title: Acoustic trapping of active matter.
abstract: 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.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Sho C
authors.initials: SC
authors.lastname: Takatori
authors.affiliation: Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, Zürich 8093, Switzerland.|Department of Chemical Engineering, KU Leuven, Leuven B-3001, Belgium.
authors.forename: Raf
authors.initials: R
authors.lastname: De Dier
authors.affiliation: Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, Zürich 8093, Switzerland.
authors.forename: Jan
authors.initials: J
authors.lastname: Vermant
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: John F
authors.initials: JF
authors.lastname: Brady
mesh_terms: D000162:Acoustics; D000388:Air / Q000032:analysis*; D003460:Crystallization; D007700:Kinetics; D007840:Latex / Q000737:chemistry*; D015233:Models, Statistical; D009038:Motion; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D052898:Optical Tweezers; D013269:Stochastic Processes; D014867:Water / Q000737:chemistry*
chemical_list: D007840:Latex; D014867:Water