Shear-induced fragmentation of Laponite suspensions

TL;DR

This study investigates how shear causes fragmentation and erosion in Laponite suspensions, revealing complex flow behaviors and a critical shear rate where erosion time diverges, advancing understanding of shear-induced solid-fluid transitions.

Contribution

It provides new insights into shear-induced fragmentation and erosion dynamics in Laponite suspensions, including a model for erosion and identification of a critical shear rate.

Findings

Shear localization occurs at short times.

Flow becomes highly heterogeneous with intermittent switching.

Erosion time diverges below a critical shear rate.

Abstract

Simultaneous rheological and velocity profile measurements are performed in a smooth Couette geometry on Laponite suspensions seeded with glass microspheres and undergoing the shear-induced solid-to-fluid (or yielding) transition. Under these slippery boundary conditions, a rich temporal behaviour is uncovered, in which shear localization is observed at short times, that rapidly gives way to a highly heterogeneous flow characterized by intermittent switching from plug-like flow to linear velocity profiles. Such a temporal behaviour is linked to the fragmentation of the initially solid sample into blocks separated by fluidized regions. These solid pieces get progressively eroded over time scales ranging from a few minutes to several hours depending on the applied shear rate $\dot{\gamma}$. The steady-state is characterized by a homogeneous flow with almost negligible wall slip. The characteristic time scale for erosion is shown to diverge below some critical shear rate $\dot{\gamma}^\star$ and to scale as $(\dot{\gamma}-\dot{\gamma}^\star)^{-n}$ with $n\simeq 2$ above $\dot{\gamma}^\star$. A tentative model for erosion is discussed together with open questions raised by the present results.