Physics (Internal)

Computing viscosity of supercooled liquids

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract:
We describe an atomistic method for computing the viscosity of supercooled liquids. A basin-filling algorithm is implemented to sample autonomously the local energy minima and saddle points in the potential energy landscape of a binary Lennard-Jones system. Analysis of a sampled trajectory shows the system is able to move from one deep minimum to another by a process that involves high activation energy and the crossing of many local minima and saddle points. Taking this to be the unit event for structural relaxation and using the relation between average local minimum and temperature from the inherent structure concepts of supercooled liquids, we deduce an effective activation barrier that can be used in an expression for the viscosity based simply on transition state theory. We show that this heuristic approach gives a non-Arrhenius temperature variation matching qualitatively the experimental data on a group of glass-forming liquids classified as fragile. Additionally we turn to linear response theory where the viscosity is given by the integral of a time-dependent stress correlation function. A network model describing the hopping between deep minima is formulated and shown to provide justification for the preceding heuristic treatment. In a companion paper we report a similar study of silica, a representative strong liquid. A comparison of the two systems gives insight into the fundamental difference between strong and fragile scaling.