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A microfluidic system includes a bubble valve for regulating fluid flow through a microchannel. The bubble valve includes a fluid meniscus interfacing the microchannel interior and an actuator for deflecting the membrane into the microchannel interior to regulate fluid flow. The actuator generates a

A microfluidic system includes a bubble valve for regulating fluid flow through a microchannel. The bubble valve includes a fluid meniscus interfacing the microchannel interior and an actuator for deflecting the membrane into the microchannel interior to regulate fluid flow. The actuator generates a gas bubble in a liquid in the microchannel when a sufficient pressure is generated on the membrane.

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1. A microfluidic device, comprising: a microfluidic substrate;a microchannel formed in the microfluidic substrate having an interior bounded by a side wall; anda valve for regulating a flow of fluid through the microchannel, the valve comprising: a gas-filled reservoir comprising a recess formed in

1. A microfluidic device, comprising: a microfluidic substrate;a microchannel formed in the microfluidic substrate having an interior bounded by a side wall; anda valve for regulating a flow of fluid through the microchannel, the valve comprising: a gas-filled reservoir comprising a recess formed in the microfluidic substrate,a flexible membrane coupled to the microfluidic substrate and forming a single flexible external wall of the microfluidic substrate and a flexible wall of the recess to seal the gas-filled reservoir, andan actuator for selectively deflecting the flexible membrane to increase an internal pressure of the gas-filled reservoir to regulate fluid flow through the microchannel. 2. The microfluidic device of claim 1, wherein the actuator deflects the flexible membrane when signaled by a detector operating on the microfluidic device that a particle having a predetermined characteristic is detected in the microchannel. 3. The microfluidic device of claim 1, further comprising a liquid in the microchannel, wherein the liquid forms a meniscus at an interface with the gas in the gas-filled reservoir. 4. The microfluidic device of claim 1, wherein the flexible membrane is disposed on a side of the gas-filled reservoir that is remote from the microchannel. 5. The microfluidic device of claim 1, wherein the actuator contacts an external surface of the flexible membrane in an unactivated state. 6. The microfluidic device of claim 1, wherein the microchannel has a substantially constant cross-section. 7. The microfluidic device of claim 1, wherein deflecting the flexible membrane creates a pressure pulse in a fluid flowing through the microchannel. 8. The microfluidic device of claim 1, wherein the recess includes an aperture formed in a side wall adjacent to the microchannel and wherein the side wall extends contiguously in a radial direction between a first side of the aperture and a second side of the aperture. 9. The microfluidic device of claim 1, wherein the actuator deflecting the flexible membrane decreases a volume of the gas-filled reservoir. 10. The microfluidic device of claim 1, further comprising a single aperture for operatively associating the gas-filled reservoir with the microchannel. 11. The microfluidic device of claim 1, wherein the microfluidic substrate comprises a first layer defining a groove forming a portion of the microchannel and a second layer disposed between the first layer and the flexible membrane to enclose the microchannel and including the recess. 12. The microfluidic device of claim 1, wherein the recess includes a contiguous rim of the recess formed in the substrate and defining a perimeter of the recess. 13. The microfluidic device of claim 1, wherein the actuator comprises a displacement actuator. 14. The microfluidic device of claim 13, wherein the actuator comprises a piezoelectric pin having a tip that selectively pushes against and deflects the flexible membrane when activated. 15. The microfluidic device of claim 1, wherein the valve is further configured to reduce the internal pressure of the gas-filled reservoir subsequent to the regulation of the fluid flow through the microchannel. 16. The microfluidic device of claim 15, wherein, subsequent to the reduction of the internal pressure of the gas-filled reservoir, the actuator selectively deflects the flexible membrane of the valve to increase the internal pressure of the gas-filled reservoir to further regulate the fluid flow through the microchannel. 17. The microfluidic device of claim 1, wherein the increased pressure in the gas-filled reservoir pushes gas from the gas-filled reservoir into the microchannel. 18. The microfluidic device of claim 17, wherein the microchannel includes a hydrophobic patch at a location wherein the gas is pushed into the microchannel. 19. The microfluidic device of claim 17, wherein the microchannel is locally shaped into a cavity at a location wherein the gas is pushed into the microchannel. 20. The microfluidic device of claim 1, wherein the actuator is external to and operated external of the substrate. 21. The microfluidic device of claim 20, wherein the microfluidic substrate comprises: a first layer having a groove forming the microchannel; anda second layer stacked on the first layer to enclose the microchannel and including an opening forming the gas-filled reservoir adjacent to the groove;wherein the flexible membrane is bonded to the second layer to seal the gas-filled reservoir. 22. The microfluidic device of claim 21, wherein the opening in the second layer includes: a main body defining the reservoir; anda slot extending from the main body to define a side channel for placing the reservoir in fluid communication with the microchannel. 23. A microfluidic device, comprising: a microfluidic substrate;a microchannel formed in the microfluidic substrate having an interior bounded by a side wall; anda valve for regulating a flow of fluid through the microchannel, the valve comprising: a gas-filled reservoir comprising a recess,a flexible membrane coupled to the microfluidic substrate and forming a single flexible external wall of the microfluidic substrate and a flexible wall of the recess to seal the gas-filled reservoir, andan actuator for selectively deflecting the flexible membrane to increase an internal pressure of the gas-filled reservoir to regulate fluid flow through the microchannel. 24. The microfluidic device of claim 23, further comprising a layer disposed between the microfluidic substrate and the flexible membrane to enclose the microchannel and including an opening associated with the gas-filled reservoir.