Directed collective motion of bacteria under channel confinement

Abstract

Dense suspensions of swimming bacteria are known to exhibit collective behaviour arising from the interplay of steric and hydrodynamic interactions. Unconfined suspensions exhibit transient, recurring vortices and jets, whereas those confined in circular domains may exhibit order in the form of a spiral vortex. Here we show that confinement into a long and narrow macroscopic 'racetrack' geometry stabilises bacterial motion to form a steady unidirectional circulation. This motion is reproduced in simulations of discrete swimmers that reveal the crucial role that bacteria-driven fluid flows play in the dynamics. In particular, cells close to the channel wall produce strong flows which advect cells in the bulk against their swimming direction. We examine in detail the transition from a disordered state to persistent directed motion as a function of the channel width, and show that the width at the crossover point is comparable to the typical correlation length of swirls seen in the unbounded system. Our results shed light on the mechanisms driving the collective behaviour of bacteria and other active matter systems, and stress the importance of the ubiquitous boundaries found in natural habitats.

Scale invariance in natural and artificial collective systems: a review

“…A good example of this can be found in microorganisms immersed in a fluid, in which diffusive signalling interactions tend to be short-range, but hydrodynamic couplings can be long-range. This was observed by Lushi et al [94] and Wioland et al [95], who have shown that the reaction to stimuli in swimming bacterial colonies can result not only from the local chemical interactions, as expected, but also from longrange hydrodynamic flows (figure 2). Similar results were presented by Cisneros et al [96].…”

Batchelor Prize Lecture Fluid dynamics at the scale of the cell

“…Intriguingly, the distribution of vorticity in these lattices can be understood using lattice field theory methods well known in statistical physics. In very recent work, we studied these same bacterial suspensions confined to annuli whose width was in the range 10-100 µm and up to several millimetres long (Wioland, Lushi & Goldstein 2016a). Here, if the transverse dimension is sufficiently small there would be spontaneous unidirectional motion around the 'racetrack', but when the channel width exceeded a size comparable to the intrinsic vortex scale (∼70 µm), the mean flow would rapidly vanish.…”

Geometry-dependent viscosity reduction in sheared active fluids

“…However, when the suspension is enclosed by a small container of dimensions comparable to Λ, individual vortices become stabilized for several minutes [11,12] and can be coupled together to form magnetically ordered vortex lattices [13]. Another form of confinement-induced symmetry breaking was observed recently in a microfluidic realization of bacterial 'racetracks' [14]. For sufficiently narrow tracks of diameter Λ, bacteria spontaneously aligned their swimming directions to form persistent unidirectional currents.…”