58 research outputs found
Directed collective motion of bacteria under channel confinement
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.Weexamine 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.This is the final published version. It first appeared at http://dx.doi.org/10.1088/1367-2630/18/7/075002
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Nonlinear concentration patterns and bands in autochemotactic suspensions
In suspensions of microorganisms, pattern formation can arise from the interplay of chemotaxis and the fluid flows collectively generated by the organisms themselves. Here we investigate the resulting pattern formation in square and elongated domains in the context of two distinct models of locomotion in which the chemoattractant dynamics is fully coupled to the fluid flows and swimmer motion. Analyses for both models reveal an aggregative instability due to chemotaxis, independent of swimmer shape and type, and a hydrodynamic instability for "pusher" swimmers. We discuss the similarities and differences between the models. Simulations reveal a critical length scale of the swimmer aggregates and this feature can be utilized to stabilize swimmer concentration patterns into quasi-one-dimensional bands by varying the domain size. These concentration bands transition to traveling pulses under an external chemoattractant gradient, as observed in experiments with chemotactic bacteria.E.L. acknowledges a New Jersey Institute of Technology faculty seed grant award. R.E.G. was supported in part by Established Career Fellowship EP/M017982/1 from the Engineering and Physical Sciences Research Council and the Schlumberger Chair Fund. M.J.S. acknowledges support from NSF Grants No. DMS-1463962 and No. DMS-1620331, as well as the NSF Grant No. DMR-1420073 awarded to the MRSEC at NYU
Collective chemotactic dynamics in the presence of self-generated fluid flows
In micro-swimmer suspensions locomotion necessarily generates fluid motion,
and it is known that such flows can lead to collective behavior from unbiased
swimming. We examine the complementary problem of how chemotaxis is affected by
self-generated flows. A kinetic theory coupling run-and-tumble chemotaxis to
the flows of collective swimming shows separate branches of chemotactic and
hydrodynamic instabilities for isotropic suspensions, the first driving
aggregation, the second producing increased orientational order in suspensions
of "pushers" and maximal disorder in suspensions of "pullers". Nonlinear
simulations show that hydrodynamic interactions can limit and modify
chemotactically-driven aggregation dynamics. In puller suspensions the dynamics
form aggregates that are mutually-repelling due to the non-trivial flows. In
pusher suspensions chemotactic aggregation can lead to destabilizing flows that
fragment the regions of aggregation.Comment: 4 page
Scattering of biflagellate micro-swimmers from surfaces
We use a three-bead-spring model to investigate the dynamics of bi-flagellate micro-swimmers near a surface. While the primary dynamics and scattering are governed by geometric-dependent direct contact, the fluid flows generated by the swimmer locomotion are important in orienting it toward or away from the surface. Flagellar noise and in particular cell spinning about the main axis help a surface-trapped swimmer escape, whereas the time a swimmer spends at the surface
depends on the incident angle. The dynamics results from a nuanced interplay of direct collisions, hydrodynamics, noise and the swimmer geometry. We show that to correctly capture the dynamics of a bi-flagellate swimmer, minimal models need to resolve the shape asymmetry.This work was supported in part by an Established Career Fellowship from the Engineering and Physical Sciences Research Council (REG)
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Directed collective motion of bacteria under channel confinement
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.Weexamine 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.This is the final published version. It first appeared at http://dx.doi.org/10.1088/1367-2630/18/7/075002
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