Signatures of motor susceptibility in the dynamics of a tracer particle in an active gel

TL;DR

This paper investigates how active forces and elastic properties in an active gel influence the motion of a tracer particle, revealing non-Gaussian displacement distributions and proposing a new microrheology probe.

Contribution

It introduces a model linking active source susceptibility and gel elasticity to the shape of the van Hove distribution of particle displacements.

Findings

Active events produce non-Gaussian displacement distributions.

Large gel elasticity and dominant active sources lead to side-peaks in distributions.

Susceptibility and duty cycle of active sources significantly alter the van Hove distribution.

Abstract

We study a model for the motion of a tracer particle inside an active gel, exposing the properties of the van Hove distribution of the particle displacements. Active events of a typical force magnitude give rise to non-Gaussian distributions, having exponential tails or side-peaks. The side-peaks appear when the local bulk elasticity of the gel is large enough and few active sources are dominant. We explain the regimes of the different distributions, and study the structure of the peaks for active sources that are susceptible to the elastic stress that they cause inside the gel. We show how the van Hove distribution is altered by both the duty cycle of the active sources and their susceptibility, and suggest it as a sensitive probe to analyze microrheology data in active systems with restoring elastic forces.