SELF-DIFFUSION AND VISCOELASTICITY OF DENSE HARD-SPHERE COLLOIDS

Brownian dynamics (BD) simulation is used to calculate the viscoelasti city of model near-hard-sphere colloidal liquids using a continuous po tential r(-n) interaction between the model colloidal particles. The e xponent n was varied between 6 and 72. The real and reciprocal compone nts of the complex shear viscosity, eta' and eta'', were computed via time-correlation functions under no-shear conditions using a. Green-Ku bo formula. Also, oscillatory shear non-equilibrium BD was employed at finite strain amplitude in the linear-response regime. We find that t he normalised stress autocorrelation function can be approximated very well by a two-parameter stretched exponential over the complete volum e-fraction range. The parameters used to specify the stretched exponen tial and also the viscosities and long-time self-diffusion coefficient s are quite sensitive to the value of n at a chosen volume fraction. T he Newtonian viscosity decreases and the long-time self-diffusion coef ficient increases with the softness of the interaction, in agreement w ith experiment. The value n = 36 gives best agreement with the experim ental data, and therefore appears to be a good 'effective' interaction which we suggest includes the time-averaged effects of the many-body hydrodynamics to some extent. The state dependence of the derived spec trum of relaxation times is determined. As for experimental systems, t he complex viscosity scales with the dimensionless ('longest') relaxat ion time, D(o)tau1/a2, where a is the radius of the particle and D(o) is the self-diffusion coefficient in the zero-density limit. Also in t he intermediate frequency regime 20 < a2omega/D(o) < 200 we find that both the real and imaginary parts of the complex shear viscosity decay as ca. omega-1/2 in agreement with experiment.