Bulk Viscosity of Interacting Hadrons

We show that first approximations to the bulk viscosity ηv are expressible in terms of factors that depend on the sound speed vs, the enthalpy, and the interaction (elastic and inelastic) cross section. The explicit dependence of ηv on the factor is demonstrated in the Chapman-Enskog approximation as well as the variational and relaxation time approaches. The interesting feature of bulk viscosity is that the dominant contributions at a given temperature arise from particles which are neither extremely nonrelativistic nor extremely relativistic. Numerical results for a model binary mixture are reported.