Self-diffusion in two-dimensional hard ellipsoid suspensions

TL;DR

This study investigates the self-diffusion behavior of colloidal ellipsoids in a monolayer near a wall, revealing anisotropic diffusion, sub-diffusive regimes, and complex relaxation dynamics influenced by particle concentration.

Contribution

It introduces a novel video microscopy tracking method and provides detailed measurements of translational and rotational diffusion in ellipsoid suspensions, highlighting non-monotonic anisotropy changes.

Findings

Diffusion coefficients depend on particle concentration.

Sub-diffusive behavior observed in intermediate times.

Diffusion anisotropy reaches minima in semi-dilute regimes.

Abstract

We studied the self-diffusion of colloidal ellipsoids in a monolayer near a flat wall by video microscopy. The image processing algorithm can track the positions and orientations of ellipsoids with sub-pixel resolution. The translational and rotational diffusions were measured in both the lab frame and the body frame along the long and short axes. The long-time and short-time diffusion coefficients of translational and rotational motions were measured as functions of the particle concentration. We observed sub-diffusive behavior in the intermediate time regime due to the caging of neighboring particles. Both the beginning and the ending times of the intermediate regime exhibit power-law dependence on concentration. The long-time and short-time diffusion anisotropies change non-monotonically with concentration and reach minima in the semi-dilute regime because the motions along long axes are caged at lower concentrations than the motions along short axes. The effective diffusion coefficients change with time t as a linear function of (lnt)/t for the translational and rotational diffusions at various particle densities. This indicates that their relaxation functions decay according to 1/t which provides new challenges in theory. The effects of coupling between rotational and translational Brownian motions were demonstrated and the two time scales corresponding to anisotropic particle shape and anisotropic neighboring environment were measured.