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

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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 κₜ and concentration susceptibility χ) as well as the shear and the bulk viscosity η s and η 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 κₜ and χ increase significantly near the coexistence line reflecting the vicinity of the critical point. Whereas η s exhibits a weak temperature dependence, η B increases significantly near the coexistence curve.