Pinning Dislocations in Colloidal Crystals with Active Particles that Seek Stacking Faults

TL;DR

This study uses computer simulations to show how active anisotropic particles can modify the mechanical properties of colloidal crystals by pinning dislocations and stacking faults, potentially enabling adaptive microrobotic materials.

Contribution

It introduces the use of anisotropic active interstitial particles to control dislocation motion and mechanical response in colloidal crystals, extending traditional pinning strategies.

Findings

Active interstitials can significantly reduce plasticity.

Particle shape and reorientation influence effectiveness.

Low concentrations of active particles can induce shear thresholds.

Abstract

There is growing interest in functional, adaptive devices built from colloidal subunits of micron size or smaller. A colloidal material with dynamic mechanical properties could facilitate such microrobotic machines. Here we study via computer simulation how active interstitial particles in small quantities can be used to modify the bulk mechanical properties of a colloidal crystal. Passive interstitial particles are known to pin dislocations in metals, thereby increasing resistance to plastic deformation. We extend this tactic by employing anisotropic active interstitials that travel super-diffusively and bind strongly to stacking faults associated with partial dislocations. We find that: 1) interstitials that are effective at reducing plasticity compromise between strong binding to stacking faults and high mobility in the crystal bulk. 2) Reorientation of active interstitials in the crystal depends upon rotational transitions between high-symmetry crystal directions. 3) The addition of certain active interstitial shapes at concentrations as low as $60$ per million host particles ($0.006\%$) can create a shear threshold for dislocation migration.