Density and pressure dependence of the equation of state and transport coefficients of soft-sphere fluids

David Heyes (1)

1 BP Institute

Molecular Dynamics, MD, simulations were used to compute physical properties of model fluids in which the particles interacted via the soft-sphere or inverse power pair potential, phi(r)=e (s/r)**n n dictates the steepness or stiffness of the potential, and e and s are a characteristic energy and distance, respectively. A wide range of n values were considered, from the hard-sphere limit down to n=3.33 (the latter for the first time). The self-diffusion coefficient, D, and shear viscosity, eta, were also calculated. At intermediate to high densities, 1/D and eta depend to a very good approximation linearly on pressure, as was found by van der Gulik (Physica A, {\bf 256}, 39 (1998)) on treatment of experimental shear viscosity data for simple molecules. Values for D and eta at fluid-solid coexistence are given as a function of $n$. We refine further simple formulae for D and eta proposed in our previous publication (Phys. Chem. Chem. Phys, {\bf 10}. 4036 (2008)). The glass transition packing fraction and pressure for the n=3.33 fluid are estimated by extrapolation of the self-diffusion coefficient data. In contrast to the n=12 case, the shear stress correlation function correlation time shows only a weak density dependence near co-existence for the very soft interactions n=3.33. It is shown that for the very soft interactions close to n=3, the increase in viscosity is largely determined by the infinite frequency shear modulus rather than the relaxation time, which hardly changes with density at high density.

Mots clés: Physical Sciences

David Heyes. Density and pressure dependence of the equation of state and transport coefficients of soft-sphere fluids. Molecular Physics, 2009, 107 (04-06), pp.309-319. ⟨10.1080/00268970802712563⟩. ⟨hal-00513249⟩