Theory of orientational ordering in colloidal molecular crystals

TL;DR

This paper develops a theoretical framework to analyze orientational order in colloidal molecular crystals, explaining experimental phenomena and predicting new phase transitions based on substrate geometry and electrostatic interactions.

Contribution

It introduces a novel theoretical approach that accounts for anisotropic electrostatic potentials and explains reentrant melting and phase transitions in colloidal crystals.

Findings

Excellent agreement with experimental and numerical results

Explanation of reentrant orientational melting

Prediction of a new phase transition with substrate aspect ratio change

Abstract

Freezing of charged colloids on square or triangular two-dimensional periodic substrates has been recently shown to realize a rich variety of orientational orders. We propose a theoretical framework to analyze the corresponding structures. A fundamental ingredient is that a non spherical charged object in an electrolyte creates a screened electrostatic potential that is anisotropic at any distance. Our approach is in excellent agreement with the known experimental and numerical results, and explains in simple terms the reentrant orientational melting observed in these so called colloidal molecular crystals. We also investigate the case of a rectangular periodic substrate and predict an unusual phase transition between orientationnaly ordered states, as the aspect ratio of the unit cell is changed.