Water slug detachment in two-phase hydrophobic microchannel flows

In this paper we present a first order study of liquid water detachment and entrainment into air flows in Hydrophobic microchannels. A silicon based microfabricated test structure was used for this purpose. It consists of a 500 μm wide by 45 μm deep U-shaped channel 23 mm in length through which air is flown. The structures are treated with a Molecular Vapor Deposition (MVD) process that renders them hydrophobic with a nominal contact angle of 108° (in situ contact angles inside the channels are measured directly during testing). Liquid water is injected through a single side slot located two-thirds of the way downstream from the air channel inlet. The side slot extends the whole depth of the air channel while its width is varied from sample to sample. Visualization of the water slugs that form as water is injected into the air channel was performed. Slug dimensions at detachment are correlated against superficial gas velocity. Proper dimensionless parameters are postulated and examined to compare hydrodynamics forces against surface tension. It is found that for Re below 200 slug detachment is dominated by pressure gradient drag arising from confinement of a viscous flow in the channel. On the other hand, for Re above 200 the predominant drag is inertial in nature with stagnation of the air due to flow obstruction by the slugs.