Collective Motion Of Motile Bacteria

Active matter is a non-equilibrium system composed of self-propelled particles that can independently generate motion by consuming energy.Active systems are not subject to thermodynamic constraints,such as fluctuation-dissipation theorem and detailed balance,and exhibit a wide range of dynamic phenomena that are not found in thermal systems.For example,collective motion widely exists in many active systems at different length scales,from bacterial swarms to bird flocks.Extended ordered structures in such systems can spontaneously arise from local interactions between individuals.Understanding the physics of collective motion is a challenging problem in soft matter and non-equilibrium statistical physics.We focus on bacterial dynamics at micrometer scale and investigate how density,cell shape and boundary conditions affect collective motion of bacteria.In a two-dimensional(2D)bacterial thin film growing on solid agar,we quantify the spatial correlation of velocity and orientation fluctuations in ordered bacterial clusters.Correlation length of fluctuations is shown to be about 30%of the spatial size of clusters L(/?0.3),and the correlation functions collapse onto a master curve after rescaling separation by.These results demonstrate that fluctuation correlations are scale-invariant in bacterial clusters.Similar scale-invariant correlations have been reported in bird flocks.Our results together those from bird flocks suggest that scale-invariant correlations may be a general feature of self-propelled systems exhibiting collective behavior.We use bacteria Serratia marcescens,living at air/liquid interface,to realize a 2D model system for collective motion,in which cell density can be tuned.S.marcescens cells with spherical cell body can aggregate into dynamic clusters.Our flow visualization experiments and fluid dynamic calculation show that bacteria orient their flagella perpendicular to the interface and generate an attractive fluid flow near the free-slip boundary that leads to cluster formation.This is a new hydrodynamic mechanism for cluster formation,which is different from self-trapping effect in self-propelled Janus particles.In systems with rod-shaped S.marcescens cells,we observe a transition from disordered active gas to turbulence-like ordered structure as the cell density increases.Correlation length and time both gradually increase with density and reach maximal values at filling fraction!"_#=0.676 and then decrease sharply.Beyond!"_#,system dynamics slows down,becomes dominated by cage effect,and shows typical glassy phenomena,including sub-diffusive mean-squared displacements and a two-step structural relaxation.Besides research on collective phenomena,we explore the possibility of utilizing motile bacteria as microscopic pumps.We use micro-fabricated structures to confine smooth-swimming bacteria in a prescribed configuration.The flagella of confined bacteria rotate to generate flow that can transport materials along designed trajectories.Different structures are combined to realize complex functions,such as collection of particles.Our method opens new ways to generate transport flow at the micrometer scale and to drive bio-hybrid devices.