Crystallization and Glassy Behaviour in Short-Range Attractive Square-Well Fluids

Abstract

Square-well systems with attractive ranges of over 25% of the particle diameter are thoroughly studied in simulations. Most of these simulations are performed with monodisperse or binary systems while experiments are most frequently performed with polydisperse systems. In systems with attractive ranges significantly shorter than the particle diameter the gas-liquid transition becomes metastable. This metastable coexistence region plays a vital role in the formation of gels and is therefore particularly interesting. Using Molecular Dynamics we measure the metastable gas-liquid coexistence and the diffusion of a polydisperse square well system near the attractive glass transition. The attractive range was chosen to be 3% of the mean particle diameter. The polydispersity was chosen to be 10 percent to avoid the formation of crystals and to allow measurements near close packing. We apply two methods for computing the densities of the two bulk phases during phase separation and compare the results. Based on the gas-liquid coexistence measurements we can make a rough estimate of the critical temperature and critical density of this system. We compare the shape of the coexistence curve to previous results for systems with longer-ranged potentials and conclude that it is unlikely the critical behavior is classically quadratic or cubic and that the critical temperature is lower for shorter attractive ranges. The long time diffusion seems to reduce to very small values near the glass transition, in agreement with previous results on a similar system with a binary mixture. We interpret our failure to equilibrate the system at attraction strengths above the glass transition predicted by MCT and the rapid decay of diffusion in the direction of the glass line as a con?rmation of a dynamic glass transition near the glass transition as obtained from MCT.