Aggregation of a quenched Lennard-Jones system under shear

The thermodynamic decomposition of an unstable thermostatted system of Lennard-Jones disks is investigated by nonequilibrium molecular dynamics. The system, first unsheared and then subjected to planar Couette flow, is studied after temperature quenches into the unstable vapor-liquid and the vapor-solid coexistence regions of the phase diagram. An interconnected morphology, characteristic of spinodal decomposition, forms after quenching. The cluster growth is found tp be temporally self-similar, and the structure factor S(q,t) obeys the dynamic scaling relation S(q,t)similar to q(m) (-df)(t)(S) over tilde[q/q(m)(t)]. Here, q is the scattered wave vector magnitude, q(m)(t) is the location of the low angle peak in S(q,t), (S) over tilde(x) is a time-independent structure function which has a maximum at x- 1, and d(f) is a fractal dimension. d(f) is relatively insensitive to the postquench state point, but may depend on the shear rate. The primary influence of shear is to accelerate the aggregation-an effect that has also been observed experimentally in dense gelling silica suspensions. The similarities between these simulations and experiment suggest that a characteristic fractal dimension of a dense gel may be determined from measurements of S(q,t).