Distribution Functions and Moments of Harmonically Trapped Inertial Run-and-Tumble Particles in a Shear-Thickening Medium

Abstract

We study the dynamics of an athermal inertial active particle confined in a harmonic potential and moving through shear-thickening media in one dimension. The activity is modeled using a symmetric dichotomous noise of strength $Σ$ and flipping rate $λ$, which mimics a run-and-tumble particle. The shear-thickening response of the medium is incorporated through a velocity-dependent viscous force of the form $g(v) \sim \tan(v)$. Starting from the Fokker-Planck equations corresponding to the particle's dynamics, we numerically compute the time-dependent joint probability density function $f(x,v,t)$ using a finite-volume upwind scheme. Next, we numerically solve the particle's equations of motion to obtain the steady-state position $P_s(x)$ and velocity $W_s(v)$ distributions and compare them with the marginal distributions obtained from $f(x,v,t)$ at sufficiently large $t$. The two approaches show excellent agreement. Both $P_s(x)$ and $W_s(v)$ exhibit several intriguing transitions as $λ$ is varied, and they eventually approach Gaussian-like profiles at sufficiently large $λ$. Our study of the time-dependent position-velocity correlation, $\langle xv \rangle(t)$, shows that larger values of $λ$ lead to faster relaxation to the steady state when all other parameters are held constant.