BROWNIAN DYNAMICS SIMULATIONS OF DOMAIN GROWTH IN LENNARD-JONES FLUIDS

The evolution of the structural, thermodynamic and rheological propert ies of a three dimensional Lennard-Jones fluid is followed as it is qu enched from a supercritical state into the two phase gas-liquid coexis tence region of the phase diagram. Domain growth is revealed in the ap pearance of a peak in the structure factor at k(max)similar to sigma(L J)(-1) which moves to lower k with time, and whose peak height S(k(max )) increases with time. k(max)(-1) and S(k(max)) show a power law depe ndence with time with exponents in the range 0.2-0.3 and 0.7-1.0, resp ectively, depending somewhat on the destination state point and broadl y consistent with previous theory and simulation. The kinetics of doma in growth depends on the value of temperature and density quenched to in the two-phase region. For quenches close to the liquid-vapour coexi stence line, an initial period marked by a lack of growth in the low k peak ('latency') is followed by power law behaviour, which is indicat ive of growth by nucleation. Quenches to well inside the unstable regi on are marked by classical spinodal decomposition power law growth. Th e influence of the box size on the phase separation has been investiga ted by carrying out simulations with N = 256 and N = 864. We have show n that system size can have a pronounced effect on growth kinetics, an d that at least N greater than or equal to 864 are advisable for studi es of this kind. Small system sizes tend to promote latency at short t imes and rapid phase separation at later times. Network formation foun d at intermediate times is manifest in a slowing down in relaxation pr ocesses which is reflected in a decrease in the self-diffusion coeffic ient and increase in shear viscosity with time from the start of the q uench.