Gas bubbles in micro-channels encountered in Lab-on-a-Chip, fuel cells and many micro-fluidic devices can present significant problems by disturbing and eventually blocking the flow introducing non-linearity and instabilities. Depending on the flow rate, the interplay of capillary forces and the surface tension and on the gas saturation indeed induce different regimes at the central inlet. In the three videos below, are shown three different flow regimes where white is liquid and black is gas: wedging, slug and bubbly.
T. Cubaud and Chih-Ming Ho, "Transport of bubbles in square microchannels", Physics of Fluid, Vol. 16, Numb. 12, 2004