Research

Sorting by Bubble

Microfluidic devices, also known as labs-on-a-chip, require clever techniques for processes like sorting particles by size. One such technique uses an oscillating bubble to sort particles. When the bubble vibrates back and forth (left) it creates what’s known as a streaming flow – large regions of recirculation (shown as gray ellipses in the right image). If the bubble is placed inside a channel, we say that two flows have been superposed; the device combines both the left-to-right flow of the channel and the recirculating streaming flow.

Introduce a micron-sized particle into this combined flow, and it will get carried to the bubble and then bounced around by its effects (left). In fact, the larger the particle is, the more the bubble deflects it relative to the flow. You can see this in the image on the right as well. Here the frame rate has been matched to the bubble’s vibration, so the bubble appears stationary, and the particle paths look smooth. The gray lines show the fluid’s path, and individual solid particles are introduced at the left. The largest particle gets strongly deflected as it passes the bubble and exits at the top-right. A fainter, smaller particle follows after it. Being smaller, the bubble’s deflection on it is weaker, and this second particle exits along a path to the center-right. The result is a fast and simple method for particle sorting. (Image and research credit: R. Thameem et al., source)

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