Diffusion of Janus Particles in Bacterial Suspensions: Second-Order Phase Transition and Counterintuitive Directional Propulsion

By developing a molecular dynamics model of bacterial chemotaxis, we present the first investigation of tracer statistics in bacterial suspensions where chemotactic effects are considered. We demonstrate that the non-Gaussian statistics of full-coated tracer arises from the athermal bacterial noise. Moreover, Janus (half-coated) tracer performs a composite random walk combining power-law-tail distributed L\'{e}vy flights with Brownian jiggling at low coating concentration, but turns to an enhanced directional transport (EDT) when coating concentration is high. Unlike conventional self-propelled particles, upon increasing coating concentration, the direction of EDT counterintuitively reverses from along to against the tracer orientation. Both these transitions are identified to be second-order, with the phase boundaries meeting at a triple point. A theoretical modeling that reveals the origin of such anomalous transport behaviors is proposed. Our findings reveal the fundamental nonequilibrium physics of active matter under external stimuli, and underscore the crucial role of asymmetrical environment in regulating the transport processes in biological systems.