Direction-dependent Dynamics of Colloidal Particle Pairs and the Stokes-Einstein Relation in Quasi-Two-Dimensional Fluids

TL;DR

This study experimentally investigates near-field hydrodynamic interactions in quasi-two-dimensional colloidal fluids, revealing direction-dependent particle dynamics and conditions under which the Stokes-Einstein relation breaks down, impacting fluid transport understanding.

Contribution

It provides the first experimental analysis of near-field hydrodynamic correlations in quasi-2D colloidal systems, linking these to the breakdown of the Stokes-Einstein relation.

Findings

Direction-dependent particle relaxation observed.

Near-field hydrodynamics influence SER breakdown.

Breakdown mitigated along directions with weaker hydrodynamic correlations.

Abstract

Hydrodynamic interactions are important for diverse fluids especially those with low Reynold's number such as microbial and particle-laden suspensions, and proteins diffusing in membranes. Unfortunately, while far-field (asymptotic) hydrodynamic interactions are fully understood in two- and three-dimensions, near-field interactions are not, and thus our understanding of motions in dense fluid suspensions is still lacking. In this contribution, we experimentally explore the hydrodynamic correlations between particles in quasi-two-dimensional colloidal fluids in the near-field. Surprisingly, the measured displacement and relaxation of particle pairs in the body frame exhibit direction-dependent dynamics that can be connected quantitatively to the measured near-field hydrodynamic interactions. These findings, in turn, suggest a mechanism for how and when hydrodynamics can lead to a breakdown of the ubiquitous Stokes-Einstein relation (SER). We observe this breakdown, and interestingly, we show that the direction-dependent breakdown of the SER is ameliorated along directions where hydrodynamic correlations are smallest. In total, the work uncovers significant ramifications of near-field hydrodynamics on transport and dynamic restructuring of fluids in two-dimensions.