Multiphase flows in polymer microfluidic systems

Abstract

Continuous delivery of segmented reagents using pressure-driven multiphase flow in microchannels is a promising technology for high throughput microfluidic bioassays. Separation and encapsulation of the target reagents with another inert fluid provide many advantages over single phase flow in microfluidic applications of biotechnology. In order to achieve these advantages and control these multiphase flows, it is necessary to understand their generation and transport characteristics as influenced by geometrical miniaturization, channel wall properties, the effects of surfactants and operating conditions. For gas-liquid two-phase flow, dry air and deionized water were driven into hot embossed PMMA microchannels with 200 μm square test microchannels. Flow regimes, flow maps and the lengths of the gas bubbles and liquid plugs in terms of the liquid volumetric flow ratio (βL) were determined. Continuous generation of regular segmented flow was also discussed. Three sub-regimes of the Segmented flow were identified based on the statistical phase length scales observed over a substantial test channel length. For the liquid-liquid segmented flow, deionized water and perfluorocarbon with a surfactant were used as test fluids in the hot embossed polycarbonate microchannels. The effects of three expansion ratios from the injection to the test channels of 2, 4, and 16 were investigated comparing the flow regimes, transitions and maps in terms of a fixed carrier fluid volumetric flow ratio. The length of the dispersed fluids and the distance between consecutive droplets or plugs in terms of the carrier fluid volumetric flow ratio (βC) were determined. Velocities of the dispersed droplets and plugs were measured using double-pulsed laser illumination and were found to be 1.46 ± 0.08 and 1.25 ± 0.05 times faster than the superficial velocity of the segmented flow, respectively. The multiphase flow pressure drops were measured for all of the flow regimes in gas-liquid two-phase and liquid-liquid segmented flows. Each flow regime identified on the basis of topological observations, including the length scale of each fluid phase and the number of the gas bubbles or dispersed droplets in unit length with respect to the volumetric flow ratio, was associated with different trends in the pressure drop variation.