Acoustic trapping of active matter.

Takatori SC; De Dier R; Vermant J; Brady JF

doi:10.1038/ncomms10694

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Journal article

Acoustic trapping of active matter.

Takatori SC Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
De Dier R Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, Zürich 8093, Switzerland.
Vermant J Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, Zürich 8093, Switzerland.
Brady JF Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.

2016-03-11

Published in:

Nature communications. - 2016

English Confinement of living microorganisms and self-propelled particles by an external trap provides a means of analysing the motion and behaviour of active systems. Developing a tweezer with a trapping radius large compared with the swimmers' size and run length has been an experimental challenge, as standard optical traps are too weak. Here we report the novel use of an acoustic tweezer to confine self-propelled particles in two dimensions over distances large compared with the swimmers' run length. We develop a near-harmonic trap to demonstrate the crossover from weak confinement, where the probability density is Boltzmann-like, to strong confinement, where the density is peaked along the perimeter. At high concentrations the swimmers crystallize into a close-packed structure, which subsequently 'explodes' as a travelling wave when the tweezer is turned off. The swimmers' confined motion provides a measurement of the swim pressure, a unique mechanical pressure exerted by self-propelled bodies.

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