Patent Publication Number: US-10780413-B2

Title: High-speed on demand microfluidic droplet generation and manipulation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 14/775,611, filed on Sep. 11, 2015, which is a U.S. 371 National Phase of PCT/US2014/026185, filed Mar. 13, 2014, which claims benefit of and priority to U.S. Ser. No. 61/798,516, filed on Mar. 15, 2013, all of which are incorporated herein by reference in their entirety for all purposes. 
    
    
     STATEMENT OF GOVERNMENTAL SUPPORT 
     This invention was made with government support under Grant Number 0901154, awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Over the last decade microfluidic systems have developed into valuable instrumental platforms for performing high throughput chemistry and biology (deMello (2006)  Nature,  442: 394-402). The ability to controllably merge droplets within segmented flow systems is of high importance when performing complex chemical or biological analyses (Shestopalov et al. (2004)  Lab Chip,  4: 316-321). Unfortunately, the controlled merging of multiple droplets in a sequential fashion is not straightforward. Although the emulsions produced in microfluidic systems are thermodynamically metastable, the process of merging has not proven to be predictable, due to subtle variations in interfacial tension, surface topography of microchannels, and fluidic properties such as of droplet size, viscosity, and velocity (see, e.g., Fuerstman et al. (2007)  Science,  315: 828-832). 
     Droplet merging is important or essential in many applications including sequential reactions (Kim et al. (2006)  Anal. Chem.,  78(23): 8011-8019), multiple step manipulation of cells (He et al. (2005)  Anal. Chem.,  77(6): 1539-1544), high-throughput bioassays (Srisa-Art et al. (2007)  Anal. Chem.,  79: 6682-6689), and the like. Additionally, the ability to merge and split droplets or bubbles in a high throughput manner cab impact the use of bubble logic systems for exchanging chemical and electronic information (Prakash and Gershenfeld (2007)  Science,  315(5813): 832-835). 
     In typical droplet merging processes, relatively large time and spatial scales are involved. For example, timescales may range from the sub-microsecond regime for some chemical reactions to many hours and even days for cell-based assays. Similarly, large spatial scales also exist, for example, between the droplets to be merged and between the droplets and the component interfaces that interact to drive the merging process. 
     Several techniques have been developed to merge droplets. These are either active and involve components such as electric fields (Priest et al. (2006)  Appl. Phys. Lett.,  89: 134101:1-134101:3; Ahn et al. (2006)  Appl. Phys. Lett.,  88: 264105), or passive and utilize the surface properties (Fidalgo et al. (2007)  Lab Chip,  7(8): 984-986) or structure (Tan et al. (2004)  Lab Chip,  4(4): 292-298) of the fluidic conduit. 
     SUMMARY 
     In various embodiments in various embodiments, a microfluidic droplet merger component (e.g., for integration into microfluidic systems such as lab-on-a-chip systems, and the like) is provided. An illustrative, but noon-limiting droplet merger structure comprises a central channel comprising a plurality of elements (e.g. a micropillar array) disposed and spaced to create a plurality of lateral passages that drain a carrier fluid out of a fluid stream comprising droplets of a first fluid contained in said carrier fluid; and a deformable lateral membrane valve disposed to control the width of said center channel. Trapping and merging of different numbers of droplets can be controlled by the spacing and arrangement of lateral passages and/or elements forming such passages (e.g., the micropillar array structure), the timing, and the constriction size of the deformable lateral membrane valve. The deformable membrane (membrane valve) forms a controllable, variable-sized constriction, e.g., at the downstream of the trapping structure. By controlling the constriction size and timing, different numbers of droplets can be trapped and merged before exiting the device. Also provided are microfluid droplet generators and devices comprising one or more microfluid droplet generators and/or one or more droplet merger structures. 
     In various aspects, the invention(s) contemplated herein may include, but need not be limited to, any one or more of the following embodiments: 
     In various aspects, the invention(s) contemplated herein may include, but need not be limited to, any one or more of the following embodiments: 
     Embodiment 1: A microfluidic droplet merger component, said component including: a central channel including a plurality of elements disposed and spaced to create a plurality of lateral passages that drain a carrier fluid out of a fluid stream including droplets of a first fluid contained in said carrier fluid; and a deformable lateral membrane valve disposed to control the width of said center channel. 
     Embodiment 2: The droplet merger component of embodiment 1, wherein said membrane valve is a pneumatically actuated lateral membrane valve. 
     Embodiment 3: The droplet merger component according to any one of embodiments 1-2, wherein the width of said central channel reduces as a function of distance downstream through said plurality of lateral passages. 
     Embodiment 4: The droplet merger component according to any one of embodiments 1-3, wherein the where the width of said lateral passages is smaller than the width of said central channel at the same location and smaller than the average diameter of a droplet in the central channel. 
     Embodiment 5: The droplet merger component according to any one of embodiments 1-4, wherein said plurality of elements comprise a micropillar array. 
     Embodiment 6: The droplet merger component according to any one of embodiments 1-5, wherein said valve is located at or downstream of the last of said plurality of elements. 
     Embodiment 7: The droplet merger component of embodiment 5, wherein said micropillar array includes pairs of pillars that form lateral channels slanted in a downstream direction. 
     Embodiment 8: The droplet merger component of embodiment 7, wherein said valve is located at or downstream of the last (downstream) pairs of pillars. 
     Embodiment 9: The droplet merger component according to any one of embodiments 1-8, wherein said pillars are configured to provide an inter-pillar spacing that ranges from about 0.1 μm to about 100 μm. 
     Embodiment 10: The droplet merger component according to any one of embodiments 1-8, wherein said pillars are configured to provide an inter-pillar spacing that ranges from about 0.1 μm to about 10 μm. 
     Embodiment 11: The droplet merger component according to any one of embodiments 1-10, wherein said deformable lateral membrane valve is configured to form a controllable, variable-sized construction at the downstream end of said plurality of elements. 
     Embodiment 12: The droplet merger component according to any one of embodiments 1-11, wherein said deformable lateral membrane valve is configured to deform horizontally. 
     Embodiment 13: The droplet merger component according to any one of embodiments 1-11, wherein said deformable lateral membrane valve is configured to deform vertically. 
     Embodiment 14: The droplet merger component according to any one of embodiments 1-13, wherein said micropillar array is formed from a material selected from the group consisting of glass, metal, ceramic, mineral, plastic, and polymer. 
     Embodiment 15: The droplet merger component according to any one of embodiments 1-13, wherein said micropillar array is formed from an elastomeric material. 
     Embodiment 16: The droplet merger component of embodiment 15, wherein said elastomeric material is selected from the group consisting of polydimethylsiloxane (PDMS), polyolefin plastomers (POPs), perfluoropolyethylene (a-PFPE), polyurethane, polyimides, and cross-linked NOVOLAC® (phenol formaldehyde polymer) resin. 
     Embodiment 17: A microfluidic droplet generator, said generator including: a first microfluidic channel containing a first fluid adjacent to a second microfluidic channel containing a second fluid wherein said first fluid is substantially immiscible in second fluid; and a cavitation channel or chamber where the contents of said cavitation channel or chamber is separated from the contents of said first microfluidic channel by a deformable channel wall or chamber wall, where said cavitation channel or chamber is configured to permit said deformable channel wall or chamber wall to deform when a bubble is formed in said cavitation channel or chamber, and where said cavitation channel or chamber is disposed above or below said first microfluidic channel. 
     Embodiment 18: The droplet generator of embodiment 17, wherein, said first microfluidic channel is in fluid communication with said second microfluidic channel via a port or a channel. 
     Embodiment 19: The droplet generator according to any one of embodiments 17-18, where a first portion of said first microfluidic channel is disposed a first distance away from said second microfluidic channel, and a second portion of said first microfluidic channel is disposed a second distance away from said second microfluidic channel and said second distance is less than said first distance. 
     Embodiment 20: The droplet generator of embodiment 19, wherein said first microfluidic channel includes a third portion disposed so that said second portion is located between said first portion and said third portion and said third portion of said microfluidic channel is located at a third distance away from said second microfluidic channel and said third distance is greater than said second distance. 
     Embodiment 21: The droplet generator according to any one of embodiments 17-20, where the maximum width of said first microfluidic channel and/or said second microfluidic channel ranges from about 0.1 μm to about 500 μm. 
     Embodiment 22: The droplet generator according to any one of embodiments 17-20, where the maximum width of said first microfluidic channel and/or said second microfluidic channel ranges from about 50 μm to about 100 μm. 
     Embodiment 23: The droplet generator according to any one of embodiments 17-20, where the width of said first microfluidic channel and/or said second microfluidic channel is about 100 μm. 
     Embodiment 24: The droplet generator according to any one of embodiments 17-23, where the maximum depth of said first microfluidic channel and/or said second microfluidic channel ranges from about 0.1 μm to about 500 μm. 
     Embodiment 25: The droplet generator according to any one of embodiments 17-23, where the maximum depth of said first microfluidic channel and/or said second microfluidic channel ranges from about 40 μm to about 80 μm. 
     Embodiment 26: The droplet generator according to any one of embodiments 17-23, where the typical depth of said first microfluidic channel and/or said second microfluidic channel is about 50 μm. 
     Embodiment 27: The droplet generator according to any one of embodiments 17-26, wherein the typical depth of said cavitation channel or chamber ranges from about 100 μm to about 150 μm. 
     Embodiment 28: The droplet generator according to any one of embodiments 17-27, wherein said droplet generator is configured to generate droplets having a volume ranging from about 1 atto L to about 1 μL. 
     Embodiment 29: The droplet generator of embodiment 28, wherein said droplet generator is configured to generate droplets having a volume ranging from about 1 pL to about 150 pL. 
     Embodiment 30: The droplet generator according to any one of embodiments 17-28, wherein said cavitation channel or chamber is a cavitation channel. 
     Embodiment 31: The droplet generator of embodiment 30, wherein said cavitation channel provides permits the contents of said channel to flow and thereby aid dissipation of a bubble formed therein. 
     Embodiment 32: The droplet generator according to any one of embodiments 17-31, wherein said cavitation channel or chamber is disposed above said first microfluidic channel. 
     Embodiment 33: The droplet generator according to any one of embodiments 17-31, wherein said cavitation channel or chamber is disposed below said first microfluidic channel. 
     Embodiment 34: The droplet generator according to any one of embodiments 17-33, wherein said cavitation channel or chamber contains a dye. 
     Embodiment 35: The droplet generator according to any one of embodiments 17-33, wherein said cavitation channel or chamber contains light-absorbing nanoparticle and/or microparticles. 
     Embodiment 36: The droplet generator according to any one of embodiments 17-35, wherein said first microfluidic channel is configured to provide said first fluid under a substantially static pressure to create a stable interface between said first fluid and said second fluid. 
     Embodiment 37: The droplet generator according to any one of embodiments 17-36, wherein said first fluid includes an aqueous fluid. 
     Embodiment 38: The droplet generator according to any one of embodiments 17-37, wherein said second fluid includes an oil or an organic solvent. 
     Embodiment 39: The droplet generator of embodiment 38, wherein said second fluid includes a solvent selected from the group consisting of carbon tetrachloride, chloroform, cyclohexane, 1,2-dichloroethane, dichloromethane, diethyl ether, dimethyl formamide, ethyl acetate, heptane, hexane, methyl-tert-butyl ether, pentane, toluene, and 2,2,4-trimethylpentane. 
     Embodiment 40: The droplet generator of embodiment 38, wherein said second fluid includes an oil. 
     Embodiment 41: The droplet generator according to any one of embodiments 17-40, wherein said port or channel includes a nozzle. 
     Embodiment 42: The droplet generator according to any one of embodiments 17-41, wherein said first and/or second microfluidic channel is formed from a material selected from the group consisting of glass, metal, ceramic, mineral, plastic, and polymer. 
     Embodiment 43: The droplet generator according to any one of embodiments 17-42, wherein said first and/or second microfluidic channel is formed from an elastomeric material. 
     Embodiment 44: The droplet generator of embodiment 43, wherein said elastomeric material is selected from the group consisting of polydimethylsiloxane (PDMS), polyolefin plastomers (POPs), perfluoropolyethylene (a-PFPE), polyurethane, polyimides, and cross-linked NOVOLAC® (phenol formaldehyde polymer) resin. 
     Embodiment 45: The droplet generator according to any one of embodiments 17-44, wherein said generator can provide on-demand droplet generation at a speed of greater than about 1,000, more preferably greater than about 2,000 droplets/sec, more preferably greater than about 4,000 droplets/sec, more preferably greater than about 6,000 droplets/sec, or more preferably greater than about 8,000 droplets/sec. 
     Embodiment 46: The droplet generator according to any one of embodiments 17-44, wherein said device can provide on-demand droplet generation at a speed ranging from zero droplets/sec, 1 droplets/sec, 2 droplets/sec, about 5 droplets/sec, about 10 droplets/sec, about 20 droplets/sec, about 50 droplets/sec, about 100 droplets/sec, about 500 droplets/sec, or about 1000 droplets/sec, up to about 1,500 droplets/sec, about 2,000 droplets/sec, about 4,000 droplets/sec, about 6,000 droplets/sec, about 8,000 droplets/sec, about 10,000 droplets/sec, about 20,000 droplets/sec, about 50,000 droplets/sec, or about 100,000 droplets/sec. 
     Embodiment 47: The droplet generator according to any one of embodiments 17-44, wherein said device can provide on-demand droplet generation at a speed of greater than about 1,000, more preferably greater than about 10,000, more preferably greater than about 20,000 droplets/sec, more preferably greater than about 40,000, more preferably greater than about 50,000 droplets/sec, more preferably greater than about 80,000, or more preferably greater than about 100,000 droplets/sec. 
     Embodiment 48: The droplet generator according to any one of embodiments 17-47, wherein said generator is present in a system including an energy source configured to form a bubble in said cavitation channel or chamber. 
     Embodiment 49: The droplet generator of embodiment 48, wherein said energy source includes an optical energy source or microwave emitter. 
     Embodiment 50: The droplet generator of embodiment 48, wherein said energy source includes a laser. 
     Embodiment 51: The droplet generator of embodiment 50, wherein said energy source includes a pulse laser. 
     Embodiment 52: The droplet generator according to any one of embodiments 17-51, wherein said generator is disposed on a substrate including a material selected from the group consisting of a polymer, a plastic, a glass, quartz, a dielectric material, a semiconductor, silicon, germanium, ceramic, and a metal or metal alloy. 
     Embodiment 53: The droplet generator according to any one of embodiments 17-52, wherein said generator is integrated with other microfluidic components. 
     Embodiment 54: The droplet generator of embodiment 53, wherein said other microfluidic components selected from the group consisting of PDMS channels, wells, valves. 
     Embodiment 55: The droplet generator of embodiment 53, wherein said generator is a component of a lab-on-a-chip. 
     Embodiment 56: The droplet generator according to any one of embodiments 17-55, wherein said first fluid includes one or more reagents for polymerase chain reaction (PCR). 
     Embodiment 57: The droplet generator of embodiment 56, wherein said first fluid includes one or more reagents selected from the group consisting of a PCR primer, a PCR template, a polymerase, and a PCR reaction buffer. 
     Embodiment 58: A device for the manipulation of microfluidic droplets, said device including a substrate carrying or including: one or more droplet merger components according to any one of embodiments 1-16; and optionally one or more droplet generators according to any one of embodiments 17-57. 
     Embodiment 59: The device of embodiment 58, wherein said device further includes a controller that controls the amount and timing of constriction of said membrane valve. 
     Embodiment 60: The device according to any one of embodiments 58-59, wherein said device includes one or more droplet generators according to any one of embodiments 17-57. 
     Embodiment 61: The device according to any one of embodiments 58-60, wherein said device includes at least two droplet generators. 
     Embodiment 62: The device of embodiment 61, wherein said device includes at least four droplet generators. 
     Embodiment 63: The device according to any one of embodiments 61-62, wherein a plurality of droplet generators are configured to share a common second microfluidic channel and to inject droplets into said common second microfluidic channel. 
     Embodiment 64: The device of embodiment 63, wherein a droplet merger component is disposed to receive and merge droplets from said common second microfluidic channel. 
     Embodiment 65: A system for the generation of droplets and/or the encapsulation of particles or cells said, said system including a droplet generator according to any one of embodiments 17-57 and an excitation source for forming gas bubbles in a fluid. 
     Embodiment 66: The system of embodiment 65, wherein said excitation source includes an optical energy source. 
     Embodiment 67: The system of embodiment 66, wherein said excitation source includes a non-coherent optical energy source. 
     Embodiment 68: The system of embodiment 66, wherein said excitation source includes a laser. 
     Embodiment 69: The system according to any one of embodiments 66-68, wherein said system includes an objective lens configured to focus optical energy into said cavitation channel or chamber. 
     Embodiment 70: The system of embodiment 69, wherein said system includes a half-wave plate. 
     Embodiment 71: The system according to any one of embodiments 69-70, wherein said system includes a polarizer. 
     Embodiment 72: The system of embodiment 71, wherein said polarizer includes a polarizing beam splitter cube. 
     Embodiment 73: The system according to any one of embodiments 66-72, wherein said system includes a controller that adjusts at least one of the timing of occurrence of light pulses emitted by the optical energy source, the frequency of occurrence of pulses emitted by the optical energy source, the wavelength of pulses emitted by the optical energy source, the energy of pulses emitted by the optical energy source, and the aiming or location of pulses emitted by the optical energy source. 
     Embodiment 74: The system according to any one of embodiments 65-73, wherein said system further includes components for detecting particles, droplets, or cells in said system. 
     Embodiment 75: The system of embodiment 74, wherein said components comprise an optical detection system, an electrical detection system, a magnetic detection system or an acoustic wave detection system. 
     Embodiment 76: The system of embodiment 74, wherein said components comprise an optical detection system for detecting a scattering, a fluorescence, or a ramen spectroscopy signal. 
     Embodiment 77: A method of combining droplets in a microfluidic system, said method including providing a plurality of droplets flowing through a microfluidic channel into the central channel(s) of one or more droplet merger components according to any one of embodiments 1-16 causing the merger of a plurality of droplets. 
     Embodiment 78: The method of embodiment 77, further including varying the constriction created by said lateral membrane valve to control the timing of droplet merger and/or the number of merged droplets. 
     Embodiment 79: The method of embodiment 78, wherein said varying the constriction includes operating a controller that pneumatically actuates said lateral membrane valve(s). 
     Embodiment 80: A method for generating droplets said method including: applying an energy source to a droplet generator according to any one of embodiments 17-57, where said energy source forms bubbles in said cavitation channel or chamber to deform said deformable channel wall or chamber wall and to inject a droplet of said first fluid into said second fluid in said second microfluidic channel. 
     Embodiment 81: The method of embodiment 80, wherein said utilizing an energy source includes utilizing a laser to excite cavitation bubbles in said cavitation channel or chamber. 
     Embodiment 82: The method of embodiment 81, wherein said method includes using a controller that adjusts at least one of the timing of occurrence of pulses emitted by a pulsed laser, the frequency of occurrence of pulses emitted by the pulsed laser, the wavelength of pulses emitted by the pulse laser, the energy of pulses emitted by the pulse laser, and the aiming or location of pulses emitted by the pulse laser. 
     Embodiment 83: The method according to any one of embodiments 80-82, further including generating a plurality of separate and additional cavitation bubbles at a frequency of at least 1000 Hz. 
     Embodiment 84: The method of embodiment 83, wherein said method is repeated at a frequency of 1 kHz or greater. 
     Embodiment 85: The method according to any one of embodiments 80-84, wherein said first fluid includes one or more reagents for polymerase chain reaction (PCR). 
     Embodiment 86: The droplet generator of embodiment 85, wherein said first fluid includes one or more reagents selected from the group consisting of a PCR primer, a PCR template, a polymerase, and a PCR reaction buffer. 
     Embodiment 87: A method of generating and combining droplets, said method including: providing a device including one or more droplet generators droplet generator according to any one of embodiments 17-57 and one or more droplet merger component(s) according to any one of embodiments 1-16, wherein at least one of said one or more droplet merger component(s) is disposed to receive droplets generated by at least one of said one or more droplet generators; applying an energy source a cavitation channel or chamber of one or more of said one or more droplet generator(s), where said energy source forms bubbles in said cavitation channel or chamber to deform said deformable channel wall or chamber wall and to inject a droplet of said first fluid into said second fluid in said second microfluidic channel; receiving a plurality of droplets generated by said one or more droplet generator(s) in at least one of said one or more droplet merger components where said droplets merger to form a combined droplet fluid. 
     Embodiment 88: The method of embodiment 87, wherein said device includes a plurality of droplet generators. 
     Embodiment 89: The method of embodiment 87, wherein said device includes at least three droplet generators. 
     Embodiment 90: The method according to any one of embodiments 87-89, wherein said device includes a plurality of droplet merger components. 
     Embodiment 91: The method of embodiment 90, wherein said device includes at least three droplet merger components. 
     Embodiment 92: The method according to any one of embodiments 87-91, wherein said utilizing an energy source includes utilizing a laser to excite cavitation bubbles in said cavitation channel or chamber. 
     Embodiment 93: The method of embodiment 92, wherein said method includes using a controller that adjusts at least one of the timing of occurrence of pulses emitted by a pulsed laser, the frequency of occurrence of pulses emitted by the pulsed laser, the wavelength of pulses emitted by the pulse laser, the energy of pulses emitted by the pulse laser, and the aiming or location of pulses emitted by the pulse laser. 
     Embodiment 94: The method according to any one of embodiments 87-93, further including generating a plurality of separate and additional cavitation bubbles at a frequency of at least 1000 Hz. 
     Embodiment 95: The method of embodiment 94, wherein said method is repeated at a frequency of 1.2 kHz or greater. 
     Embodiment 96: The method according to any one of embodiments 87-95, wherein said first fluid includes one or more reagents for polymerase chain reaction (PCR). 
     Embodiment 97: The method of embodiment 96, wherein said first fluid includes one or more reagents selected from the group consisting of a PCR primer, a PCR template, a polymerase, and a PCR reaction buffer. 
     Embodiment 98: The method according to any one of embodiments 87-97, wherein said device comprise or is integrated with other microfluidic components. 
     Embodiment 99: The method of embodiment 98, wherein said other microfluidic components selected from the group consisting of PDMS channels, wells, valves. 
     Embodiment 100: The method of embodiment 98, wherein said device includes or is integrated with a lab-on-a-chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic illustration of a droplet merger module  100  with deformable lateral membrane valves  106 . Fluid containing droplets  108  flows in flow direction  112  through a central channel  102 . A plurality of elements  104  form lateral channels  114  that drain fluid from between trapped droplets  108  which then merge into a merged droplet  116  and leave behind the drained fluid, e.g. as droplets  110 . A pair of pneumatically actuated lateral membrane valves  106  located at the end of the plurality of elements (e.g., pillar structure) is used to change the width of the center channel to control the number of trapped droplets. When a threshold number of droplets is reached, the hydraulic pressure on the merged droplet becomes bigger than the surface tension to release it. No movement of membrane is required, which is the key to enable high speed merging. 
         FIG. 2  schematically illustrates pulse laser induced on-demand membrane valve droplet generation. A PDMS thin membrane is used to totally separate the pulse laser induced contamination. The induced bubble can deform the membrane into the aqueous channel, break the stable water-oil interface, and squeeze out a picoliter droplet into the oil channel. 
         FIG. 3  shows a schematic illustration of an on-demand droplet generation and fusion platform, that integrates a pulse laser induced on-demand membrane valve droplet generator and a lateral membrane valve controlled droplet merging module. 
         FIG. 4  shows snapshots of the droplet generation process. 
         FIG. 5  shows time-resolved images of an illustrative droplet merging process. Up to six droplets have been experimentally trapped and merged on this passive but tunable merging module. In this illustrated embodiment, the merged droplet releases automatically when the number of droplets is larger than 6. 
         FIG. 6  schematically illustrates one embodiment of a platform with parallel droplet generators and the downstream droplet merger for generating droplets with multiplexed drug/chemical combinations 
         FIG. 7  schematically illustrates one embodiment of a platform integrating uFACS and multiple droplet generators for high-speed single cell encapsulation and single cell analysis. 
         FIG. 8  illustrates multiple droplet trapping and merging with a vertically deformed membrane valve. 
         FIGS. 9A through 9ZB  depict, via simplified cross-sectional views, various stages of a manufacturing technique for producing multi-layer PDMS structures. 
     
    
    
     DETAILED DESCRIPTION 
     Two-phase flow (or multi-phase) systems that can manipulate picoliter (pL) volume droplets in closed channels have broad lab on chip applications. Small droplets allow reduction of reagent consumption, high sensitivity detection, and large scale analysis. Thousands of droplets can be easily generated in a simple two-phase flow channel, and tens or hundreds of channels can be structured in parallel to increase throughput. Droplets can also be programmed to merge, split, and mix at high speed for rapid screening. Commercial systems could achieve 10 million droplet reactions or screening per hour. Applications include a variety of PCR techniques, such as digital PCR, RT-PCR, PCR, single cell analysis, combinatorial chemical synthesis, and the like. 
     I. On-Demand Lateral Membrane Valve for Passive Droplet Trapping and Merging 
     In various embodiments a tunable droplet trapping and merging module is provided. One embodiment of such a component/module  100  is illustrated in  FIG. 1 . As illustrated therein the droplet merger component comprises a central channel  102  comprising a plurality of elements  104  (e.g., an array of pillar structures) disposed and spaced to create a plurality of lateral passages  114  that drain a carrier fluid out of a fluid stream comprising droplets  108  of a first fluid contained in the carrier fluid; and a deformable lateral membrane valve  106  disposed to control the width of the center channel. 
     Droplets in, for example, a microchannel  118  move in a downstream direction  112 . Droplets flowing downstream are trapped in the droplet mering module. The plurality of elements  104  provides lateral passages  114  are used to drain out fluid oil between droplets to merge them. A pair of pneumatically actuated lateral membrane valves  106  located at the downstream end of the plurality of elements can be used to change the width of the flow channel. In various embodiments the width of the central channel reduces as a function of distance downstream through a plurality of lateral passages. In certain embodiments the width ranges from 10 μm to about 1 mm at the upstream end of the component to a width that ranges about 1 μm to about 900 μm where the downstream width is smaller than the upstream width. In certain embodiments the width of the lateral passages is smaller than the width of the central channel at the same location and smaller than the average diameter of a droplet in the central channel. 
     In certain embodiments the plurality of elements comprise a micropillar array. In certain embodiments the micropillar array comprises pairs of pillars that define the central channel. In certain embodiments the valve is located at or downstream of the last of the plurality of elements (e.g. downstream of the last (downstream) pair of pillars). In certain embodiments the pillars are configured to provide an inter-pillar spacing that ranges from about 0.1 μm to about 100 μm. In certain embodiments the pillars are configured to provide an inter-pillar spacing that ranges from about 0.1 μm to about 10 μm. In certain embodiments the deformable lateral membrane valve is configured to form a controllable, variable-sized construction at the downstream end of said plurality of elements. In certain embodiments the deformable lateral membrane valve is configured to deform vertically. 
     In various embodiments the deformation of the lateral membrane can be tuned by changing the pneumatic pressure though the vias underneath the deformation chamber. It is noted that in certain embodiments, deformation of the lateral membrane can be regulated by mechanical actuators. For example, in certain embodiments, piezo-linear actuators, electrostatic actuators, and the like can be used to control deformation of the lateral membrane. 
     The number of droplets trapped in the merging module can be tuned by degree of deformation of the lateral membrane valve. When the number of trapped droplets reaches to its trapping threshold, the fused droplet releases automatically without the need to mechanically deform the membrane valve. Such passive type droplet merger can have high speed since there is no need to deform the mechanical membrane. As illustrated in  FIG. 5 , six droplets can be passively trapped, merged and released. In some cases where more droplets are required to merge (for example, creating a large combinatorial library), the membrane valve, which in this illustration is deformed vertically can be fully closed to hold more droplets. Up to 15 droplets have been trapped and merged using this mode, as shown in  FIG. 8 . It will be noted, however, that the methods and devices are not limited to trapping six or 15 droplets. Accordingly in certain embodiments, at least 2, or 3, or 4, or 5, or 6, or 7, or 8, or 9, or 10, or 11, or 12, or 13, or 14, or 15, or 16, or 17, or 18, or 19, or 20, or 21, or 22, or 23, or 24, or 25, or 26, or 27, or 28, or 29, or 30 or more droplets are trapped and merged. 
     In certain embodiments, where the membrane closing process is slow, the throughput may be lower than about 10 merged droplets/sec. 
     One illustrative, but non-limiting schematic of an integrated droplet generation and merging modules for high speed production of multiplexed droplets is shown in  FIG. 3 . 
     In certain embodiments the devices described herein utilize a thin-layer soft lithography process to produce certain structures (e.g., valve membranes). The fabrication of thin layers of, e.g., PDMS is enabled by a novel Pt-PDMS thin film process described in provisional application No. 61/616,385, filed on Mar. 27, 2012, and in copending provisional application entitled “CONTINUOUS WHOLE-CHIP 3-DIMENSIONAL DEP CELL SORTER AND RELATED FABRICATION METHOD”, filed on Mar. 15, 2013, both of which are incorporated herein for the PtPDMS thin film fabrication processes described therein. 
     In particular one implementation of this ptPDMS fabrication process is depicted via simplified cross-sectional views in  FIGS. 9A through 9ZB . The structure that is being constructed in  FIGS. 9A through 9ZB  is a portion of a three-dimensional DEP cell sorter, e.g., the features within the DEP separation region of such a cell sorter, however the same fabrication method is readily applied to fabrication of the devices described herein (e.g., on-demand lateral membrane valve).  FIGS. 9A through 9ZB  are not drawn to scale. In  FIGS. 9A through 9P , the Figures depict two different manufacturing streams—the steps in the streams may be largely the same, but the molds used may have different feature sizes. For example, the cross-sections on the left side of each Figure depict the formation of a PDMS layer that may be used to provide a first passage or a second passage, e.g., of a cell sorter, droplet injector, and the like, and the cross-sections on the right side of each Figure may depict the formation of a PDMS layer that may be used to provide a sorting passage of the cell sorter, injection passage of a droplet former, and the like.  FIGS. 9Q through 9ZB  depict the assembly of the layers into an assembled cell sorter (e.g., as described in provisional application No. 61/616,385, filed on Mar. 27, 2012,). 
     As illustrated in  FIG. 9A , a hard substrate may be prepared for etching by depositing or providing a photo-patternable or photo-resistive material on the substrate. 
     Such a material may be, for example, negative photoresist SU8 or positive photoresist AZ4620, and the substrate may, for example, be silicon or glass, although other photoresists or photo-patternable materials may be used as well, as well as other substrate materials. As illustrated in  FIG. 9B , an etching operation can remove material from the hard substrate to form a master mold. Alternatively, the raised features on the master mold can be formed by deposition instead of etching. In certain embodiments both etching and deposition can be used to form features on the master mold. As shown in in  FIG. 9C , the master mold is coated with a conformal silane surface treatment to facilitate later removal of cured PDMS from the molds. In  FIG. 9D , uncured PDMS (or other soft lithographic material) may be poured onto the master mold and cured to form a complementary PDMS mold. In  FIG. 9E , the PDMS mold may then be separated from the master mold. In  FIG. 9F , the PDMS mold may be coated with a conformal silane surface treatment. 
     As illustrated in  FIG. 9G , the PDMS mold may be temporarily set aside and another hard substrate, e.g., silicon or glass, may be prepared by pouring uncured PDMS onto the substrate. In  FIG. 9H , the PDMS mold may be retrieved, and in  FIG. 9I , the PDMS mold may be pressed into the uncured PDMS on the substrate and the uncured PDMS may then be cured. In  FIG. 9J , the PDMS mold may be removed from the cured PDMS on the substrate. The resulting PDMS structure on the substrate may be an exact, or near-exact, duplicate of the master mold and may be referred to herein as a PDMS master mold. It will be recognized that a hard master mold or a “soft” (e.g., PDMS) master mold can be used. A hard master mold will reduce thin film distortion during the molding process. In standard soft lithography, people use SU-8 mold (a hard master mold) to make PDMS structures. 
     In  FIG. 9K , the PDMS master mold may be coated with a CYTOP™ surface treatment to assist in later removal of cast PDMS parts. 
     Uncured PDMS may be applied to the PDMS master mold (see, e.g.,  FIG. 9L ). The steps discussed above with respect to  FIGS. 9A through 9L  are similar, in large part, to existing PDMS layer fabrication processes. 
       FIG. 9M , however, depicts a step that deviates from existing fabrication techniques. In existing fabrication techniques, a PDMS stamping, e.g., a large, flat, featureless base, may be used to compress the uncured PDMS into the PDMS master mold. In one embodiment of the present fabrication technique, however, the PDMS stamping has been modified to include a plate of material within the PDMS that has a much higher modulus than the PDMS (e.g., is stiffer than the PDMS). The plate is located such that a very thin layer of PDMS exists between the plate and the uncured PDMS and the PDMS master mold. This thin layer may be, for example, on the order of 500 microns or less in thickness. In practice, thicknesses of 10 to 30 microns have been found to work well. The plate may be plastic, glass, or other material with a substantially higher modulus than that of PDMS. In practice, plastic plates have proven to be more robust than glass plates. Without being bound to a particular theory, the plate may act as an intermediate load spreader within the PDMS stamping to distribute a compression load across the PDMS master mold and the uncured PDMS. The thin layer of PDMS can allow for very small localized deflections that facilitate full contact between the PDMS mold and the stamping while avoiding the creation of large edge ridges that may appear when a traditional stamping is used. 
     In one illustrative, but non-limiting embodiment, the embedded-plate stamping shown may be provided by spin-coating the plate with PDMS. However, it was discovered that PDMS exhibits inconsistent curing behavior when applied in too thin a layer. Indeed, in many thin film situations, it was observed that the PDMS does not cure at all and remains in a liquid state. Thus, it was discovered that the PDMS will not reliably set at thicknesses such as those discussed above, resulting in an unreliable manufacturing technique. It was a surprising discovery that if the PDMS that forms the thin layer on the stamping is doped with a catalyst (e.g., a platinum catalyst), however, the PDMS will set reliably regardless of thickness. While catalysts have been used to accelerate cure rate it is believed that such catalysts have not been previously used to reverse a non-cure or inconsistent cure situation, or to prepare PDMS high-temperature processing. Thus, in certain embodiments, the fabrication techniques contemplated herein may include preparing a stamping (this step is not shown) by coating a substantially rigid plate with a thin layer of platinum-doped PDMS. It was discovered that providing platinum ions to the uncured PDMS insures consistent, relatively uniform, curing. It was also surprisingly discovered that with addition of enough platinum ions, the PDMS cured in a short time even at room temperature. In certain embodiments the catalyst is platinum-divinyltetramethyldisiloxane (C 8 H 18 OPtSi 2 ). 
     The stamping may also have a thicker layer of PDMS on the opposite side of the plate to allow for easy handling or integration with existing equipment, although such a thicker layer is not strictly necessary. The thin layer of PDMS (or the entire PDMS stamping) may be treated with a silane surface treatment, e.g., trichloro (1H,1H,2H,2H-perfluorooctyl) silane (also referred to as “PFOCTS”). 
     In  FIG. 9N , the stamping has been compressed against the uncured PDMS and the PDMS master mold and then cured. In  FIG. 9O , the cured PDMS layer is removed from the PDMS master mold by pulling the stamping away from the PDMS master mold. Due to the higher bond strength in silane-treated surfaces as compared with CYTOP-treated surfaces, the PDMS layer will stay bonded to the stamping, allowing for easy transfer to other structures. 
       FIG. 9P  depicts the removed PDMS layer bonded to the stamping; the PDMS layer may be treated with an oxygen plasma to facilitate later bonding with a glass or PDMS structure. In  FIG. 9Q , one of the PDMS layers is positioned over a prepared glass substrate; the glass substrate may, for example, be prepared by coating it with an electrically-conductive coating such as ITO so that it may act as an electrode layer of a DEP cell sorter. In  FIG. 9R , the PDMS layer may be directly bonded to the glass substrate as a result of the oxygen plasma treatment of the PDMS layer. In  FIG. 9S , the stamping may be removed—due to the higher bond strength of the direct bonding via oxygen plasma treatment as compared with the bond across the silane-treated surfaces, the PDMS layer may separate from the stamping and remain cleanly attached to the glass substrate. The PDMS layer placed on the substrate in this case corresponds to a sublayer of an electrically-insulating layer in a DEP cell sorter having a first or second passage in it. 
     In  FIG. 9T , another PDMS layer (in one embodiment time corresponding with a sublayer of an electrically-insulating layer in a DEP cell sorter having a sorting passage in it), may be positioned over the previously-placed PDMS layer using the stamping to which it is attached. This second PDMS layer may also be treated with an oxygen plasma to facilitate direct bonding to the previously-placed PDMS layer. In  FIG. 9U , the second PDMS layer may be directly bonded to the first PDMS layer by compressing it into the first PDMS layer with the stamping. In  FIG. 9V , the stamping may be removed in much the same manner as in  FIG. 9S . 
     In  FIG. 9W , a third PDMS layer, in this case similar to the first PDMS layer, may be positioned over the first and second PDMS layers. The third PDMS layer, as with the other PDMS layers, may be treated with an oxygen plasma. In  FIG. 9X , the third PDMS layer may be directly bonded to the second PDMS layer to form a three-layer stack of PDMS layers that are fused into one, essentially contiguous, structure. In  FIG. 9Y , the stamping may be removed, leaving the 3-layer PDMS structure behind. In  FIG. 9Z , the exposed top of the PDMS structure may be prepared for bonding to another hard substrate, e.g., glass. In  FIG. 9ZA , the hard substrate may be positioned over the assembled PDMS stack, and in  FIG. 9ZB , the hard substrate may be bonded to the stack. 
     In various embodiments the resulting structure provides very clean inter-layer via features, and is particularly well-suited for microfluidic devices. The above technique may be modified as needed to omit certain steps, add other steps, and otherwise tailor the technique for particular design requirements. For example, it may be possible to form features with stepped cross-sections in the molds, thus reducing the number of individual layers that must be made and bonded together. While the depicted technique was shown for a 3-layer stack of PDMS layers, more or less PDMS layers may be manufacturing in this manner and assembled into a PDMS layer stack. 
     II. Pulse Laser Induced On-Demand Membrane Valve Droplet Generation 
       FIG. 2  schematically illustrates an embodiment of a pulse laser driven membrane valve droplet generator. A thin membrane (e.g., a PDMS membrane) is used to separate a dye channel for laser excitation from the sample channel to prevent potential contamination from pulse laser induced reactive chemicals. 
     Each laser pulse can trigger a cavitation bubble to deform the membrane to squeeze out a droplet through the nozzle into the oil phase. Instead of using continuous phase flow that consumes huge amount of reagents, a static pressure source can be used to maintain the stable (e.g., water-oil) interface. This approach dramatically reduces the consumption of expensive and precious reagents. After a single droplet is ejected into the oil phase, the interface can automatically recover to its original location in a very short time period since the membrane is only partially and locally deformed. The droplet generation rate can go up to hundreds of Hz. The volume of droplets produced on certain embodiments of this platform is around 80 pL as shown in  FIG. 4 . 
     Various embodiments of the devices described herein incorporate microchannels (microfluidic channels). The terms “microfluidic channel” or “microchannel” are used interchangeably and refer to a channel having at least one characteristic dimension (e.g., width or diameter) less than 1,000 μm, more preferably less than about 900 μm, or less than about 800 μm, or less than about 700 μm, or less than about 600 μm, or less than about 500 μm, or less than about 400 μm, or less than about 300 μm, or less than about 250 μm, or less than about 200 μm, or less than about 150 μm, or less than about 100 μm, or less than about 75 μm, or less than about 50 μm, or less than about 40 μm, or less than about 30 μm, or less than about 20 μm. 
     In certain embodiments the methods and devices described herein may utilize immiscible fluids. In this context, the term “immiscible” when used with respect to two fluids indicates that the fluids when mixed in some proportion, do not form a solution. Classic immiscible materials are water and oil. Immiscible fluids, as used herein also include fluids that substantially do not form a solution when combined in some proportion. Commonly the materials are substantially immiscible when they do not form a solution if combined in equal proportions. In certain embodiments immiscible fluids include fluids that are not significantly soluble in one another, fluids that do not mix for a period of time due to physical properties such as density or viscosity, and fluids that do not mix for periods of time due to laminar flow. 
     In addition, such fluids are not restricted to liquids but may include liquids and gases. Thus, for example, where the droplets are to be formed comprising an aqueous solvent (such as water) any number of organic compounds such as carbon tetrachloride, chloroform, cyclohexane, 1,2-dichloroethane, dichloromethane, diethyl ether, dimethyl formamide, ethyl acetate, heptane, hexane, methyl-tert-butyl ether pentane, toluene, 2,2,4-trimethylpentane, and the like are contemplated. Various mutually insoluble solvent systems are well known to those skilled in the art (see e.g. Table 1). In another example, droplets of aqueous buffer containing physiologically normal amounts of solute may be produced in a dense aqueous buffer containing high concentrations of sucrose. In yet another example, droplets of an aqueous buffer containing physiologically normal amounts of solute may be produced in a second aqueous buffer containing physiologically normal amounts of solute where the two buffers are segregated by laminar flow. In still another example, droplets of a fluid may be produced in a gas such as nitrogen or air. 
     Table 1 illustrates various solvents that are either miscible or immiscible in each other. The solvent on left column does not mix with solvents on right column unless otherwise stated. 
     
       
         
           
               
               
             
               
                   
               
               
                 Solvents 
                 Immiscibility 
               
               
                   
               
             
            
               
                 Acetone 
                 can be mixed with any of the solvents listed in the column at left 
               
               
                 Acetonitrile 
                 cyclohexane, heptane, hexane, pentane, 2,2,4-trimethylpentane 
               
               
                 carbon 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                 tetrachloride 
                 water 
               
               
                 chloroform 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                   
                 water 
               
               
                 cyclohexane 
                 acetonitrile, dimethyl formamide, dimethyl sulfoxide, methanol, water 
               
               
                 1,2- 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                 dichloroethane 
                 water 
               
               
                 dichloromethane 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                   
                 water 
               
               
                 diethyl ether 
                 dimethyl sulfoxide, water 
               
               
                 dimethyl 
                 cyclohexane, heptane, hexane, pentane, 2,2,4-trimethylpentane, water 
               
               
                 formamide 
               
               
                 dimethyl 
                 cyclohexane, heptane, hexane, pentane, 2,2,4-trimethylpentane, diethyl 
               
               
                 solfoxide 
                 ether 
               
               
                 1,4-dioxane 
                 can be mixed with any of the solvents listed in the column at left 
               
               
                 ethanol 
                 can be mixed with any of the solvents listed in the column at left 
               
               
                 ethyl acetate 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                   
                 water 
               
               
                 heptane 
                 acetonitrile, dimethyl formamide, dimethyl sulfoxide, methanol, water 
               
               
                 hexane 
                 acetonitrile, dimethyl formamide, dimethyl sulfoxide, methanol, acetic 
               
               
                   
                 acid, water 
               
               
                 methanol 
                 cyclohexane, heptane, hexane, pentane, 2,2,4-trimethylpentane 
               
               
                 methyl-tert-butyl 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                 ether 
                 water 
               
               
                 pentane 
                 acetonitrile, dimethyl formamide, dimethyl sulfoxide, methanol, water, 
               
               
                   
                 acetic acid 
               
               
                 1-propanol 
                 can be mixed with any of the solvents listed in the column at left 
               
               
                 2-propanol 
                 can be mixed with any of the solvents listed in the column at left 
               
               
                 tetrahydrofuran 
                 can be mixed with any of the solvents listed in the column at left 
               
               
                 toluene 
                 can be mixed with any of the solvents listed in the column at left except 
               
               
                   
                 water 
               
               
                 2,2,4- 
                 acetonitrile, dimethyl formamide, dimethyl sulfoxide, methanol, water 
               
               
                 trimethylpentane 
               
               
                 water 
                 carbon tetrachloride, chloroform, cyclohexane, 1,2-dichloroethane, 
               
               
                   
                 dichloromethane, diethyl ether, dimethyl formamide, ethyl acetate, 
               
               
                   
                 heptane, hexane, methyl-tert-butyl ether, pentane, toluene, 2,2,4- 
               
               
                   
                 trimethylpentane 
               
               
                   
               
            
           
         
       
     
     In certain embodiments the first fluid and second fluid need not be immiscible in each other. In such embodiments, injected droplets can be kept separate from each other simply by adjusting flow rates in the microchannels and rate of bubble formation to form separated bubbles. 
     In various embodiments the droplets generated by the devices and methods described herein can contain or encapsulate a wide variety of materials. In some embodiments the droplets may contain test samples, cells, organelles, proteins, nucleic acids, enzymes, PCR or other testing reagents, biochemicals, dyes, or particulates (for example polymeric microspheres, metallic microparticles, or pigments). In still other embodiments a droplet may encapsulate one or more previously generated droplets. In addition, the invention need not be limited to aqueous droplet systems. For example, such droplet generating methods and devices may be used in nanoparticle coating, where materials in organic solvents can be used to deposit layers on or encapsulate nanoparticles. 
     As noted above, in some embodiments an opening in a fluid channel can be configured as a nozzle. The depth, inner diameter, and outer diameter of such a nozzle can be optimized to control droplet size, droplet uniformity, mixing at the fluid interface, or a combination of these. 
     In certain embodiments the droplet generation and/or droplet merger components described herein may be provided on a substrate that differs from the material that comprises the fluid channels. For example, the fluid channels may be fabricated using an elastomeric material that is disposed upon a rigid surface. Suitable fluid channel materials include but are not limited to flexible polymers such as PDMS, plastics, and similar materials. Fluid channels may also be comprised of nonflexible materials such as rigid plastics, glass, silicon, quartz, metals, and similar material. Suitable substrates include but are not limited to transparent substrates such as polymers, plastic, glass, quartz, or other dielectric materials. Other suitable substrate materials include but are not limited to nontransparent materials such as opaque or translucent plastics, silicon, metal, ceramic, and similar materials. 
     The parameters described above and in the Examples (e.g., flow rate(s), laser intensity, laser frequency/wavelength, channel dimensions, port/nozzle dimensions, channel wall stiffness, location of cavitation bubble formation, and the like) can be varied to optimize droplet formation and/or droplet/particle/cell encapsulation for a particular desired application. 
     There are a number of formats, materials, and size scales that may be used in the construction of the droplet generating devices described herein and in microfluidic devices that may incorporate them. In some embodiments the droplet generating devices and the connecting fluid channels are comprised of PDMS (or other polymers), and fabricated using soft lithography. PDMS is an attractive material for a variety of reasons, including but not limited to low cost, optical transparency, ease of molding, and elastomeric character. PDMS also has desirable chemical characteristics, including compatibility with both conventional siloxane chemistries and the requirements of cell culture (e.g. low toxicity, gas permeability). In an illustrative soft lithography method, a master mold is prepared to form the fluid channel system. This master mold may be produced by a micromachining process, a photolithographic process, or by any number of methods known to those with skill in the art. Such methods include, but are not limited to, wet etching, electron-beam vacuum deposition, photolithography, plasma enhanced chemical vapor deposition, molecular beam epitaxy, reactive ion etching, and/or chemically assisted ion beam milling (Choudhury (1997)  The Handbook of Microlithography, Micromachining, and Microfabrication , Soc. Photo-Optical Instru. Engineer.; Bard &amp; Faulkner,  Fundamentals of Microfabrication ). 
     Once prepared the master mold is exposed to a pro-polymer, which is then cured to form a patterned replica in PDMS. The replica is removed from the master mold, trimmed, and fluid inlets are added where required. The polymer replica may be optionally be treated with a plasma (e.g. an O 2  plasma) and bonded to a suitable substrate, such as glass. Treatment of PDMS with O 2  plasma generates a surface that seals tightly and irreversibly when brought into conformal contact with a suitable substrate, and has the advantage of generating fluid channel walls that are negatively charged when used in conjunction with aqueous solutions. These fixed charges support electrokinetic pumping that may be used to move fluid through the device. While the above described fabrication of a droplet generating device using PDMS, it should be recognized that numerous other materials can be substituted for or used in conjunction with this polymer. Examples include, but are not limited to, polyolefin plastomers, perfluoropolyethylene, polyurethane, polyimides, and cross-linked phenol/formaldehyde polymer resins. 
     In some embodiments single layer devices are contemplated. In other embodiments multilayer devices are contemplated. For example, a multilayer network of fluid channels may be designed using a commercial CAD program. This design may be converted into a series of transparencies that is subsequently used as a photolithographic mask to create a master mold. PDMS cast against this master mold yields a polymeric replica containing a multilayer network of fluid channels. This PDMS cast can be treated with a plasma and adhered to a substrate as described above. 
     As noted above, the methods and devices described herein are particularly suitable for use in microfluidic devices. In some embodiments therefore the fluid channels are microchannels. Such microchannels have characteristic dimensions ranging from about 100 nanometers to 1 micron up to about 500 microns. In various embodiments the characteristic dimension ranges from about 1, 5, 10, 15, 20, 25, 35, 50 or 100 microns up to about 150, 200, 250, 300, or 400 microns. In some embodiments the characteristic dimension ranges from about 20, 40, or about 50 microns up to about 100, 125, 150, 175, or 200 microns. In various embodiments the wall thickness between adjacent fluid channels ranges from about 0.1 micron to about 50 microns, or about 1 micron to about 50 microns, more typically from about 5 microns to about 40 microns. In certain embodiments the wall thickness between adjacent fluid channels ranges from about 5 microns to about 10, 15, 20, or 25 microns. 
     In various embodiments the depth of a fluid channel ranges from 5, 10, 15, 20 microns to about 1 mm, 800 microns, 600 microns, 500 microns, 400 microns, 300 microns, 200 microns, 150 microns, 100 microns, 80 microns, 70 microns, 60 microns, 50 microns, 40 microns, or about 30 microns. In certain embodiments the depth of a fluid channel ranges from about 10 microns to about 60 microns, more preferably from about 20 microns to about 40 or 50 microns. In some embodiments the fluid channels can be open; in other embodiments the fluid channels may be covered. 
     As noted above, some embodiments a nozzle is present. In certain embodiments where a nozzle is present, the nozzle diameter can range from about 0.1 micron, or about 1 micron up to about 300 microns, 200 microns, or about 100 microns. In certain embodiments the nozzle diameter can range from about 5, 10, 15, or 20 microns up to about 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or about 80 microns. In some embodiments the nozzle diameter ranges from about 1, 5, 10, 15, or 20 microns to about 25, 35, or 40 microns. 
     In some embodiments the methods and devices described herein can generate droplets at a rate ranging from zero droplets/sec, about 2 droplets/sec, about 5 droplets/sec, about 10 droplets/sec, about 20 droplets/sec, about 50 droplets/sec, about 100 droplets/sec, about 500 droplets/sec, or about 1000 droplets/sec, up to about 1,500 droplets/sec, about 2,000 droplets/sec, about 4,000 droplets/sec, about 6,000 droplets/sec, about 8,000 droplets/sec, about 10,000 droplets/sec, about 20,000 droplets/sec, about 50,000 droplets/sec, and about 100,000 droplets/sec. 
     In various embodiments the devices and methods described herein can generate droplets having a substantially continuous volume. Droplet volume can be controlled to provide volumes ranging from about 0.1 fL, about 1 fL, about 10 fL, and about 100 fL to about 1 microliter, about 500 nL, about 100 nL, about 1 nL, about 500 pL or about 200 pL. In certain embodiments volume control of the droplet ranges from about 1 pL to about 150 pL, about 200 pL, about 250 pL, or about 300 pL. 
     As indicate above, the microchannel droplet formation/merger injection devices described herein can provide a system integrated with other processing modules on a microfluidic “chip” or in flow through fabrication systems for microparticle coating, microparticle drug carrier formulation, and the like. These uses, however, are merely illustrative and not limiting. 
     Microfluidic devices can manipulate volumes as small as several nanoliters. Because the microfluidic reaction volume is close to the size of single mammalian cells, material loss is minimized in single-cell mRNA analysis with these devices. The ability to process live cells inside microfluidic devices provides a great advantage for the study of single-cell transcriptomes because mRNA is rapidly degraded with cell death. A highly integrated microfluidic device, having 26 parallel 10 nL reactors for the study of gene expression in single human embryonic stem cells (hESC) has been reported (Zhong et al. (2008)  Lab on a Chip,  8: 68-74; Zhong et al. (2008)  Curr. Med. Chem.,  15: 2897-2900). In various microfluidic devices all systems for obtaining single-cell cDNA including cell capture, mRNA capture/purification, cDNA synthesis/purification, are performed inside the device. The present devices and methods offer effective means of encapsulating and and/or separating individual cells for, e.g., further processing, 
     Any of a number of approaches can be used to convey the fluids, or mixtures of droplets, particles, cells, etc. along the channels of the devices described herein. Such approaches include, but are not limited to gravity flow, syringe pumps, peristaltic pumps, electrokinetic pumps, bubble-driven pumps, and air pressure driven pumps. 
     In certain illustrative but non-limiting embodiments two major applications of the platforms described herein are contemplated. These include: 
     1. Rapid production of a large combinatorial cocktail drug library: A 2D scanning mirror coupled with a high repetition rate pulse laser can support parallel droplet generators deployed on the same microfluidic chip (see, e.g.,  FIG. 6 ). The 3D microfluidic fabrication technique described herein can solve the cross interconnect issues found in 2D microfluidics and enable 3D microchannel routing to support up to hundreds of sample channels on the same chip for producing a large multiplexed combinatorial library. Producing a large library with 1 million different chemical combinations may take less than 3 hours (100 combinations/sec). 
     2. The droplet generation and merging platform described herein can be readily integrated with our PLACS system described in U.S. Patent Publication No: 2012/0236299 to enable high speed single cell encapsulation and downstream merging of the cell captured droplets with other biochemical reagents such as cell lysing buffers, primers, and other PCR buffers for single cell PCR analysis ( FIG. 7 ). This integrated system will enable the first high throughput FACS system that can simultaneously provide not only optical signatures of single cells but also the molecular level analysis data such as mRNA expression levels. 
     While various implementations have been described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the implementations described herein, but should be defined only in accordance with the following and later-submitted claims and their equivalents. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes. However, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. The terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.