Liquid-Liquid Segmented Flows in Polymer Microfluidic Channels

Liquid-liquid segmented flows in microchannels fabricated on polymer test chips were investigated experimentally. Polymer test chips were prepared using hot embossing of polycarbonate (PC) sheets with micro-milled brass mold inserts. Three different configurations of microchannels were prepared with injection to test channels expansion ratios of 16, 4 and 2 and a fixed test channel geometry. Deionized water with blue food-coloring dye (1% v/v) was used as a dispersed fluid at flow rates (Q_D) between 0.5 and 60 μl/min. The carrier fluid was perfluorocarbon (FC 3283) with nonionic fluorous-soluble surfactant (Perfluorooctanol, 10% v/v) at flow rates (Q_C) between 3 and 25 μl/min. The two fluids were injected separately into the chips. Droplet and Plug flows with transient Irregular Segmented flows between two flow regimes were mainly observed in the test channels of the three different chips. Flow pattern maps and transitions between flow regimes were determined in terms of a fixed homogeneous carrier fluid volumetric flow ratio (β_C) to compare the effect of the expansion ratios from the injection to the test channels. The droplet and plug regimes were shifted to higher carrier and lower dispersed fluid superficial velocities and the plug flow regime was broader with the lower expansion ratio channels. The transient irregular segmented flow was favored in the higher expansion channel ratio and the interval of transient irregular segmented flow between droplet and plug flow regimes were shorter for the low expansion channel ratios. This is evidence that flow regime maps in micro-channels are not universal and depend on the configuration part of the micro-injection system. The length of the dispersed segmented flows and the distance between consecutive droplets or plugs as a function of β_C were determined by image processing of frames acquired via CCD camera with bright field illumination. The average length of the dispersed fluid was shown to scale approximately with β_C to the −1.2 power. Velocities of the dispersed droplet and plug flows were measured using double-pulsed laser illumination and were found to be 1.25 ± 0.049 and 1.46 ± 0.077 times faster than the superficial velocity of the segmented flow respectively. Two-phase pressure drop measurements were also carried out for all flow regimes and associated trends were correlated with changes in flow topology. Comparisons of experimental pressure drop with the predictions for a modified Lockhart-Martinelli correlation were also made.