Depinning dynamics of confined colloidal dispersions under oscillatory shear

TL;DR

This study explores how confined colloidal dispersions respond to oscillatory shear, revealing depinning transitions, temporary depinning, and complex dynamics, with implications for controlling such systems in rheological applications.

Contribution

It provides a combined particle-level and analytical investigation of depinning behavior under oscillatory shear, including a functional expression for the critical shear amplitude.

Findings

Identification of a critical shear amplitude for depinning

Observation of temporary depinning near the critical amplitude

Diverse dynamical structures with stability depending on shear parameters

Abstract

Strongly confined colloidal dispersions under shear can exhibit a variety of dynamical phenomena, including depinning transitions and complex structural changes. Here, we investigate the behaviour of such systems under pure oscillatory shearing with shear rate $\dot{\gamma}(t) = \dot{\gamma}_0 \cos(\omega t)$, as it is a common scenario in rheological experiments. The colloids' depinning behaviour is assessed from a particle level based on trajectories, obtained from overdamped Brownian Dynamics simulations. The numerical approach is complemented by an analytic one based on an effective single-particle model in the limits of weak and strong driving. Investigating a broad spectrum of shear rate amplitudes $\dot{\gamma}_0$ and frequencies $\omega$, we observe complete pinning as well as temporary depinning behaviour. We discover that temporary depinning occurs for shear rate amplitudes above a frequency-dependent critical amplitude $\dot{\gamma}_0^\mathrm{crit}(\omega)$, for which we attain an approximate functional expression. For a range of frequencies, approaching $\dot{\gamma}_0^\mathrm{crit}(\omega)$ is accompanied by a strongly increasing settling time. Above $\dot{\gamma}_0^\mathrm{crit}(\omega)$, we further observe a variety of dynamical structures, whose stability exhibits an intriguing ($\dot{\gamma}_0, \omega$) dependence. This might enable new perspectives for potential control schemes.