Abstract
Micron-sized objects having asymmetric boundaries can rectify the chaotic motions of an active bacterial suspension and perform geometrically biased random walks. Using numerical simulations in a planar geometry, we show that arrow-shaped micro-shuttles, constrained to move in one dimension (1D) in a bath of self-propelled micro-organisms, spontaneously perform unidirectional translational motions with a strongly shape-dependent speed. Relaxing the 1D constraint, a random motion in the whole plane sets in at long times, due to random changes in shuttle orientation caused by bacterial collisions. The complex dynamics arising from the mechanical interactions between bacteria and the object boundaries can be described by a Gaussian stochastic force with a shape-dependent mean and a self-correlation decaying exponentially on the timescale of seconds.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Transporting objects in the microscopic world is a task that has not yet been efficiently solved with artificial motors, while over the course of billions of years many ingenious solutions have been developed in the natural world. The possibility of using motile bacteria as propellers for microstructures is fascinating and relies on an exciting mixture of microbiology, statistical mechanics and nanotechnolgy.
Main results. By using numerical simulations, we demonstrate that a micron-sized shuttle with an asymmetric shape can be transported in a preferential direction by simple immersion in an active bacterial bath. We discuss the efficiency of this transport mechanism with respect to the shuttle's shape and the micro-organisms' properties. We show that this geometrically biased random walk can be described in terms of a Langevin equation with effective random forces.
Wider implications. The reported findings provide new insights into the physical description of transport in active suspensions and are potentially interesting for a wide range of applications, from drug-delivery to lab-on-chip technology.
Figure. Snapshots taken from simulations of two differently shaped micro-shuttles immersed in bacterial baths. The preferential alignment of bacteria along the symmetry axis of the shuttle gives rise to a net pushing unidirectional force towards the right.