SOLID FLUID INTERFACES IN NONLINEAR STEADY-STATES/

A molecular dynamics (MD) method for investigating the steady-state co existence of solidifying systems is described. The model system may be applicable to the formation of non-equilibrium crystals or glasses un der highly non-linear conditions such as splat quenching in external f ields, 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 sys tem is characterised by the usual two intensive thermodynamic state va riables, density and temperature (or a Stokes friction constant for co lloidal systems), and two additional steady-state variables, a uniform driving force and an interface velocity. At a sufficient driving forc e 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 inte rface velocity (upsilon(b)), relative to the driving force (F-s) deter mines the properties of the nonequilibrium defect crystal or glassy so lid phase. Preliminary results suggest this simple idealised steady-st ate system exhibits a rich phase diagram, showing a kind of phase beha viour, analogous to two-phase coexistence and critical behaviour, with interfacial characteristics reminiscent of thermodynamic equilibria.