Viscoelastic behaviour of fluid with steeply repulsive potentials

We analyse the shear stress, C-s(t) and pressure or 'bulk', C-b(t) time-cor relation functions for steeply repulsive inverse power fluids (SRP) in whic h the particles interact via a pair potential with the analytic form, phi(r ) = epsilon(sigma/r)(n), in a new approach to the understanding of their vi scoelastic properties. We show analytically, and confirm by molecular dynam ics simulations, that close to the hard-sphere limit both these time-correl ation functions have the analytic form, C-s(t)/C-s(0) and C-b(t)/C-b(0) = 1 - T*(nt*)(2) + O((nt*)(4)), where T* = k(B)T/epsilon, is the reduced tempe rature, k(B) is Boltzmann's constant and t* = (epsilon/m sigma(2))(1/2)t is the reduced time. This leads to an alternative and much simpler derivation of formulae for the shear and bulk viscosities which for the limiting case of hard spheres are numerically very close to the traditional Enskog relat ions. These simple relations for the finite and continuous SRP interaction are in satisfactory agreement with the essentially exact molecular dynamics simulation results for ca, n greater than or equal to 18.