Encounter-based model of a run-and-tumble particle

TL;DR

This paper extends encounter-based diffusion models to run-and-tumble particles, linking boundary local time to absorption events, and provides solutions for various absorption models including geometric distributions.

Contribution

It introduces a novel encounter-based framework for RTPs, where boundary local time counts collisions, enabling analysis of diverse absorption mechanisms.

Findings

Derived boundary value problems for RTPs with local time

Connected geometric distribution of collisions to Robin boundary conditions

Provided solutions for joint probability densities of position and local time

Abstract

In this paper we extend the encounter-based model of diffusion-mediated surface absorption to the case of an unbiased run-and-tumble particle (RTP) confined to a finite interval $[0,L]$ and switching between two constant velocity states $\pm v$ at a rate $\alpha$. The encounter-based formalism is motivated by the observation that various surface-based reactions are better modeled in terms of a reactivity that is a function of the amount of time that a particle spends in a neighborhood of an absorbing surface, which is specified by a functional known as the boundary local time. The effects of surface reactions are taken into account by identifying the first passage time (FPT) for absorption with the event that the local time crosses some random threshold $\widehat{\ell}$. In the case of a Brownian particle, the local time $\ell(t)$ is a continuous non-decreasing function of the time $t$. Taking $\Psi(\ell)\equiv \P[\widehat{\ell}>\ell]$ to be an exponential distribution, $\Psi[\ell]=\e^{-\kappa_0\ell}$, is equivalent to imposing a Robin boundary condition with a constant rate of absorption $\kappa_0$. One major difference in the encounter-based model of an RTP is that the boundary local time $\ell(t)$ is a now a discrete random variable that counts the number of collisions of the RTP with the boundary. Given this modification, we show that in the case of a geometric distribution $\Psi(\ell)=z^{\ell}$, $z=1/(1+\kappa_0/v)$, we recover the RTP analog of the Robin boundary condition. This allows us to solve the boundary value problem (BVP) for the joint probability density for particle position and the local time, and thus incorporate more general models of absorption based on non-geometric distributions $\Psi(\ell)$.