Structural Responses of Quasi-2D Colloid Fluids to Excitations Elicited by Nonequilibrium Perturbations

TL;DR

This study examines how dense quasi-2D colloid suspensions respond to localized perturbations, revealing anisotropic stress propagation, flow patterns, and the importance of particle size and structure in the response.

Contribution

It combines experiments and simulations to analyze the microscopic mechanisms of stress and flow propagation in dense colloid suspensions under nonequilibrium perturbations.

Findings

Density wave and wake formation observed

Flow patterns consistent with dipolar fields reconstructed

Finite particle size significantly affects flow decay

Abstract

We investigate the response of a dense monodisperse quasi-two-dimensional (q2D) colloid suspension when a particle is dragged by a constant velocity optical trap. Consistent with microrheological studies of other geometries, the perturbation induces a leading density wave and trailing wake, and we use Stokesian Dynamics (SD) simulations to parse direct colloid-colloid and hydrodynamic interactions. We go on to analyze the underlying individual particle-particle collisions in the experimental images. The displacements of particles form chains reminiscent of stress propagation in sheared granular materials. From these data, we can reconstruct steady-state dipolar flow patterns that were predicted for dilute suspensions and previously observed in granular analogs to our system. The decay of this field differs, however, from point Stokeslet calculations, indicating that the finite size of the colloids is important. Moreover, there is a pronounced angular dependence that corresponds to the surrounding colloid structure, which evolves in response to the perturbation. Put together, our results show that the response of the complex fluid is highly anisotropic owing to the fact that the effects of the perturbation propagate through the structured medium via chains of colloid-colloid collisions.