Particle sorting using an oscillating microbubble

The ability to sort suspended matter within complex fluid samples is a key part of the functionality of Lab-on-a-chip devices. This study investigates the use of microbubbles to achieve this task. Bubbles which vibrate due to acoustic excitation are very effective at concentrating energy which can cause strong acoustic microstreaming. This fluid motion brings particles very close to the bubbles' surface. When in close proximity to the bubble the Bjerknes force arising between the particle and bubble can be large enough to pull the particle out of the swirling flow which characterizes acoustic streaming. If this occurs the particle is captured on the bubble surface, otherwise the force balance, which is size and density dependent, is such that the particle will remain in the streaming flow, apparently rejected by the bubble. The bubbles can be excited at their resonance or by the presence of an acoustic standing wave which can be created by exciting the fluid chamber at resonance. In the latter case, the interaction between the standing wave and particles can be used to bring more particles into the vicinity of the bubble.