STRUCTURAL EVOLUTION OF PHASE-SEPARATING MODEL COLLOIDAL LIQUIDS BY BROWNIAN DYNAMICS COMPUTER-SIMULATION

Brownian dynamics, (BD), simulation has been used to follow the phase separation of Lennard-Jones-type particles quenched from a supercritic al state point into the vapor-liquid or vapor-solid co-existence parts of their phase diagrams. Calculations were performed with spherical p articles interacting via 12:6, 24:12, and 36:18 interaction laws at su bcritical temperatures and low-volume fractions (phi less than or equa l to 0.2). Structural properties were followed as the systems evolved using pictures of the configurations, radial distribution function, an d the low-angle scattering peak of the structure factor. The time depe ndence of the interaction energy was also followed. The scaling behavi or of these quantities as a function of time was found to be similar t o that observed in light scattering experiments during the phase separ ation of real colloidal systems. The aggregate structure that develope d with time was sensitive to the range of the attractive part of the p otential and its underlying phase diagram (the 36:18 system does not h ave a liquid phase). The 12:6 systems soon formed compact structures, whereas the systems generated using the shorter-ranged potentials pers isted in a more diffuse, tenuous network for the duration of the simul ations. Apart from at very short times for all potential laws, the onl y convincing evidence for a long-lived fractal structure was for the 3 6:18 systems at the lowest quench temperatures (k(T)/epsilon= 0.3, whe re epsilon is the depth of the potential). The local structure in the dense regions of the network was sensitive to the range of the potenti al, exhibiting in the vapor-solid co-existence part of the phase diagr am glassylike features for the 12:6 systems and crystalline local orde r for the 24:12 and 38:18 states. The 12:6 systems close to the metast able region of the vapor-liquid two-phase part of the phase diagram ex hibited latency in the appearance and growth of the small angle scatte ring peak. The 24:12 and 36:18 systems also displayed latency at the h igher temperatures both in the growth of the peak height and its movem ent to lower scattering vectors. (C) 1998 American Institute of Physic s. [S0021-9606(98)51741-2].