Manipulation of confined bubbles in a thin microchannel: Drag and acoustic Bjerknes forces

Bubbles confined between the parallel walls of microchannels experience an increased drag compared to freestanding bubbles. We measure and model the additional friction from the walls, which allows the calibration of the drag force as a function of velocity. We then develop a setup to apply locally acoustic waves and demonstrate the use of acoustic forces to induce the motion of bubbles. Because of the bubble pulsation, the acoustic forces-called Bjerknes forces-are much higher than for rigid particles. We evaluate these forces from the measurement of bubble drift velocity and obtain large values of several hundreds of nanonewtons. Two applications have been developed to explore the potential of these forces: asymmetric bubble breakup to produce very well controlled bidisperse populations and intelligent switching at a bifurcation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3579263]