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

Abstract

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 $\mathrm{(\eta )}$ and the self-diffusion ${\mathrm{(D}}_i)$ 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 ${\mathrm{(T}}_c)$ for both the two dynamical properties are significantly higher than the corresponding critical temperature $T_0^{\eta }$ obtained by fitting to the Vogel–Fulcher–Tammann (VFT) equation. (iii) The critical temperature for viscosity ${\mathrm{(T}}_0^{\eta })$ is considerably larger than that for the diffusion coefficients ${\mathrm{(T}}_0^{D_i})$ 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.