Computer simulation of rheological phenomena in dense colloidal suspensions with dissipative particle dynamics

The rheological properties of colloidal suspensions of spheres and rods have been studied using dissipative particle dynamics (DPD). We have measured the viscosity as a function of shear rate and volume fraction of the suspended particles. The viscosity of a 30 vol% suspension of spheres displays characteristic shear-thinning behaviour as a function of shear rate. The values for the lowand high-shear viscosity are in good agreement with experimental data. For higher paniculate densities, good results are obtained for the high-shear viscosity, although the viscosity at low shear rates shows a dependence on the size of the suspended spheres. Dilute suspensions of rods show an intrinsic viscosity which is in excellent agreement with theoretical results. For concentrated rod suspensions, the viscosity increases with the third power of the volume fraction. We find the same scaling behaviour as Doi and Edwards for the semidilute regime, although the explanation is unclear. The DPD simulation technique therefore emerges as a useful tool for studying the rheology of paniculate suspensions. The design of colloidal suspensions with the desired rheological properties is a key issue for industrial research. To study the effects of non-spherical or polydisperse particles and colloidal interactions on suspension rheology, computer simulations offer a powerful alternative to experiments. Simulation techniques used to date are based on a continuum model for the solvent, such as Brownian and Stokesian dynamics. To calculate manybody hydrodynamic interactions, however, it would be computationally more efficient to employ a particle-based simulation of the solvent. Inspired by this idea, Hoogerbrugge and Koelman [1] proposed a novel particle-based method to simulate complex fluid systems, called dissipative particle dynamics (DPD). In DPD, the system is updated in discrete time steps St consisting of an instantaneous collision followed by a free propagation substep of duration St. In the collision phase the momenta are simultaneously updated according to the stochastic rule p,-a + <$0=P/( f ) + ]rn,-./e,v (1) j where e,j is the unit vector pointing from particle j to particle ('. The change in momentum Œ,; can be written as a> = W(\ r, r i I ) {FT,, a>(p, -pj). e , j } . (2) W (r) is adimensionless 'weight' function which is zero beyond the interaction range rc = 1. The first stochastic term within the braces on the right-hand side of equation (2) causes the system to heat up, while the second dissipative term tends to relax any relative motion. Both terms acting together have the effect of a thermostat. The fluid particles in DPD should 0953-8984/96/479509+04$19.50 © 1996 IOP Publishing Ltd 9509