Simulation of fluid-solid coexistence in finite volumes: A method to study the properties of wall-attached crystalline nuclei

TL;DR

This paper introduces a simulation method for studying wall-attached crystalline nuclei in colloid-polymer mixtures, enabling detailed analysis of nucleation and phase coexistence in finite systems.

Contribution

It presents a novel simulation approach to analyze heterogeneous nucleation and phase coexistence by observing wall-attached crystalline clusters in confined geometries.

Findings

Agreement with Young's equation for contact angles.

Identification of the droplet evaporation-condensation transition.

Observation of slab states with two crystalline domains.

Abstract

The Asakura-Oosawa model for colloid-polymer mixtures is studied by Monte Carlo simulations at densities inside the two-phase coexistence region of fluid and solid. Choosing a geometry where the system is confined between two flat walls, and a wall-colloid potential that leads to incomplete wetting of the crystal at the wall, conditions can be created where a single nanoscopic wall-attached crystalline cluster coexists with fluid in the remainder of the simulation box. Following related ideas that have been useful to study heterogeneous nucleation of liquid droplets at the vapor-liquid coexistence, we estimate the contact angles from observations of the crystalline clusters in thermal equilibrium. We find fair agreement with a prediction based on Young's equation, using estimates of interface and wall tension from the study of flat surfaces. It is shown that the pressure versus density curve of the finite system exhibits a loop, but the pressure maximum signifies the "droplet evaporation-condensation" transition and thus has nothing in common with a van der Waals-like loop. Preparing systems where the packing fraction is deep inside the two-phase coexistence region, the system spontaneously forms a "slab state", with two wall-attached crystalline domains separated by (flat) interfaces from liquid in full equilibrium with the crystal in between; analysis of such states allows a precise estimation of the bulk equilibrium properties at phase coexistence.