Microfluidic systems are important in a variety of applications such as DNA sequencing, cell separation, environmental monitoring, heat transportation in spacecraft and space station [1–2]. Micromixers are the most important component in microfluidic systems and they can be classified as active and passive micromixers [3]. In micro dimensions, surface forces dominate over body forces requiring special attention for problems involving two-phase flows with free surfaces which are often driven by capillary forces [4]. In the present work the flow and mixing process in a passive microchannel mixer were numerically investigated. Effects of velocity, surface tension, and contact angle on the two phase flow were studied. Numerical results are obtained with a finite volume CFD code and using structured grids. Different liquids-gas Reynolds number ratios (Reliquid/Regas) were used ranging from 4 to 42. In addition, five values of the contact-angle (0 – 120) and seven values of the liquid surface tension (0.02 – 0.1) were used. Results show that increasing Reliquid increases the rate of the development of the air bubble. Increased surface tension resulted in increased bubble length. Bubble length decreased with increased contact angle till 75 degrees and further increase resulted in increased bubble length.

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