Probability densities of a forced probe particle in glass: results from mode coupling theory and simulations of active microrheology

TL;DR

This study combines mode coupling theory and simulations to analyze how a probe particle in a glass responds to external forces, revealing anisotropic displacement distributions and strain softening effects.

Contribution

It provides a comparative analysis of theoretical predictions and simulations for active microrheology in glasses, highlighting anisotropic distributions and softening behavior.

Findings

Probe density becomes anisotropic under force

Distribution shifts and skews towards force direction

Softening occurs at a few thermal energies per particle radius

Abstract

We investigate the displacements of a probe particle inside a glass, when a strong external force is applied to the probe (active nonlinear microrheology). Calculations within mode coupling theory are presented for glasses of hard spheres and compared to Langevin and Brownian dynamics simulations. Under not too strong forces where the probe remains trapped, the probe density distribution becomes anisotropic. It is shifted towards the direction of the force, develops an enhanced tail in that direction (signalled by a positive skewness), and exhibits different variances along and perpendicular to the force direction. A simple model of an harmonically trapped probe rationalizes the low force limit, with strong strain softening setting in at forces of the order of a few thermal energies per particle radius.