Depinning and heterogeneous dynamics of colloidal crystal layers under shear flow

TL;DR

This paper combines simulations and an analytical model to study the complex shear-induced dynamics of confined colloidal layers, revealing depinning transitions and heterogeneous transport phenomena influenced by system parameters.

Contribution

It introduces an effective single-particle model for shear-driven colloidal layers and explores heterogeneity effects in incommensurate systems, linking to the FK model.

Findings

Critical shear rate for depinning depends on pore width and interaction strength.

Heterogeneous systems show density excitations similar to FK model kinks.

Incommensurate layers exhibit asymmetric kink-antikink dynamics.

Abstract

Using Brownian dynamics (BD) simulations and an analytical approach we investigate the shear-induced, nonequilibrium dynamics of dense colloidal suspensions confined to a narrow slit-pore. Focusing on situations where the colloids arrange in well-defined layers with solidlike in-plane structure, the confined films display complex, nonlinear behavior such as collective depinning and local transport via density excitations. These phenomena are reminiscent of colloidal monolayers driven over a periodic substrate potential. In order to deepen this connection, we present an effective model which maps the dynamics of the shear-driven colloidal layers to the motion of a single particle driven over an effective substrate potential. This model allows to estimate the critical shear rate of the depinning transition based on the equilibrium configuration, revealing the impact of important parameters such as the slit-pore width and the interaction strength. We then turn to heterogeneous systems where a layer of small colloids is sheared with respect to bottom layers of large particles. For these incommensurate systems we find that the particle transport is dominated by density excitations resembling the so-called "kink" solutions of the Frenkel-Kontorova (FK) model. In contrast to the FK model, however, the corresponding "antikinks" do not move.