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Hollow sphere swims like a jellyfish thanks to sound waves.

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Powering microrobots inside a human organism would be useful for delivering small quantities of drugs at the right place, thus increasing their efficiency and reducing the possible side effects.

To do so, in this experiment, the simplest geometry ever is used: a hollow sphere. Under pressure, such a sphere becomes unstable and collapses. While this instability is generally seen as a mechanical failure, this property is used here to propel the sphere.

An upscaled experiment have shown that the energy contained in echographic sound waves is sufficient to make such a sphere deflate and reinflate periodically, which induces swimming. The motion mainly depends on the intensity of the sound and on the thickness of the shell surrounding the bubble; at small scale and high frequency it should be fast enough to go against the stream in blood vessels. A conveyor made of a few of such microbubbles with different shell thicknesses, might constitute a microrobot whose 3D displacement can be remotely controlled by an echographic device - a relatively cheap and widely available tool in the hospitals.

An illustration movie :

See focus of APS.

View online : A. Djellouli, P. Marmottant, H. Djeridi, C. Quilliet, and G. Coupier, Phys. Rev. Lett. 2017