In microchannel flow, gas-liquid interface behavior will be important for developing a wide range of microfluidic applications, especially in micro reactors. In this paper, we discuss some topics related to capillary action and two-phase fluid behavior in a microchannel. One of the topics is interface motion in the flow driven only by capillary action. We examined circular and rectangular microchannel with diameter of 50 μm and 100 μm × 67 μm, respectively. For the circular channel, experiments well agreed with the previous theory in the case of ethyl alcohol as the test liquid. The effects of inner surface condition are found to be critical for interface motion on a microscopic scale. We have extended our theory to a rectangular microchannel. We obtained the same formula of relation between non-dimensional time and interface position as that of the circular channel. We compared predictions with experimental results of a PDMS microchannel. They agreed qualitatively, but not quantitatively. The difference was considered to be caused by contact angle estimation.

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