Brownian dynamics simulations of domain growth in Lennard-Jones fluids

The evolution of the structural, thermodynamic and rheological properties of a three dimensional Lennard-Jones fluid is followed as it is quenched from a supercritical state into the two phase gas-liquid coexistence region of the phase diagram. Domain growth is revealed in the appearance of a peak in the structure factor at k(max)similar to sigma(LJ)(-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 dependence with time with exponents in the range 0.2-0.3 and 0.7-1.0, respectively, depending somewhat on the destination state point and broadly consistent with previous theory and simulation. The kinetics of domain growth depends on the value of temperature and density quenched to in the two-phase region. For quenches close to the liquid-vapour coexistence line, an initial period marked by a lack of growth in the low k peak ('latency') is followed by power law behaviour, which is indicative of growth by nucleation. Quenches to well inside the unstable region are marked by classical spinodal decomposition power law growth. The influence of the box size on the phase separation has been investigated by carrying out simulations with N = 256 and N = 864. We have shown that system size can have a pronounced effect on growth kinetics, and that at least N greater than or equal to 864 are advisable for studies of this kind. Small system sizes tend to promote latency at short times and rapid phase separation at later times. Network formation found at intermediate times is manifest in a slowing down in relaxation processes which is reflected in a decrease in the self-diffusion coefficient and increase in shear viscosity with time from the start of the quench.