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 to be temporally self-similar, and the structure factor S(q,t) obeys the dynamic scaling relation S(q,t)∼${\mathit{q}}_{\mathit{m}}^{\mathrm{-}{\mathit{d}}_{\mathit{f}}}$(t)S̃[q/${\mathit{q}}_{\mathit{m}}$(t)]. Here, q is the scattered wave vector magnitude, ${\mathit{q}}_{\mathit{m}}$(t) is the location of the low angle peak in S(q,t), S̃(x) is a time-independent structure function which has a maximum at x=1, and ${\mathit{d}}_{\mathit{f}}$ is a fractal dimension. ${\mathit{d}}_{\mathit{f}}$ 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). © 1996 The American Physical Society.