Pressure and temperature dependence of viscosity and diffusion coefficients of a glassy binary mixture

Extensive isothermal-isobaric ~NPT! molecular dynamics simulations at many different temperatures and pressures have been carried out in the well-known Kob–Andersen binary mixture model to monitor the effect of pressure ~P! and temperature ~T! on the dynamic properties such as the viscosity (h) and the self-diffusion (Di) coefficients of the binary system. The following results have been obtained: ~i! Compared to temperature, pressure is found to have a weaker effect on the dynamical properties. Viscosity and diffusion coefficients are found to vary exponentially with pressure up to a certain high pressure after which the nature of exponential dependence changes. This change is rather sharp. ~ii! With temperature, on the other hand, both viscosity and diffusion show super-Arrhenius dependence. Viscosity and diffusion coefficients fit well also to the mode coupling theory ~MCT! prediction of a power law dependence on the temperature. The MCT critical temperature (Tc) for both the two dynamical properties are significantly higher than the corresponding critical temperature T0 h obtained by fitting to the Vogel–Fulcher–Tammann ~VFT! equation. ~iii! The critical temperature for viscosity (T0 ) is considerably larger than that for the diffusion coefficients (T0 i) implying the decoupling between diffusion and viscosity in deeply supercooled liquid. ~iv! The nature of the motion of small particles change from continuous to hopping dominated once the larger ones are frozen. ~v! The potential energy of the system shows a minimum against density at a relatively high density when the latter is changed by applying pressure at a constant temperature.