Solid/fluid interfaces in non-linear steady states

Abstract

A molecular dynamics (MD) method for investigating the steady-state coexistence of solidifying systems is described. The model system may be applicable to the formation of non-equilibrium crystals or glasses under highly non-linear conditions such as splat quenching in external fields, or to the formation of colloidal crystals or colloidal glasses in uniaxial compaction processes.

As a first step towards determining constitutive relations for these processes, computer simulations have been undertaken for the continuous steady-state uniaxial compaction of soft-sphere model particles in one, two and three dimensions. The system is characterised by the usual two intensive thermodynamic state variables, density and temperature (or a Stokes friction constant for colloidal systems), and two additional steady-state variables, a uniform driving force and an interface velocity. At a sufficient driving force these conditions lead spontaneously to, and completely characterise, coexisting low- and high-density uniform phases, the ‘feed’ and the ‘bed’, and an interface between them. For rigid spheres, a reduced interface velocity (v_b), relative to the driving force (F_s) determines the properties of the non-equilibrium defect crystal or glassy solid phase.

Preliminary results suggest this simple idealised steady-state system exhibits a rich phase diagram, showing a kind of phase behaviour, analogous to two-phase coexistence and critical behaviour, with interfacial characteristics reminiscent of thermodynamic equilibria.