Thinning and thickening in active microrheology

TL;DR

This paper introduces a three-time-scales picture to unify the understanding of thinning and thickening behaviors in active microrheology, supported by simulations and microscopic analysis.

Contribution

It proposes the TTSP framework that links probe friction behavior to three distinct bath particle time scales, advancing the microscopic understanding of thinning and thickening.

Findings

The TTSP explains the transition between thinning and thickening behaviors.

Maxwellian velocity distribution works at low Re but fails at high Re.

Microscopic mechanisms of thickening are understood in the zero-temperature limit.

Abstract

When pulling a probe particle in a many-particle system with fixed velocity, the probe's effective friction, defined as average pulling force over its velocity, $\gamma_{eff}:=\langle F_{ex}\rangle/u$, first keeps constant (linear response), then decreases (thinning) and finally increases (thickening). We propose a three-time-scales picture (TTSP) to unify thinning and thickening behaviour. The points of the TTSP are that there are three distinct time scales of bath particles: diffusion, damping, and single probe-bath (P-B) collision; the dominating time scales, which are controlled by the pulling velocity, determine the behaviour of the probe's friction. We confirm the TTSP by Langevin dynamics simulation. Microscopically, we find that for computing the effective friction, Maxwellian distribution of bath particles' velocities works in low Reynolds number (Re) but fails in high Re. It can be understood based on the microscopic mechanism of thickening obtained in the $T=0$ limit. Based on the TTSP, we explain different thinning and thickening observations in some earlier literature.