Layering, freezing and re-entrant melting of hard spheres in soft confinement

TL;DR

This study explores how the softness of a confining potential influences the phase behavior of hard spheres, revealing layering, re-entrant melting, and phase separation phenomena through simulations.

Contribution

It introduces the phase diagram of hard spheres under soft harmonic confinement, highlighting the effects of confinement stiffness on structural phases and transitions.

Findings

Layered structures form depending on confinement stiffness

Re-entrant liquid phases occur under weak confinement

Phase separation happens under strong confinement

Abstract

Confinement can have a dramatic effect on the behavior of all sorts of particulate systems and it therefore is an important phenomenon in many different areas of physics and technology. Here, we investigate the role played by the softness of the confining potential. Using grand canonical Monte Carlo simulations, we determine the phase diagram of three-dimensional hard spheres that in one dimension are constrained to a plane by a harmonic potential. The phase behavior depends strongly on the density and on the stiffness of the harmonic confinement. Whilst we find the familiar sequence of confined hexagonal and square-symmetric packings, we do not observe any of the usual intervening ordered phases. Instead, the system phase separates under strong confinement, or forms a layered re-entrant liquid phase under weaker confinement. It is plausible that this behavior is due to the larger positional freedom in a soft confining potential and to the contribution that the confinement energy makes to the total free energy. The fact that specific structures can be induced or suppressed by simply changing the confinement conditions (e.g. in a dielectrophoretic trap) is important for applications that involve self-assembled structures of colloidal particles.