Effective Langevin equations for a polar tracer in an active bath

TL;DR

This paper develops a coarse-grained Langevin model for a polar tracer in an active particle bath, capturing its dynamics and displacement distributions through simulations and parameter analysis.

Contribution

It introduces a novel Langevin framework incorporating active forces and noise, providing insights into tracer behavior in active matter systems.

Findings

Tracer experiences a finite average force along its polar axis.

Displacement distribution transitions from non-Gaussian to Gaussian over time.

Tracer dynamics show a crossover from ballistic to diffusive motion.

Abstract

We study a polar tracer, having a concave surface, immersed in a two-dimensional suspension of active particles. Using Brownian dynamics simulations, we measure the distributions and auto-correlation functions of forces and torque exerted by active particles on the tracer. The tracer experiences a finite average force along its polar axis, while all the correlation functions show exponential decay in time. Using these insights we construct the full coarse-grained Langevin description for tracer position and orientation, where the active particles are subsumed into an effective self-propulsion force and exponentially correlated noise. The ensuing mesoscopic dynamics can be described in terms of five dimensionless parameters. We perform a thorough parameter study of the mean squared displacement, which illustrates how the different parameters influence the tracer dynamics, which crosses over from a ballistic to diffusive motion. We also demonstrate that the distribution of tracer displacements evolves from a non-Gaussian shape at early stages to a Gaussian behavior for sufficiently long times.