Steady state and relaxation dynamics of run and tumble particles in contact with a heat bath

TL;DR

This paper investigates the relaxation behavior of a run and tumble particle in a linear potential, revealing a two-mode distribution with distinct relaxation times and a propagating relaxation front, influenced by thermal noise and active motion.

Contribution

It introduces a detailed analysis of the coupled two-mode distribution and their relaxation dynamics in a run and tumble particle system with thermal contact, highlighting the role of thermal noise and active motion.

Findings

The position distribution is described by two coupled modes with exponential tails.

The relaxation times of the modes depend on their decay rates and thermal noise.

A propagating relaxation front moves outward with a constant speed related to temperature and relaxation time.

Abstract

We study the relaxation dynamics of a run and tumble particle in a one-dimensional piecewise linear potential $U(x)=b|x|$, from delta-function initial conditions at $x=0$ to steady state. In addition to experiencing active telegraphic noise, the particle is in contact with a heat bath at temperature $T$ that applies white thermal noise. We find that the position distribution of the RTP is described by a sum of two distributions ("modes"), each of which of the form $P(x,t\to\infty)\sim e^{-\lambda_i|x|}$ ($i=1,2$) at steady state. The two modes are dynamically coupled: At very short times ($t\to 0$), each mode stores half of the probability, and exhibits thermal diffusive spreading with a Gaussian profile. With progressing time and evolution toward steady state, the partition of probability between the modes becomes increasingly uneven and, depending on the model parameters, the mode with the smaller value of $\lambda_i$ may carry an overwhelming majority of the probability. Moreover, we identify that the characteristic relaxation time of each mode is $\tau_i=(\lambda_i^2T)^{-1}$, which implies that the minority mode also relaxes much faster than the dominant one. A more detailed analysis reveals that $\tau_i$ is characteristic of the mode relaxation only close to the origin at the core of the distribution, while further away it increases linearly with $|x|$ as if a relaxation front is propagating at constant speed $v_i^*=2\sqrt{T/\tau_i}$ in the system. The rate of non-equilibrium entropy production can be related to the two-mode splitting of the probability distribution and be expressed in terms of their correlation-lengths $\lambda_i$ and their contributions to the steady state distribution.