast, upon application of an electric field, droplet formation became generally synchronized, ensuring that pairs of identically sized droplets reached the point of confluence simultaneously. Moreover, the droplets were oppositely charged, causing them to traverse the stream lines and contact each other thereby causing them to coalesce. The synchronization of droplet formation resulted from coupling of the break-off of the two droplets as mediated by the electric field; the magnitude of the electric field varied as the separation between the leading edges of the two droplets changes and the frequency of droplet break-off is mode locked to the electric field. A minimum charge is required to cause droplets to coalesce in this example, presumably because of the stabilizing effects of the surfactant coating; the E field depends on the percentage of droplets that contacted each other that actually coalesce. 
     In one embodiment, droplets having opposite sign of electrostatic charge can be generated by applying a voltage across the two aqueous streams. In another embodiment, in the absence of the field, the frequency and timing of droplet formation at the two nozzles may be independent, and each nozzle may produce a different size droplet at a different frequency; infusion rates are the same at both nozzles. After the confluence of the two streams, droplets from the upper and lower nozzles stay in their respective halves of the stream. Due to surfactant, there are no coalescence events even in the case of large slugs that fill the channel width. In yet another embodiment, with an applied voltage of 200V across the 500 micron separation of the nozzles, the droplets that simultaneously break-off from the two nozzles are essentially identical; simultaneous droplet formation can be achieved for unequal infusion rates of the aqueous streams even up to a factor of two difference in volumes. The fraction of the droplets that encounter each other and coalesce increases linearly above a critical field when a surfactant, sorbiton-monooleate 3% is present. 
     The use of oppositely charged droplets and an electric field to combine and mix reagents was extremely robust, and nearly 100% of the droplets from the two streams coalesced with their partners from the opposite stream. However, after they coalesced the resultant droplets carried essentially no electrostatic charge. While it is convenient to charge droplets during formation, other methods may be employed in any robust droplet-based microfluidic system to recharge the mixed droplets, if necessary, for further processing. This may be accomplished, for example, through the use of extensional flow to split neutral droplets in the presence of an electric field which polarizes the droplets resulting in two oppositely charged daughter droplets. In one embodiment, neutral droplets enter a bifurcation and split into charged daughter droplets. In some cases, the asymmetric stretching of the charged droplets in the electric field can be observed. The vertical dashed lines indicate the edge of the electrodes where the droplets returned to their symmetric spherical shape. The electric field also allowed precision control of the droplet splitting, providing the basis of a robust droplet division module which allows the splitting of the contents into two or more aliquots of identical reagent facilitating multiple assays on the contents of the same microreactor. 
     In another embodiment, neutral droplets can be recharged by breaking them in the presence of an electric field. Uncharged droplets (q=0) were polarized in an electric field (ES not equal to 0), provided ES was sufficiently large, and the droplets break into two oppositely charged daughter drops in the extensional flow at a bifurcation. The charged droplets were stretched in the electric field ES, but returned to spherical on contacting the electrodes. 
     Another component useful for the construction of microfluidic droplet reaction systems is a droplet sorter. The contents of individual may must be probed, and selected droplets may be sorted into discreet streams. Such sorting in microfluidic devices can be accomplished, as shown in this example, through the use of mechanical valves. The use of electrostatic charging of droplets may provide an alternate means that can be precisely controlled, can be switched at high frequencies, and requires no moving parts. Electrostatic charge on the droplets may enable drop-by-drop sorting based on the linear coupling of charge to an external electric field. A T-junction bifurcation that splits the flow of carrier fluid equally will also randomly split the droplet population equally into the two streams. However, a small electric field applied at the bifurcation may precisely dictate which channel the droplets enter; varying the direction of the field varies the direction of the droplet sorting. The large forces that can be imparted on the droplets and the high switching frequency make this a fast and robust sorting engine with no moving parts; thus the processing rate is limited primarily by the rate of droplet generation. 
     In one embodiment, charged droplets alternately entered the right and left channels when there was no field applied (ES=0). When an electric field is applied to the right, the droplets entered the right branch at the bifurcation; they entered the left branch when the field is reversed. After the bifurcation, the distance between droplets is reduced to half what it was before, indicating the oil stream is evenly divided. In some cases, the deformation in the shape of a highly charged drop in an electric field can be observed. 
     The enhanced functionality that electrostatic charge brings to droplets in microfluidic devices enables an expansive list of microfluidics applications. This toolkit of techniques for manipulating droplets will enable modular integration of systems for transporting and reacting small numbers of molecules. High throughput screening, combinatorial chemistry, and the search for rare biological function in libraries could all potentially benefit from electrostatic manipulation of droplets in microchannels. For instance, droplet based microfluidic technology can also be used to develop a chip-scale fluorescence activated cell sorter (FACS) with enhanced activation functionality that goes beyond fluorescence to include multiple reagent-based assays between the droplet formation and sorting steps. Moreover by using femtoliter drops, which are a few microns in diameter, even a single biomolecule represents a concentrations of &gt;&gt;1 nM, sufficient for efficient chemical reactivity and single molecule assays. 
     Many of the uses of droplet-based microfluidic devices are driven by a need to encapsulate a varied population or library of molecules, cells or particles into microreactors, perform an assay on the contents, perhaps through the addition of reagents, and then, finally, to selectively remove specific microreactors from the collection in a search for rare events. This requires a processing rate of 103 per second to sort through the smallest libraries in a reasonable time while rates on the order 105 per second are desirable for larger libraries. These rates are feasible, as discussed herein. Moreover, because the microfluidic devices may be produced using stamping techniques, e.g., as described herein, parallel flow streams or fluidic systems can be fabricated, further enhancing the total throughput. Combined, the advantages of droplets and high throughput manipulation provide significant opportunity for widespread application. 
     While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the scope of the present invention. 
     All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of”, when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. 
     This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.