Shear zones and wall slip in the capillary flow of concentrated colloidal suspensions

TL;DR

This study visualizes the flow of concentrated colloidal suspensions in a capillary, revealing shear zones and wall slip that align with granular flow theories rather than traditional yield-stress models.

Contribution

It demonstrates that the flow behavior of dense colloidal suspensions can be explained by stress fluctuation theory, challenging conventional yield-stress fluid assumptions.

Findings

Flow consists of a central plug with shear near walls

Velocity profiles match stress fluctuation theory for granular media

Behavior is consistent across different boundary conditions

Abstract

We image the flow of a nearly random close packed, hard-sphere colloidal suspension (a `paste') in a square capillary using confocal microscopy. The flow consists of a `plug' in the center while shear occurs localized adjacent to the channel walls, reminiscent of yield-stress fluid behavior. However, the observed scaling of the velocity profiles with the flow rate strongly contrasts yield-stress fluid predictions. Instead, the velocity profiles can be captured by a theory of stress fluctuations originally developed for chute flow of dry granular media. We verified this behavior both for smooth and rough boundary conditions.