Langevin Dynamics simulations of a 2-dimensional colloidal crystal under confinement and shear

TL;DR

This study uses Langevin Dynamics simulations to explore how shear affects a 2D colloidal crystal confined by walls, revealing layered motion, shear banding, and structural disorder at various shear velocities and confinements.

Contribution

It provides new insights into shear-induced structural changes and disorder in confined colloidal crystals, including the effects of misfit and high shear rates.

Findings

Inner layers move with slow shear, outer layers tilt.

High shear causes crystal breakup and disorder.

Misfit introduces soliton structures that are disrupted by shear.

Abstract

Langevin Dynamics simulations are used to study the effect of shear on a two-dimensional colloidal crystal confined by structured parallel walls. When walls are sheared very slowly, only two or three crystalline layers next to the walls move along with them, while the inner layers of the crystal are only slightly tilted. At higher shear velocities, this inner part of the crystal breaks into several pieces with different orientations. The velocity profile across the slit is reminiscent of shear-banding in flowing soft materials, where liquid and solid regions coexist; the difference, however, is that in the latter case the solid regions are glassy while here they are crystalline. At even higher shear velocities, the effect of the shearing becomes smaller again. Also the effective temperature near the walls (deduced from the velocity distributions of the particles) decreases again when the wall velocity gets very large. When the walls are placed closer together, thereby introducing a misfit, a structure containing a soliton staircase arises in simulations without shear. Introducing shear increases the disorder in these systems until no solitons are visible any more. Instead, similar structures like in the case without misfit result. At high shear rates, configurations where the incommensurability of the crystalline structure is compensated by the creation of holes become relevant.