Phase behavior of hard spheres confined between parallel hard plates: Manipulation of colloidal crystal structures by confinement

TL;DR

This study uses computer simulations to map the phase diagram of hard spheres confined between parallel plates, revealing how confinement influences crystal structures and phase transitions, with implications for colloidal material design.

Contribution

The paper provides the first comprehensive equilibrium phase diagram for confined hard spheres across various densities and separations, detailing structural transitions and intermediate phases.

Findings

Identified a first-order fluid-solid transition influenced by confinement.

Mapped a sequence of crystal structures from triangular to square with increasing plate separation.

Discovered intermediate phases like prism, buckled, and rhombic structures at high densities.

Abstract

We study the phase behavior of hard spheres confined between two parallel hard plates using extensive computer simulations. We determine the full equilibrium phase diagram for arbitrary densities and plate separations from one to five hard-sphere diameters using free energy calculations. We find a first-order fluid-solid transition, which corresponds to either capillary freezing or melting depending on the plate separation. The coexisting solid phase consists of crystalline layers with either triangular or square symmetry. Increasing the plate separation, we find a sequence of crystal structures from n triangular to (n+1) square to (n+1) triangular, where n is the number of crystal layers, in agreement with experiments on colloids. At high densities, the transition between square to triangular phases are intervened by intermediate structures, e.g., prism, buckled, and rhombic phases.