DYNAMIC MODULI OF CONCENTRATED DISPERSIONS BY BROWNIAN DYNAMICS

The viscoelastic properties of model stable colloidal dispersions are calculated using Brownian dynamics simulations with a free-draining mo del for the hydrodynamic interactions. In this report we describe a nu mber of alternative routes to the dynamic moduli, making a comparison of the relative merits of the approaches. These include the use of the Green-Kubo formulas (used widely by the molecular simulation communit y but little used so far in colloid modeling), which gives the linear stress relaxation function from stress fluctuations in an unsheared mo del colloidal liquid. We also consider the direct application of an os cillating shear strain, as used in experiment, changing the frequency and strain amplitude in discrete jumps. This method is developed to co nsider a continuously varying sweep through frequency (constant strain amplitude) or strain amplitude (constant frequency). This route avoid s problems associated with equilibration at each frequency. We avoid e rrors associated with truncation of the Fourier transform by cutting i t off smoothly with a broad Gaussian. We concentrate on the real and i maginary parts of the complex storage modulus G, the storage modulus G', and loss modulus G'', being the most frequently cited viscoelastic functions. For very large amplitudes, we find oscillatory-shear-induc ed crystallization to a fcc structure.