Amorphous and ordered states of concentrated hard spheres under oscillatory shear

TL;DR

This study uses Brownian dynamics simulations to explore how concentrated hard sphere colloids form different crystal structures under oscillatory shear, revealing anisotropic behaviors and the role of particle displacements in shear-induced ordering.

Contribution

It provides new insights into the microscopic mechanisms of shear-induced crystallization and the anisotropic nature of the resulting crystal structures in colloidal glasses.

Findings

Parallel FCC crystals are more efficient at reducing stress at larger strains.

Perpendicular FCC crystals exhibit less stress and displacement at smaller strains.

Shear-induced ordering involves large out-of-cage particle displacements exploring energy minima.

Abstract

Hard sphere colloidal particles are a basic model system for general research into phase behavior, ordering and out-equilibrium glass transitions. Experimentally it has been shown that oscillatory shearing of a monodisperse hard sphere glass, produces two different crystal orientations; a Face Centered Cubic (FCC) crystal with the close packed direction parallel to shear at high strains and an FCC crystal with the close packed direction perpendicular to shear at low strains. Here, using Brownian dynamics simulations of hard sphere particles, we have examined high volume fraction shear-induced crystals under oscillatory shear as well as the same volume fraction glass counterparts. We find that, while the displacements under shear of the glass are isotropic, the sheared FCC crystal structures oriented parallel to shear, are anisotropic due to the cooperative motion of velocity-vorticity layers of particles sliding over each other. These sliding layers generally result in lower stresses and less overall particle displacements. Additionally, from the two crystal types, the perpendicular crystal exhibits less stresses and displacements at smaller strains, however at larger strains, the sliding layers of the parallel crystal are found to be more efficient in minimizing stresses and displacements, while the perpendicular crystal becomes unstable. The findings of this work suggest that the process of shear-induced ordering for a colloidal glass is facilitated by large out of cage displacements, which allow the system to explore the energy landscape and find the minima in energy, stresses and displacements by configuring particles into a crystal oriented parallel to shear.