Abstract:
A system for monodispersed microdroplet generation and trapping including providing a flow channel in a microchip; producing microdroplets in the flow channel, the microdroplets movable in the flow channel; providing carrier fluid in the flow channel using a pump or pressure source; controlling movement of the microdroplets in the flow channel and trapping the microdroplets in a desired location in the flow channel. The system includes a microchip; a flow channel in the microchip; a droplet maker that generates microdroplets, the droplet maker connected to the flow channel; a carrier fluid in the flow channel, the carrier fluid introduced to the flow channel by a source of carrier fluid, the source of carrier fluid including a pump or pressure source; a valve connected to the carrier fluid that controls flow of the carrier fluid and enables trapping of the microdroplets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/038,543 filed on Mar. 21, 2008 entitled “method for monodisperse microdroplet generation and stopping without coalescence for interrogation, chemical reaction, or sorting,” the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND 
     1. Field of Endeavor 
     The present invention relates to microfluidic devices and more particularly to microfluidic devices for generating and trapping monodisperse microdroplets in a microfluidic channel. 
     2. State of Technology 
     Microfluidic devices are poised to revolutionize environmental chemical, biological, medical, and pharmaceutical detectors and diagnostics. “Microfluidic devices” loosely describes the new generation of instruments that mix, react, count, fractionate, detect, and characterize complex gaseous or liquid-solvated samples in a micro-optical-electro-mechanical system (MOEMS) circuit manufactured through standard semiconductor lithography techniques. These techniques allow mass production at low cost as compared to previous benchtop hardware. The applications for MOEMS devices are numerous, and as diverse as they are complex. 
     As sample volumes decrease, reagent costs plummet, reactions proceed faster and more efficiently, and device customization is more easily realized. By reducing the reaction volume, detection of target molecules occurs faster through improved sensor signal to noise ratio over large, cumbersome systems. However, current MOEMS fluidic systems may only be scratching the surface of their true performance limits as new techniques allow for repeatable generation and manipulation of nano-scale and pico-scale microfluidic reactors, loosely termed “microdroplets.” Some popular monodisperse (same size) microdroplet generating techniques include flow focusing and the T-junction, both of which employ the water-in-oil emulsion method for generating discrete aqueous chemical and/or biological reactors, at volumes previously unheard of. For example, droplets tens of microns in diameter contain a volume in the tens of picoliters. These tiny volumes, when properly controlled, enable revolutionary science, such as: single cell isolation and analysis, single molecule detection, nucleic acid amplification from single genome copies, in-vitro protein translation, microdroplet protein crystallization, and other novel techniques. The ability to generate monodisperse droplets has been crucial for optical calibration and droplet manipulation since a polydisperse distribution of droplets changes the optical interrogation performance, as well as altering the chemical kinetics of reactions within the droplets due to differing analyte quantities. 
     To date, the limitation of monodisperse microdroplet generation has been that it is a steady-state phenomenon, generating typically hundreds to thousands of microdroplets per second. This causes a problem when the stream of droplets needs to be slowed down or stopped for subsequent on-chip manipulation, energy deposition, chemical reaction, or optical interrogation and analysis. Prior art has been limited to employing droplets only in analyses that can happen at very short timescales, which excludes many interesting problems that would benefit from the perfect isolation the spherical aqueous microreactors can provide. 
     SUMMARY 
     Features and advantages of the present invention will become apparent from the following description. Applicants are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the invention. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention. The scope of the invention is not intended to be limited to the particular forms disclosed and the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. 
     The present invention provides a system that includes a microfluidic channel, a fluid in microfluidic channel, and micro droplets in the microfluidic channel, wherein an emulsion is formed of the micro droplets in the fluid. An emulsion is a mixture of two immiscible liquids. One liquid (the dispersed phase) is dispersed in the other (the continuous phase). A commonly used emulsion is an oil/water emulsion, where oil droplets are dispersed in an aqueous mixture. In other cases aqueous droplets are dispersed in an oil, which is a water/oil emulsion. Unless otherwise specified, we use the term “Emulsion Fluid” to designate the continuous phase, or “carrier” fluid, and we use the term “Emulsion Source” to designate the “Emulsion Fluid source.” 
     The present invention provides apparatus and methods for on-chip microreactor generation and trapping for subsequent optical or electromagnetic interrogation, chemical reaction, microdroplet sorting, and/or archival. In one embodiment the present invention provides apparatus and methods for on-chip monodisperse microdroplet generation and trapping for subsequent microdroplet optical or electromagnetic interrogation, chemical reaction, microdroplet sorting, and/or archival. The present invention also describes a system for channel washing, if desired, between experiments. In one embodiment the present invention provides an apparatus for monodispersed microdroplet generation and trapping including a microchip; a flow channel in the microchip; a droplet maker that generates microdroplets, the droplet maker connected to the flow channel; a carrier fluid in the flow channel, the carrier fluid introduced to the flow channel by a source of carrier fluid, the source of carrier fluid including a pump or pressure source; a valve connected to the carrier fluid that controls flow of the carrier fluid and enables trapping of the microdroplets. In another embodiment the present invention provides a method of monodispersed microdroplet generation and trapping that includes the steps of providing a flow channel in a microchip; producing microdroplets in the flow channel, the microdroplets movable in the flow channel; providing carrier fluid in the flow channel using a pump or pressure source; controlling movement of the microdroplets in the flow channel and trapping the microdroplets in a desired location in the flow channel. 
     The present invention has uses in biowarfare detection applications for identifying, detecting, and monitoring bio-threat agents that contain nucleic acid signatures, such as spores, bacteria, viruses etc. The present invention has uses in biomedical applications for tracking, identifying, and monitoring outbreaks of infectious disease including emerging, previously unidentified and genetically engineered pathogens; for automated processing, amplification, and detection of host or microbial and viral DNA or RNA in biological fluids for medical purposes; for automated processing and detection of proteomic signatures in biological fluids; for cell cytometry or viral cytometry in fluids drawn from clinical or veterinary patients for subsequent analysis; for high throughput genetic screening for drug discovery and novel therapeutics; for in-vitro protein translation; for on-chip cell isolation; and for genetic sequencing and SNP detection. 
     The present invention has uses in forensic applications for automated processing, amplification, and detection DNA in biological fluids for forensic purposes. The present invention has uses in food and beverage safety for automated food testing for bacterial or viral contamination. The present invention has uses in chemical analysis and synthesis for protein crystallography, emulsification, fluid partitioning, and nanoparticle synthesis. The present invention has uses in biochemical detection for contraband detection, explosives detection, nanoscale reactor generation, picoscale reactor generation. 
     The invention is susceptible to modifications and alternative forms. Specific embodiments are shown by way of example. It is to be understood that the invention is not limited to the particular forms disclosed. The invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate specific embodiments of the invention and, together with the general description of the invention given above, and the detailed description of the specific embodiments, serve to explain the principles of the invention. 
         FIG. 1  is an illustration of monodispersed microdroplets generated and trapped on a chip. 
         FIGS. 2A ,  2 B, and  2 C illustrate three embodiments of systems for generating a monodisperse stream of microdroplets and subsequently stopping the stream of microdroplets without droplet coalescence. 
         FIGS. 3A-3H  illustrate the stopping action of a droplet in the flow channel. 
         FIG. 4  is a graph illustrating droplet velocity profile verses time during droplet stopping. 
         FIG. 5  illustrates an example of 11 stopped droplets containing MS2 bacteriophage that were subjected to RT-PCR real-time amplification and detection. 
         FIG. 6  illustrates forming an emulsion using two different fluids. 
         FIG. 7  illustrates a system for droplet stopping with mirodroplets containing magnetic beads. 
         FIG. 8  a system for droplet stopping with a double T-junction generator for producing alternating droplets of solution A and solution B respectively. 
         FIGS. 9A and 9B  provide additional details of the system for droplet stopping with a double T-junction generator for producing alternating droplets of solution A and solution B respectively. 
         FIG. 10  illustrates a system for droplet stopping with mirodroplets containing non magnetic particles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, to the following detailed description, and to incorporated materials, detailed information about the invention is provided including the description of specific embodiments. The detailed description serves to explain the principles of the invention. The invention is susceptible to modifications and alternative forms. The invention is not limited to the particular forms disclosed. The invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. 
     Referring now to the drawings and in particular to  FIG. 1 , an illustration of monodispersed microdroplets generated and trapped on a chip is illustrated. The system illustrated in  FIG. 1  includes the following structural elements: microfluidic device  100 , chip  102 , microfluidic channel  104 , cover plate  106 , micro droplets  108  and a fluid  110  that creates an emulsion of the micro droplets  108  in the fluid  110 . The system provides generation of a monodisperse stream of microdroplets and subsequent stopping the stream of microdroplets without droplet coalescence. The article “Monodisperse droplet generation and rapid trapping for single molecule detection and reaction kinetics measurement” by Neil Reginald Beer, Klint Aaron Rose and Ian M. Kennedy (Article citation: Neil Reginald Beer,  Lab Chip,  2009, DOI: 10.1039/b818478j) provides additional information about generating a monodisperse stream of microdroplets and subsequently stopping the stream of microdroplets without droplet coalescence. The article “Monodisperse droplet generation and rapid trapping for single molecule detection and reaction kinetics measurement” by Neil Reginald Beer, Klint Aaron Rose and Ian M. Kennedy (Article citation: Neil Reginald Beer,  Lab Chip,  2009, DOI: 10.1039/b818478j) is incorporated herein in its entirety by this reference. 
     The present invention provides apparatus and methods for generating a monodisperse stream of microdroplets and subsequently stopping the stream of microdroplets without droplet coalescence for as long as the droplets need to be stopped to allow chemical reaction, energy deposition, and optical interrogation to occur, such as in the on-chip Polymerase Chain Reaction process. The flow may then be restarted, seconds, minutes, hours, or days later to continue the experiment, sort or archive microdroplets, refresh the microfluidic channels, etc. The present invention enables much longer photon acquisition times for single molecule/low light detection, it allows for non-instantaneous chemical reactions (the majority), and it allows for sorting and archival at a pace that matches data acquisition and control system capability. The microdroplets in the micro-nano-pico or femtoliter range can be chemically reacted, heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This greatly expands the use of microdroplet chemical and optical analysis of microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Referring now to  FIGS. 2A ,  2 B, and  2 C, three embodiments of systems for generating a monodisperse stream of microdroplets and subsequently stopping the stream of microdroplets without droplet coalescence constructed in accordance with the present invention are illustrated. The three embodiments provide methods and apparatus for generating monodisperse microdroplets and trapping them indefinitely in the same microfluidic channel without droplet coalescence or addition of surfactants (which alter droplet chemistry). In some embodiments of the present invention surfactants are added. These microdroplets in the micro-nano-pico or femtoliter range can then be chemically reacted, heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This ability will greatly expand the use of microdroplet chemical and optical analysis of microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Furthermore, microfluidic device designs will change, eliminating complex serpentine channels (developed to lengthen path lengths to keep droplets on the chip and in-view longer), as they are no longer needed with the ability to stop droplets and interrogate them at-will in real time. This will free up valuable real estate on the chip surface, to be used for other important functions. Additionally, exciting new studies in single molecule detection, which require long optical exposure times to collect enough photons for detection above the background noise, can now be studied in stationary, monodisperse, aqueous microdroplets, whereas in the past they were either limited in photon collection times or performed on fixed substrates—a significant limitation to the chemistries involved. 
     Flow in microfluidic devices is governed by Stoke&#39;s flow. (Stoke&#39;s flow is a linearized solution of the Navier-Stokes equations and accounts for the insignificance of inertia affects to flow in micron-scale channels.)
 
∇ p=μ∇   2   {right arrow over (V)}   [Equation 1]
 
Which reduces to:
 
 F= 6 πμU   0   a   [Equation 21]
 
     Where F is the force on the droplet, μ is the fluid media viscosity, U 0  is the mean flow rate, and a is the particle radius. For emulsions, this art can employ an upstream microdroplet generator, such as a T-junction or a flow focusing microdroplet generator to create microdroplet reactors (aqueous droplets in an oil carrier flow or oil droplets in an aqueous flow) that are partitioned from each other and the fluid medium by the oil/water interface. This provides a method to generate and trap monodisperse droplets on-chip. This is possible by the dynamics of the Stoke&#39;s flow equation. Note there is no inertia term in this flow regime, so once the supply pressure or flow rate is stopped (valved) the flow stops instantaneously. This effect, correctly applied, provides the capability to increase the signal to noise or improve the instrument&#39;s limit of detection (LOD) proportional to the amount of time droplets are interrogated before restarting the flow. This allows collection of light (or signal) to be limited only by photobleaching of the fluorophore or the dynamics of the reaction directly under observation. This presents an ideal case for the optical laboratory on a chip, the ability to interrogate at-will individually partitioned microreactors of constant size (monodisperse) for as long as desired before restarting the flow and resuming the experiment on a different field of microdroplets. This method will quickly find itself in instruments that perform PCR, binding assays, chemical detection, nanoparticle synthesis, crystallization, and any other systems where near real-time performance or highly sensitive limits of detection are beneficial. This performance enhancement may provide the transformational impetus needed to move many biomedical instruments from the laboratory to point-of-care locations, and hence multiply the number of instruments needed. Furthermore, this method works well with magnetic nanoparticles solvated in the aqueous solution, allowing microdroplet reactors with the bead substrate to be generated, stopped, and operated identically to simple aqueous microdroplets, without the need for magnetic trapping. 
     This method specifically entails a single fast valve actuation to simultaneously close all lines to and from the microfluidic device sharing channel connectivity. By employing a custom multiport valve configuration, but rotating the valve rotor between ports through multiposition stepper motor actuation, all lines to and from the device may be taken from open (droplet generation) to closed (droplet trapping) in milliseconds. Given the noninertial character of Stoke&#39;s flow, this method has proven to trap monodisperse microdroplets on the device indefinitely. This method works on soft device substrates such as PolyDiMethylSiloxane (PDMS), and is even more effective on harder substrates due to the lack of pressure capacitance in the fluid path. A glass or silicon device substrate, such as those made from the common photolithography chip manufacturing process, is ideal, and can be coupled to the valve through PEEK, stainless steel, polycarbonate or other relatively inelastic lines. 
     Referring now to  FIG. 2A  a system for droplet stopping with a T-junction generator is illustrated. The system is designated generally by the reference numeral  200 . The system  200  includes the following structural elements: multiport valve  202 , ports  202   a - 202   f , emulsion fluid source  204 , emulsion fluid  204   a , pump or pressure source  206 , line  208  (emulsion fluid), microfluidic channel  210 , T-junction  212 , aqueous solution  214 , pump or pressure source  216 , line (Aqueous)  218 , directional arrow  220 , droplets or microreactors  222 , return line  224 , and waste container  226 , and microreactor maker  228 . The sample to be analyzed or otherwise acted upon is usually contained in the aqueous solution  214   a.    
     In operation an aqueous fluid with the desired chemical reagents that may or may not include magnetic nanoparticles is injected into the cross-channel flow of oil carrier fluid (T-junction shearing) as illustrated in  FIG. 2A . Pump  206  delivers emulsion fluid  204   a  from the emulsion fluid source  204  through line  208  to port  202   e  on multiport valve  202 . Emulsion fluid  204   a  exits valve  202  at port  202   f  and flows through line  208  and enters microfluidic channel  210 . 
     Pump  216  delivers aqueous solution  214  through line  218  to valve  202  at port  202   a . The aqueous solution  214  exits valve  202  at port  202   b  and flows thru line  218  to T-Junction  212 . The emulsion fluid  204   a  flowing in microfluidic channel  210  in the direction shown by arrow  220  interacts with aqueous solution  214   a  at T-Junction  212  and by a shearing action droplets  222  are formed producing an emulsion of droplets  222  in fluid  204   a . The aqueous solution droplets  222  formed of the aqueous solution  214   a  do not mix with fluid  204   a  and remain suspended as discrete droplets. By repositioning valve  202  all ports can be shut effectively stopping all motion in microfluidic channel  210 . This essentially freezes droplets  222  in some pre-determined position in order to perform some task on the droplets  222 , Upon completion of the process on the droplets  222  the valve  202  can be repositioned and the droplets  222  can flow along return line  222  and pass thru ports  202   c  &amp;  202   d  on valve  202  and on to waste container  226 . 
     The system  200  provides apparatus and methods for generating a monodisperse stream of microdroplets  222  and subsequently stopping the stream of microdroplets  222  without droplet coalescence for as long as the droplets need to be stopped to allow chemical reaction, energy deposition, and optical interrogation to occur, such as in the on-chip Polymerase Chain Reaction process. The flow may then be restarted, seconds, minutes, hours, or days later to continue the experiment, sort or archive microdroplets, refresh the microfluidic channels, etc. The system  200  enables much longer photon acquisition times for single molecule/low light detection, it allows for non-instantaneous chemical reactions (the majority), and it allows for sorting and archival at a pace that matches data acquisition and control system capability. The microdroplets in the micro-nano-pico or femtoliter range can be chemically reacted, heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This greatly expands the use of microdroplet chemical and optical analysis of microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Referring now to  FIG. 2B  a system for droplet stopping with flow focusing is illustrated. The system is designated generally by the reference numeral  300 . The system  300  includes the following structural elements: multiport valve  302 , ports  302   a - 302   f , emulsion fluid source  304 , emulsion fluid  304   a , pump or pressure source  306 , line  308  (emulsion fluid), microfluidic channel  310 , flow focusing ports  312 , aqueous solution  314 , pump or pressure source  316 , line (Aqueous)  318 , directional arrow  320 , droplets  322 , return line  324 , and waste container  326 . 
     In operation an aqueous fluid with the desired chemical reagents that may or may not include magnetic nanoparticles is injected into the flow focusing area of cross-channel flow of oil carrier fluid as illustrated in  FIG. 2B . Pump  306  delivers emulsion fluid  314   a  thru line  308  to port  302   e  on multiport valve  302 . Emulsion fluid  314   a  exits valve  302  at port  302   f  and flows along line  308  and enters flow focusing ports  312 . Pump  316  delivers aqueous solution  314  thru line  318  to port  302   a  on valve  302 . The aqueous solution  314  exits valve  302  at port  302   b  and flows thru line  318  and enters microfluidic channel  310 . 
     The aqueous solution  314   a  flowing in microfluidic channel  310  in the direction of arrow  320  meets the emulsion fluid  304   a  entering microfluidic channel  310  thru flow focusing ports  312 . The emulsion fluid  304   a  and aqueous solution  314   a  do not want to mix so at the interface  328  of emulsion fluid &amp; aqueous solution  314   a  a flow focusing action occurs and droplets  322  are formed and separate into discrete droplets  322  which are entrained and suspended as an emulsion of droplets  322  in aqueous solution  314   a . By repositioning valve  302  all the ports can be shut effectively stopping all motion in the microfluidic channel  310 . This essentially freezes droplets  322  at some predetermined position on the chip in order to perform some task on the droplets  322 . Upon completion of the process on the droplets  322  the valve  302  can be repositioned and the droplets  322  can flow along return line  324  and pass thru ports  302   c  &amp;  302   d  on valve  302  and on to waste container  326 . 
     Referring now to  FIG. 2C  a system for droplet stopping with a shear action slug generator is illustrated. The system is designated generally by the reference numeral  400 . The system  400  includes the following structural elements: multiport valve  402 , ports  402   a - 402   f , emulsion fluid source  404 , emulsion fluid  404   a , pump or pressure source  406 , line  408 , microfluidic channel  410 , channels for shear action slug generation  412 , aqueous solution  414 , pump or pressure source  416 , line (Aqueous)  418 , directional arrow  420 , slugs  422 , return line  424 , waste container  426 , and interface aqueous solution and emulsion  428 . 
     In operation an aqueous fluid with the desired chemical reagents that may or may not include magnetic nanoparticles is injected into the cross-channel flow of oil carrier fluid as illustrated in  FIG. 2C . In operation an aqueous fluid with the desired chemical reagents that may or may not include magnetic nanoparticles is injected into the cross-channel flow of oil carrier fluid as illustrated in  FIG. 2C . Pump  406  delivers emulsion fluid  404   a  from emulsion fluid source  404  through line  408  to port  402   e  on multiport valve  402 . Emulsion fluid  404  exits valve  402  at port  402   f  and flows through line  408  and enters microfluidic channel  410  through the two flow focusing ports  412 . 
     Pump  416  delivers aqueous solution  414   a  from the aqueous solution source  414  through line  418  to valve  402  at port  402   a . The aqueous solution  414   a  exits valve  402  at port  402   b  and flows thru line  418  to the channels for shear action slug generation  412 . The emulsion fluid  404   a  flowing in microfluidic channel  410  interacts with aqueous solution  414   a  at the channels for shear action slug generation (double T-Junction)  412  and by a shearing action slugs  422  are formed. The aqueous solution slugs  422  formed of the aqueous solution  414   a  do not mix with fluid  404   a  and are transported in the direction of arrow  420  as discrete slugs, i.e. as an emulsion of aqueous solution  414 , in fluid  404   a . By repositioning valve  402  all ports can be shut effectively stopping all motion in microfluidic channel  410 . This essentially freezes slugs  422  in some pre-determined position in order to perform some task on the slugs  422 . Upon completion of the process on the slugs  422  the valve  402  can be repositioned and the slugs  422  can flow along return line  422  and pass thru ports  402   c  &amp;  402   d  on valve  402  and on to waste container  426 . 
     The system  400  provides apparatus and methods for generating a monodisperse stream of microslugs  422  and subsequently stopping the stream of microslugs  422  without droplet coalescence for as long as the slugs need to be stopped to allow chemical reaction, energy deposition, and optical interrogation to occur, such as in the on-chip Polymerase Chain Reaction process. The flow may then be restarted, seconds, minutes, hours, or days later to continue the experiment, sort or archive microslugs, refresh the microfluidic channels, etc. The system  400  enables much longer photon acquisition times for single molecule/low light detection, it allows for non-instantaneous chemical reactions (the majority), and it allows for sorting and archival at a pace that matches data acquisition and control system capability. The microslugs can be chemically reacted, heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This greatly expands the use of microdroplet or micro slug chemical and optical analysis of microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Referring now to  FIGS. 3A-3H , the stopping action of a droplet in the flow channel is illustrated. The illustrations of  FIGS. 3A-3H  represent droplet stopping action images taken by a camera. The droplet stopping in action images were taken at 0.5 ms intervals by a MotionPro HS-4 CMOS camera with a 10× objective during droplet stopping. Every tenth image is shown in  FIGS. 3A-3H . Images after 30 ms were identical and omitted for clarity. Droplets suspended in emulsion  502  are moving thru microfludic channel  504  in the direction shown by arrow  506 . The droplet of interest  508  is shown cross hatched for clarity. The line  510  defines the droplets stopped position. Time to is approximate. Note the same droplet is tracked through the frames to show the deceleration. Flow in the channel is from left to right. Initial oil emulsifier flow rate is 10 μL/min, aqueous flow rate is 0.3 μL/min. Channel cross section is elliptical, 150 μm wide by 60 pm deep. Computed droplet stopping time is 38 ms. 
     Referring now to  FIG. 4  a graph illustrates droplet velocity profile verses time during droplet stopping. Initial oil emulsifier flow rate is 10 μL/min, aqueous flow rate is 0.3 μL/min. Average droplet velocity prior to stopping is 26.1 mm/sec. Note the ˜38 ms stopping timer defined as the time taken for the velocity to fall from 26.1 mm/sec to 0.3% of that value. A considerable contribution to the noise prior to droplet stopping is due to droplet production. Noise due to image analysis is evident after ˜900 ms when the droplets are stopped and is primarily from an ˜285 Hz arc lamp light source intensity fluctuation. 
     Referring now to  FIG. 5  an example of 11 stopped droplets containing MS2 bacteriophage that were subjected to subsequent on-chip RT-PCR real-time amplification and detection is illustrated. Droplets  608  are illustrated in a microfluidic channel  610  carried by an emulsion fluid  604 . Note the constant droplet separation and droplet monodispersity even after the rigorous thermal cycling regimen. Droplet size is ˜32 μm (17 pL). Coalescence is notably absent even though no surfactants were employed. Droplets remain at channel center due to the hydrophobic surface coatings at the channel walls. Channel cross section is elliptical, 150 μm wide by 60 μm deep. In some embodiments of the present invention surfactants are added. 
     Referring now to  FIG. 6  a system for droplet stopping with a T-junction generator is illustrated. The system is designated generally by the reference numeral  600 . The system  600  illustrates forming an emulsion using two different fluids. An emulsion is a mixture of two immiscible liquids. One liquid (the dispersed phase) is dispersed in the other (the continuous phase). A commonly used emulsion is an oil/water emulsion. 
     In the system  600  the emulsion fluid used to form the droplets contains the sample and the aqueous solution functions as the carrier fluid. The system  600  includes the following structural elements: multiport valve  602 , ports  602   a - 602   f , emulsion fluid source  604 , emulsion fluid  604   a , pump or pressure source  606 , line  608  (Emulsion Fluid), microfluidic channel  610 , T-junction  612 , aqueous solution source  614 , aqueous solution  614   a , pump or pressure source  616 , line (Aqueous)  618 , directional arrow  620 , droplets  622 , return line  624 , and waste container  626 . 
     In operation a fluid with the desired chemical reagents that may or may not include magnetic nanoparticles and contains the sample to be analyzed or acted upon is injected into the cross-channel flow of aqueous carrier fluid (T-junction shearing) as illustrated in  FIG. 6 . Pump  606  delivers emulsion fluid from an emulsion fluid source  604  through line  608  to port  602   a  on multiport valve  602 . Emulsion fluid  604  exits valve  602  at port  602   b  and flows through the line  608  to produce droplets  622 . 
     Pump  616  delivers aqueous solution  614   a  from aqueous solution source  614  through line  618  to valve  602  at port  602   e . The aqueous solution  614   a  exits valve  602  at port  602   f  and flows into the flow channel  610 . The aqueous solution flowing in microfluidic channel  610  in the direction shown by arrow  620  interacts with the emulsion fluid at T-junction  612  and droplets  622  are formed by a shearing action. The droplets  622  do not mix with aqueous solution  614   a  and remain suspended in the aqueous solution  614   a  as discrete droplets. By repositioning valve  602  all ports can be shut effectively stopping all motion in microfluidic channel  610 . This essentially freezes droplets  622  in some pre-determined position in order to perform some task on the droplets  622 . Upon completion of the process on the droplets  622  the valve  602  can be repositioned and the droplets  622  can flow along return line  622  and pass thru ports  602   c  &amp;  602   d  on valve  602  and on to waste container  626 . 
     The system  600  provides apparatus and methods for generating a monodisperse stream of microdroplets  622  and subsequently stopping the stream of microdroplets  622  without droplet coalescence for as long as the droplets need to be stopped to allow chemical reaction, energy deposition, and optical interrogation to occur, such as in the on-chip Polymerase Chain Reaction process. The flow may then be restarted, seconds, minutes, hours, or days later to continue the experiment sort or archive microdroplets, refresh the microfluidic channels, etc. The system  600  enables much longer photon acquisition times for single molecule/low light detection, it allows for non-instantaneous chemical reactions (the majority), and it allows for sorting and archival at a pace that matches data acquisition and control system capability. The microdroplets in the micro-nano-pico or femtoliter range can be chemically reacted, heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This greatly expands the use of microdroplet chemical and optical analysis is microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Referring now to  FIG. 7  a system for droplet stopping with a T-junction generator is illustrated. The system is designated generally by the reference numeral  700 . The system  700  is similar to the system show in  FIG. 2A . The system  700  additionally incorporates a source of nano-magnetic beads to which the sample to be analyzed will adhere and be contained in the droplets formed at the T-junction. When the droplet stream is stopped the magnets can be activated and the beads will be pulled free of the droplets and retained by the magnets for further processing. The system  700  includes the following structural elements: multiport valve  702 , ports  702   a - 702   f , emulsion fluid source  704 , emulsion fluid  704   a , pump or pressure source  706 , line  708  (Emulsion Fluid), micofluidic channel  710 , T-junction  712 , aqueous solution  714 , pump or pressure source  716 , line (Aqueous)  718 , directional arrow  720 , droplets  722 , return line  724 , waste container  726 , magnetic particle source  728 , magnetic particles  730 , and electro magnets  732 . 
     In operation an aqueous fluid with the desired chemical reagents that may or may not include magnetic nanoparticles is injected into the cross-channel flow of carrier fluid (T-junction shearing) as illustrated in  FIG. 7 . Pump  706  delivers emulsion fluid  704  through line  708  to port  702   e  on multiport valve  702 . Emulsion fluid  704  exits valve  702  at port  702   f  and flows through line  708  and enters microfluidic channel  710 . 
     Pump  716  delivers aqueous solution  714  through line  718  to valve  702  at port  702   a . The aqueous solution  714  exits valve  702  at port  702   b  and flows thru line  718  to T-function  712 . The emulsion fluid  704  flowing in microfluidic channel  710  in the direction shown by arrow  720  interacts with aqueous solution  714  at T-Junction  712  and by a shearing action droplets  722  are formed. The aqueous solution droplets  722  formed of the aqueous solution  714  do not mix with emulsion fluid  704  and remain suspended as discrete droplets. By repositioning valve  702  all ports can be shut effectively stopping all motion in microfluidic channel  710 . This essentially freezes droplets  722  in some pre-determined position in order to perform some task on the droplets  722 . Upon completion of the process on the droplets  722  the valve  702  can be repositioned and the droplets  722  can flow along return line  722  and pass thru ports  702   c  &amp;  702   d  on valve  702  and on to waste container  726 . 
     The system  700  includes a source  728  of nano-magnetic beads  730  to which the sample to be analyzed will adhere and be contained in the droplets  722  formed at the T-junction  712 . When the droplet stream is stopped the magnets  732  can be activated and the beads will be pulled free of the droplets and retained by the magnets for further processing. 
     The system  700  provides apparatus and methods for generating a monodisperse stream of microdroplets  722  and subsequently stopping the stream of microdroplets  722  without droplet coalescence for as long as the droplets need to be stopped to allow chemical reaction, energy deposition, and optical interrogation to occur, such as in the on-chip Polymerase Chain Reaction process. The flow may then be restarted, seconds, minutes, hours, or days later to continue the experiment sort or archive microdroplets, refresh the microfluidic channels, etc. The system  700  enables much longer photon acquisition times for single molecule/low light detection, it allows for non-instantaneous chemical reactions (the majority), and it allows for sorting and archival at a pace that matches data acquisition and control system capability. The microdroplets in the micro-nano-pico or femtoliter range can be chemically reacted, heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This greatly expands the use of microdroplet chemical and optical analysis is microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Referring now to  FIG. 8  a system for droplet stopping with a double T-junction generator for producing alternating droplets of solution A and solution B respectively is illustrated. The system is designated generally by the reference numeral  800 . The system  800  includes the following structural elements: multiport valve  802 , ports  802   a - 802   h , emulsion fluid source  804 , emulsion fluid  804   a , pump or pressure source  806 , line  808  (emulsion), microfluidic channel  810 , T-junctions  812   a  and  812   b , aqueous solution  814 , aqueous solution  816 , pump or pressure source  818 , pump or pressure source  820 , line (Aqueous Solution A)  822 , directional arrow  826 , droplet containing aqueous solution A  828 , droplet containing aqueous solution B  830 , return line  832 , and waste container  834 . 
     In operation an aqueous fluid (aqueous solution A reference numeral  828  and aqueous solution B reference numeral  830 ) with the desired chemical reagents that may or may not include magnetic nanoparticles is injected into the cross-channel flow of oil carrier fluid (Double T-junction Shearing at  812   a  and  812   b ) as illustrated in  FIG. 8  producing alternate microdroplets  828  and  830 . Pump  806  delivers emulsion fluid  804  through line  808  to port  802   f  on multiport valve  802 . Emulsion fluid  804  exits valve  802  at port  802   e  and flows through line  808  into microfluidic channel  810 . 
     Pump  818  delivers an aqueous solution (Aqueous Solution A)  814  to valve  802  at port  802   c . The aqueous solution A  814  exits valve  802  at port  802   d  and flows thru line  822  to T-Junction  812   a . The emulsion fluid  804  flowing in microfluidic channel  810  in the direction shown by arrow  826  interacts with aqueous solution A  814  at T-Junction  812   a  and by a shearing action droplets  828  are formed. The aqueous solution droplets  828  formed of the aqueous solution A  814  do not mix with emulsion fluid  804  and remain suspended in the emulsion fluid  804  as discrete droplets of aqueous solution A  814 . 
     Pump  820  delivers an aqueous solution (Aqueous Solution B)  816  to valve  802  at port  802   a . The aqueous solution B  814  exits valve  802  at port  802   b  and flows thru line  824  to T-junction  812   b . The emulsion fluid  804  flowing in microfluidic channel  810  in the direction shown by arrow  826  interacts with aqueous solution B  816  at T-Junction  812   b  and by a shearing action droplets  830  are formed. The aqueous solution B droplets  830  formed of the aqueous solution B  814  do not mix with emulsion fluid  804  and remain suspended in the emulsion fluid  804  as discrete droplets of aqueous solution B  816 . 
     By repositioning valve  802  all ports can be shut effectively stopping all motion in microfluidic channel  810 . This essentially freezes droplets  828  and  830  in some pre-determined position in order to perform some task on the droplets  828  and/or  830 . Upon completion of the process on the droplets  828  and  830  the valve  802  can be repositioned and the droplets  828  and  830  can flow along return line  822  and pass thru ports  802   g  &amp;  802   h  on valve  802  and on to waste container  834 . 
     The system  800  provides apparatus and methods for generating a monodisperse stream of microdroplets  828  and  830  and subsequently stopping the stream of microdroplets  828  and  830  without droplet coalescence for as long as the droplets need to be stopped to allow chemical reaction, energy deposition, and optical interrogation to occur, such as in the on-chip Polymerase Chain Reaction process. The flow may then be restarted, seconds, minutes, hours, or days later to continue the experiment, sort or archive microdroplets, refresh the microfluidic channels, etc. The system  800  enables much longer photon acquisition times for single molecule/low light detection, it allows for non-instantaneous chemical reactions (the majority), and it allows for sorting and archival at a pace that matches data acquisition and control system capability. The microdroplets in the micro-nano-pico or femtoliter range can be chemically reacted heated, cooled, optically interrogated, sorted and analyzed for as long as desired before channel flow is restarted. This greatly expands the use of microdroplet chemical and optical analysis is microfluidics because reactions will no longer have to take place within the millisecond time frames that moving droplets are in view of a device&#39;s optical window. 
     Referring now to  FIGS. 9A and 9B  additional detail of the system  800  for droplet stopping with a double T-junction generator for producing alternating droplets of solution A and solution B respectively are illustrated. In operation the aqueous fluid (aqueous solution A reference numeral  828  and aqueous solution B reference numeral  830 ) is injected into the flow channel  810  of oil carrier fluid  804  using the double T-junction shearing at  812   a  and  812   b  to produce alternate microdroplets  828  and  830 . Microdroplets  828  contain solution A and microdroplets  830  contain solution B 
     The aqueous solution A flows thru line  822  to T-Junction  812   a . The emulsion fluid  804  flowing in microfluidic channel  810  in the direction shown by arrow  826  interacts with aqueous solution A from line  822  at T-Junction  812   a  and by a shearing action forms droplets  828 . The aqueous solution droplets  828  formed of the aqueous solution A  814  do not mix with emulsion fluid  804  and remain suspended in the emulsion fluid  804  as discrete droplets of aqueous solution A. 
     The aqueous solution B flows thru line  824  to T-Junction  812   b . The emulsion  804  flowing in microfluidic channel  810  in the direction shown by arrow  826  interacts with aqueous solution B from line  824  at T-Junction  812   b  and by a shearing action forms droplets  830 . The aqueous solution droplets  830  formed of the aqueous solution B do not mix with emulsion fluid  804  and remain suspended in the emulsion fluid  804  as discrete droplets of aqueous solution B. 
     Referring now to  FIG. 10  a system for droplet stopping with mirodroplets with non magnetic particles is illustrated. The system is designated generally by the reference numeral  1000 . The system  1000  is similar to the system show in  FIG. 2A . The system  1000  additionally incorporates a source of non magnetic particles to which the sample to be analyzed will adhere and be contained in the droplets formed at the T-junction. The system  1000  includes the following structural elements: multiport valve  1002 , ports  1002   a - 1002   f , emulsion fluid source  1006 , emulsion fluid  1004   a , pump or pressure source  1004 , line  1008  (Emulsion Fluid), microfluidic channel  1010 , T-junction  1012 , aqueous solution  1014 , pump or pressure source  1016 , line (Aqueous)  1018 , directional arrow  1020 , droplets  1022 , return line  1024 , waste container  1026 , non magnetic particles source  1028 , and non magnetic particles  1030 . 
     In operation an aqueous fluid  1014  that includes non magnetic nanoparticles  1030  is injected into the cross-channel flow of emulsion  1004   a  and subjected to T-junction shearing at  1012  as illustrated in  FIG. 10 . Pump  1004  delivers emulsion fluid  1004   a  through line  1008  to port  1002   e  on multiport valve  1002 . Emulsion fluid  1004   a  exits valve  1002  at port  1002   f  and flows through line  1008  and enters microfluidic channel  1010 . 
     Pump  1016  delivers aqueous solution  1014  through line  1018  to valve  1002  at port  1002   a . The aqueous solution  1014  exits valve  1002  at port  1002   b  and flows thru line  1018  to T-Junction  1012 . The emulsion fluid  1004   a  flowing in microfluidic channel  1010  in the direction shown by arrow  1020  interacts with aqueous solution  1014   a  at T-Junction  1012  and by a shearing action droplets  1022  are formed. The system  1000  includes a source of non magnetic particles  1028  to which the sample to be analyzed will adhere and be contained in the droplets  1022  formed at the T-junction  1012 . 
     The aqueous solution droplets  1022  containing the non magnetic particles  1030  formed of the aqueous solution  1014   a  do not mix with emulsion fluid  1004   a  and remain suspended in the emulsion fluid  1004   a  as discrete droplets. By repositioning valve  1002  all ports can be shut effectively stopping all motion in microfluidic channel  1010 . This essentially freezes droplets  1022  in some pre-determined position in order to perform some task on the droplets  1022 . Upon completion of the process on the droplets  1022  the valve  1002  can be repositioned and the droplets  1022  can flow along return line  1022  and pass thru ports  1002   c  &amp;  1002   d  on valve  1002  and on to waste container  1026 . 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.