Capillary instability, squeezing, and shearing in head-on microfluidic devices

We investigate two-phase (oil and water) flow in head-on microfluidic devices, which consist of two identical channels as inlets and the long leg as a constriction channel leading to a wider outlet section. Over an exceptionally broad range of flow rates of 10(-4)-10 mu l/min in 10-100 mu m (hydraulic diameter) microchannels, corresponding to capillary numbers of 10(-6)-10(-1), a two-phase flow map is presented. A rich flow behavior was found. The flow patterns observed were dripping, jetting, and threading. These phenomena are interpreted as caused by capillary instability, squeezing, and shearing by considering the contribution of different forces acting at the oil/water interface. This device provides us with a broad choice to generate droplets of different sizes and frequencies by modifying either the geometrical design or the flow rates.