An interfacial pressure balance model was proposed andverified for the elucidation of the physical mechanism of micro countercurrent flow in a hydrophilic-hydrophobic selective-modification microchannel. We considered the conditions of the microflow phase separation, where the phase separation entails a single phase flow in each output of the microchannel. In this pressure balance model, the pressure difference between the two phases due to pressure loss in each phase is balanced by the Laplace pressure generated by the interfacial tension at the liquid-liquid interface between the separated phases. When the pressure difference between the two phases is sufficiently low, the contact line between the two phases is pinned at the boundary between the hydrophilic and the hydrophobic surfaces. Since the contact angle is restricted to values between the advancing and receding contact angles, the Laplace pressure has a limit. When the pressure difference between the two phases exceeds the limiting Laplace pressure, one of the phases leaks into the output channel of the other phase, and the phase separation fails. In order to experimentally verify this physical picture, a microchip with an asymmetric cross section, whose hydraulic diameters were 19 and 102 (mu)m, was used. In the microchip, a phase separation of a water-toluene micro countercurrent flow was achieved under pressure differences between an upper limit of 6.9 kPa and a lower limit of -9.3 kPa. The upper limit agreed well with the proposed model. The model is also applicable to cocurrent flows, so that it is useful for general multiphase microflows in continuous-flow chemical processing.
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