# Ordering of Binary Colloidal Crystals by Random Potentials

## TL;DR

This study demonstrates how a random optical potential can be used to control and engineer the defect density in 2D binary colloidal crystals, revealing a non-monotonic relationship between defect density and particle composition.

## Contribution

It introduces a novel experimental method to manipulate defect densities in colloidal crystals using random potentials and confirms findings with particle-based simulations.

## Key findings

- Defect density can be tuned by the fraction of strong particles.
- Crystalline order initially decreases then recovers at high strong particle fractions.
- Simulations explain the non-monotonic behavior through particle interactions and potential competition.

## Abstract

Structural defects are ubiquitous in condensed matter, and not always a nuisance. For example, they underlie phenomena such as Anderson localization and hyperuniformity, and they are now being exploited to engineer novel materials. Here, we show experimentally that the density of structural defects in a 2D binary colloidal crystal can be engineered with a random potential. We generate the random potential using an optical speckle pattern, whose induced forces act strongly on one species of particles (strong particles) and weakly on the other (weak particles). Thus, the strong particles are more attracted to the randomly distributed local minima of the optical potential, leaving a trail of defects in the crystalline structure of the colloidal crystal. While, as expected, the crystalline ordering initially decreases with increasing fraction of strong particles, the crystalline order is surprisingly recovered for sufficiently large fractions. We confirm our experimental results with particle-based simulations, which permit us to elucidate how this non-monotonic behavior results from the competition between the particle-potential and particle-particle interactions.