Brownian dynamics simulations of Lennard-Jones gas/liquid phase separation and its relevance to gel formation

Abstract

Brownian dynamics computer simulations have been used to follow the phase separation behaviour of model adhesive colloidal systems quenched in temperature from a supercritical state into the two-phase coexistence region, CR, of the phase diagram. The systems studied consisted of monodisperse spherical particles interacting through either 12–6, 24–12 or 36–18 pair potentials. Systems quenched to state points just inside the two-phase boundary initially exhibit negligible domain growth or a period of ‘latency’ before slow decomposition, a feature associated with phase separation by nucleation. Systems quenched further into the two-phase region are characterised by immediate decomposition and more rapid growth of a characteristic length scale, behaviour characteristic of phase separation via a spinodal decomposition mechanism. Both domain size and peak height followed a power law growth with time over a period of the separation and some systems exhibit dynamic scaling of the structure factors. For systems at a volume fraction in excess of ca. 0.1 percolating interconnecting networks formed which manifested gel-like properties such as an increasing infinite frequency shear modulus, the appearance of a low-frequency shear modulus and more pronounced power-law behaviour of the stress time autocorrelation functions. The LJ clusters consist of compact regions separated by thin branches, whereas the shorter ranged potentials formed more tenuous gel-like structures. Phase separation was slower for the shorter range attractive potentials.