The FLOW-3D simulation below shows air bubble generation in a simple co-flowing micro channel. Water and air enter the microchannel from the same side (left), separated by a barrier. There are two steps in the bubble formation process. First the gas phase expands downwards and horizontally until a neck is formed, then the neck propagates downstream and its diameter decreases until it finally breaks.
*Experimental results taken from Renqiang Xiong, Mo Bai, Yun Whan Na, Jacob N Chung "Bubble generation in the micro-channel with a barrier," Proceedings of FEDSM2007 5th Joint ASME/JSME Fluids Engineering Conference July 30-August 2, 2007 San Diego, California USA.