Single-particle level access to hydrodynamic and frictional coupling between spheres in dense colloidal suspensions

TL;DR

This paper introduces a novel method to measure both translational and rotational motion of individual colloidal spheres in dense suspensions, revealing hydrodynamic and frictional interactions at the single-particle level.

Contribution

It develops uniform colloidal spheres with embedded cores to access rotational dynamics and uncovers interparticle hydrodynamic and frictional coupling effects.

Findings

Hydrodynamic rotational coupling observed in charged colloidal crystals.

Higher local crystallinity increases rotational diffusivity.

Nearly arrested particles show stick-slip rotational motion due to friction.

Abstract

The rotational Brownian motion of colloidal spheres in dense suspensions reflects local hydrodynamics and friction, both key to non-linear rheological phenomena such as shear-thickening and jamming, and transport in crowded environments, including intracellular migration and blood flow. To fully elucidate the role of rotational dynamics experimentally, it is crucial to measure the translational and rotational motion of all spheres simultaneously. Here, we develop compositionally uniform colloidal spheres with an off-centre, fully embedded core with a different fluorophore to the particle body, allowing access to rotational motion for all particles at the single-particle level. We reveal interparticle hydrodynamic rotational coupling in charged colloidal crystals. We also find that higher local crystallinity in denser crystals enhances rotational diffusivity, and that nearly arrested particles exhibit a stick-slip rotational motion due to frictional coupling. Our method sheds new light on the largely-unexplored local rotational dynamics of spherical particles in dense colloidal materials.