The bulk viscosity of a symmetrical Lennard--Jones mixture above and at liquid--liquid coexistence: A computer simulation study

A Lennard--Jones model of a binary dense liquid (A,B) with a symmetrical miscibility gap is investigated by means of computer simulation methods. Semigrand--canonical Monte Carlo simulations yield the phase diagram in the $T$--$x$ plane ($T$: temperature, $x$: concentration of A or B particles) as well as equilibrated configurations at coexistence. Then Molecular Dynamics simulations use these configurations to determine static properties (isothermal compressibility $\kappa_T$ and concentration susceptibility $\chi$) as well as the shear and the bulk viscosity $\eta_{\rm s}$ and $\eta_{\rm B}$, respectively. The latter quantities are calculated along a path approaching the coexistence line from high temperatures in the one--phase region and ending at a state at the coexistence line about 15% below the critical point. We find that $\kappa_T$ and $\chi$ increase significantly near the coexistence line reflecting the vicinity of the critical point. Whereas $\eta_{\rm s}$ exhibits a weak temperature dependence, $\eta_{\rm B}$ increases significantly near the coexistence curve.