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Microfluidic bubbles for novel applications: acoustic laser and ultrasonically controlled swimming microrobots

Final Report Summary - BUBBLEBOOST (Microfluidic bubbles for novel applications: acoustic laser and ultrasonically controlled swimming microrobots)

The project consisted in using miniature bubbles for novel achievements powered by acoustic waves.
The first achievement was to take advantage of their strong acoustic strong resonance to obtain powerful effects in microfluidic circuits such as the ones where very small chemical or biological samples are analysed in lab-on-a-chip devices. We showed how the acoustic resonance of bubbles is modified in confined environment. We found that at resonance powerful streaming fluid currents are generated, especially when several bubbles are interacting. Such currents are helpful to mix fluids, which is a traditionally difficult operation at small scales. The bubbles are free to move and also damped within channels making it difficult to obtain our initial goal of an acoustic laser. In order to keep bubbles at precise positions we discovered an original way to hold them in place, which works also in whole water. We 3D-fabricated small open cubes, acting as cages to trap bubbles. In this way we could produce many bubbles and study their acoustic coupling, with a better resonance. This paves the way to new developments such as metamaterial with exceptional absorption properties underwater. It also gives a better candidate method for an acoustic laser that would emit coherent sounds through the non-linear response of an array of many bubbles.
The second main achievement was that miniature bubbles are able to act as propellers. Bubbles can be either kept into an open rigid shell or in a flexible shell. In an open rigid shell, the bubble vibrates at the opening, starting a very strong propulsion jet. These shells are small, down to 10 micrometers, close to the minimum size of capillary vessels, but they could swim at velocities up to a few mm/s. We could design very complex geometries, with a "cargo" space to transport materials. In a flexible shell, the bubble vibrates differently, since they suddenly buckle under applied pressure. We call this propulsion "swimming by buckling": it is based on the sudden buckling of the shell under repeated applied pressure, which generates a momentum impulse. The advantage of flexible shells is that they can keep the gas for a long time. The application of these swimming bubbles is the delivery of drugs in the human circulation, where bubbles would be powered by ultrasonic waves such as the ones used in ultrasound echography.