Hydrodynamic cascade drives tumbling in sheared colloidal rod suspensions

TL;DR

This paper reveals that hydrodynamic interactions cause a cascade of tumbling events in sheared colloidal rod suspensions, significantly affecting flow behavior and challenging existing models.

Contribution

It uncovers a hydrodynamically-driven cascade mechanism causing tumbling in colloidal rods, which was previously underestimated in semi-dilute regimes.

Findings

Hydrodynamic interactions induce a cascade of tumbling events.

Collective tumbling disrupts flow alignment and increases viscosity.

Results align qualitatively with recent experimental observations.

Abstract

Modeling the dynamics of colloidal rods remains a central challenge in soft-matter physics due to the anisotropic and long-ranged nature of their interactions. Hydrodynamic interactions in rods suspensions are often assumed to be screened or too week to play any role in semi-dilute regimes, yet we find here these assumptions to break down at shear rates and concentrations that are often attained in experiments. Using particle-based simulations and scaling analysis, we uncover a cascade of tumbling events driven by hydrodynamic coupling among neighboring rods. This collective dynamics disrupts flow alignment and leads to a pronounced increase in viscosity and normal stress differences, in qualitative agreement with recent experiments. The discovery of this hydrodynamically-promoted cascade effect calls for a revision of existing constitutive models for colloidal rods and highlights hydrodynamic coupling as a key mechanism governing collective dynamics in highly anisotropic suspensions.