Valve-based flow focusing for drop formation

Picoliter drops made in microfluidic devices can serve as individual compartments for chemical reactions and can be processed at kilohertz rates with high precision. This combination of speed and containment is very useful for highthroughput screening, for discovering novel drugs, for sorting analytes and worms, and for directed evolution of enzymes and cells. For applications in microfluidics, precision control of the drops is essential. In droplet based screening, drops must be formed, merged, and sorted in a particular sequence and with precise timing. This has stimulated the development of microfluidic systems that provide exceptional control of drops. Drop-on-demand microfluidics allows drop formation to be actively controlled by creating drops as needed. This allows operations between drops, such as drop merger, to be carefully timed and controlled. However, drop-on-demand devices are difficult to operate; they require sophisticated external devices, such as automated pumps, actuators, and computers, to carefully monitor and execute all operations. The intermittent nature of the devices also significantly limits their speed and their usefulness for high-throughput screens. Alternatively, continuous-flow microfluidics can operate at very high throughput. As with drop-on-demand devices, continuous-flow microfluidics can be structured like an assembly line, enabling complex tasks to be efficiently performed. With soft lithography, devices can be reproducibly engineered and easily fabricated. This allows implementation of passive forms of control. For example, channel dimensions can be carefully selected to cause drops to flow as pairs in preparation for electrically induced merger. Drops can be steered based on size, slowed in large incubation chambers, and ordered into a linear array for individualized detection. However, continuous-flow microfluidics can be difficult to operate robustly. Coupled devices can display long-lived or chaotic oscillations in which flows vary uncontrollably over time and never reach a steady state. To increase control, sensors can monitor flows and automatically adjust pumps to compensate for variations. However, due to compliance of the pumps and devices, flow rate changes controlled at the pump can have a slow response time, lagging by seconds, or even minutes, on different parts of the device. For typical drop production frequencies of 1 kHz, this is a significant limitation. Moreover, changes in flow rate affect the performance of the device as a whole and cannot control individual components in a microfluidic circuit. An optimal system would combine the simplicity and throughput of continuous-flow microfluidics, with the timing and control of drop-on-demand devices. In this paper, we introduce a simple system that provides localized, fast response-time control of drops in continuousflow microfluidics. We use single-layer membrane valves to adjust the local dimensions of the drop formation junction. We introduce two methods to control flow focusing with valves. First, we use membrane valves to control the dimensions of the drop formation orifice, thereby enabling drop size to be controlled. Second, we also use membrane valves to control the dimensions of the flow-focus side channels, thereby enabling drop frequency to be controlled. Both forms of control are localized at the flow-focus junction. Neither requires adjustment of the total flow rate. Together, they enable drop formation to be controlled in real time, over a wide range, and on a single, simple to fabricate microfluidic device. The microfluidic devices are fabricated using singlelayer soft lithography in poly dimethylsiloxane PDMS . We produce water drops in HFE-7500 fluorocarbon oil with 5% vol/vol 1H ,1H ,2H ,2H-perfluoro-1-octanol Sigma , stabilized by 1.8 wt % of the fluorosurfactant ammonium carboxylate of DuPont Krytox 157. For the valves we use single-layer membrane valves, which exist in the same plane as the microfluidic channels they control, thereby enabling the entire microfluidic device to be fabricated conveniently in a single-layer mold. For optimal valve performance, the PDMS membrane must be as thin and flexible as possible. To make thin membranes that can be reliably fabricated, we design the membrane width to be 20 m; due to limited resolution of the fabrication process, they reliably turn out at approximately 13 m in width in the PDMS device. To make the membranes very flexible, we use a 12-to-1 PDMSto-cross-linker ratio and bake the devices at 65 °C for 1 h. To actuate the valves, we pressurize the valve control channels. For the size control device, we pressurize the channels with a hand-held syringe filled with water. For the frequency control device, we use an electronically controlled pressure regulator Parker Hannefin, VSO series ; this allows highfrequency actuations to be applied to the valve, which we use to impose high-frequency signals onto the drop trains. We introduce two forms of valve-based flow focusing. In the first, we use valves to control drop size. For a flow-focus drop maker in the dripping regime, the drop size is proportional to the width of the flow-focus orifice and inversely proportional to the continuous phase flow speed. This is evident in the approximation for a channel with circular cross section, cucdd do, where c is the viscosity of the continuous phase, uc is the flow rate of the continuous phase, dd a Electronic mail: [email protected]. APPLIED PHYSICS LETTERS 94, 023503 2009