Mechanical, rheological and transport properties of soft particle fluids

In this report, we investigate the physical and dynamical properties of model fluids whose constituent particles have their softness varied in a systematic manner. Molecular dynamics ( MD) computer simulation is applied to inverse power or soft-sphere fluids, in which the particles interact through the pair potential, phi(r) = epsilon(sigma/r)(n) where n measures the steepness or stiffness of the potential. We have investigated the properties of model fluids with 4 <= n <= n 1152 over a wide density range. Attention is paid to local structural properties, the elastic moduli and the transport coefficients ( principally the shear viscosity, eta(s) and self-diffusion coefficient, D). We note that this is the first time mechanical and transport coefficient data have been reported for n = 4: It was found that the Batschinski - Hildebrand expressions, in which D and eta(-1)(s) are assumed to have a linear dependence on the molar volume, represent the data quite well for all n. The density for which, on extrapolation, each of these quantities are zero, increases with the softness of the interaction (or similar to n(-1)), suggesting that the effective hard sphere diameter decreases with increasing softness in the small n limit. This treatment leads to simple empirical formulas for the effect of density ( in an intermediate range) and n on the two transport coefficients of these fluids. As n decreases so do the relative fluctuations in the pressure and force on a particle. The local coordination number as measured up to the first minimum in the radial distribution function does not increase significantly above similar or equal to 13 up to the co-existence packing fluid, even for the softest of particles (i:e: n = 4). If we assume that the effective hard sphere diameter is approximately the position of the first peak in the radial distribution function, the Stokes - Einstein expression reproduces the simulation data reasonably well, with the boundary condition being typically between the stick and slip limits, and approaching the latter with increasing density. For the softer particles and with increasing packing fraction, the shear viscosity increases more rapidly than the self-diffusion coefficient decreases. For the softer systems, the bulk viscosity is relatively low compared to the shear viscosity, in contrast to the trend for the corresponding infinite frequency moduli. The softer particles are more "rubbery" in their response ( relatively high bulk modulus compared to the shear modulus), which leads us to conclude that auxetic behaviour is more likely to arise when the building units are quite hard.