Shape of Dynamical Heterogeneities and the Stokes-Einstein and Stokes-Einstein-Debye Relations in Suspensions of Colloidal Ellipsoids

TL;DR

This study investigates how the shape of dynamical heterogeneities affects the validity of the Stokes-Einstein and Stokes-Einstein-Debye relations in colloidal ellipsoid suspensions, revealing shape-dependent breakdowns and restorations of these relations.

Contribution

It demonstrates that the shape change of dynamical heterogeneities from string-like to compact causes the breakdown of SE and SED relations, and shows how attractions can preserve these relations.

Findings

Shape change from string-like to compact causes relation breakdown.

Attractions maintain relation validity even in supercooling.

Numerical predictions are consistent with experimental observations.

Abstract

We examine the role of shape of dynamical heterogeneities on the validity of the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations in quasi-two-dimensional suspensions of colloidal ellipsoids. For ellipsoids with repulsive interactions, although the orientational relaxation time remains coupled to the structural one, the SED relation by the Einstein formalism shows a breakdown. Strikingly, we find that it is the change in the shape of the dynamical heterogeneities from string-like to compact and not just their presence that results in the breakdown of both the SE and SED relations. On introducing a short-range depletion attraction between the ellipsoids, associated with the lack of morphological evolution of dynamical heterogeneities, the SE and SED relations remain valid even for deep supercooling. Our observations are consistent with numerical predictions.