Bi-axial shear of confined colloidal suspensions: the structure and rheology of the vorticity-aligned string phase

TL;DR

This study uses a novel bi-axial confocal rheoscope to explore how shear-induced microstructures in confined colloidal suspensions influence their rheological behavior, revealing isotropic viscosity despite anisotropic structures.

Contribution

It introduces a bi-axial shear protocol and measurement device to manipulate and analyze microstructure and rheology simultaneously in colloidal suspensions.

Findings

Vorticity-aligned string phase observed under uniaxial shear.

Suspension viscosity remains isotropic despite microstructural anisotropy.

Hydrodynamic effects dominate the suspension response.

Abstract

Using a novel bi-axial confocal rheoscope, we investigate the structure and rheology of sheared colloidal suspensions under confinement. Consistent with previous work [X. Cheng \textit{et al., Proc. Natl. Acad. Sci. U. S. A.}, 2011, 109, 63], we observe a vorticity-aligned string phase in moderate concentrated colloidal suspensions under uniaxial shear. Using bi-axial shear protocols, we directly manipulate the orientation and morphology of the string structures. Simultaneously, we measure the suspension rheology along both the flow and vorticity directions with a bi-axial force measurement device. Our results demonstrate that despite the highly anisotropic microstructure, the suspension viscosity remains isotropic and constant over the shear rates explored. These results suggest that hydrodynamic contributions dominate the suspension response. In addition they highlight the capabilities of bi-axial confocal rheoscopes for elucidating the relationship between microstructure and rheology in complex fluids.