Transient colloidal gels by Brownian dynamics computer simulation

Brownian dynamics, BD, simulation has been used to model the structural evo lution, phase separation dynamics and rheology of transient particle colloi dal gels during formation, by quenching model monodisperse attractive spher ical colloidal particles from a supercritical state point into the vapour/l iquid or vapour/solid parts of their phase diagrams. Calculations were perf ormed with particles interacting via 12 : 6, 24 : 12 and 36 : 18 Lennard-Jo nes type interaction laws at sub-critical temperatures k(B) T/epsilon, wher e epsilon is the depth of the potential well, down to 0.01 and low volume f ractions (phi less than or equal to 0.2). These systems developed a gel-lik e morphology during the simulations, with the aggregate morphology and rheo logy sensitive to the range of the attractive part of the potential and the position in the phase diagram of the quench. The long-range 12 : 6 potenti al induced compact structures with thick filaments, whereas the systems gen erated using the shorter-ranged 24 : 12 and 36 : 18 potentials persisted in a more diffuse network for the duration of the simulations and evolved mor e slowly with time. The rheology of these systems was characterized using t he linear shear stress relaxation function, C-s(t), computed using the Gree n-Kubo fluctuation formula. The rheology of many of the systems displayed g el-like viscoelastic features, especially for the long-range attractive int eraction potentials, which manifested a non-zero plateau in Cs(t), the so-c alled equilibrium modulus, G(eq), useful indicator of a gel, which suggests also the presence of an apparent yield stress. A formal statistical mechan ical definition of G(eq) is presented. The infinite frequency shear rigidit y modulus G(infinity) is extremely sensitive to the form of the potential. Despite being the most short-lived, the 12 : 6 potential systems gave the m ost pronounced gel-like rheological features, which suggests that the tradi tional picture of a particle gel as being formed by thin filametary network s might require reconsideration.