Two-phase flow studies have been carried out extensively over the years. However, there have been a relatively small amount of publications dealing with micro-channels when compared with those for ordinarily sized channels. Capillary force is likely to play an important role for two-phase flow characteristics in microchannels, resulting in flow phenomena significantly different from those observed in ordinarily sized channels. Serizawa et al. [1], for instance, investigated the visualization of the two-phase flow pattern in circular micro-channels. The flowing mixture of air and water in channels of 20, 25 and 100 lm in diameter and that of steam and water in a channel of 50 lm in diameter were conducted experimentally. The study confirmed that the surface wettability had a significant effect on the two-phase flow patterns in very small channels. The discrepancies between micro-scale and macro-scale flows have been reported in the literature [2–7].