Influence of Confining Walls on the Dynamics of Supercooled Simple Liquids

Abstract

The relaxation dynamics of supercooled liquids in the bulk shows many features that are not seen in the dynamics of liquids at elevated temperatures, such as a very slow decay of the time-correlation functions, stretching, etc. The dynamics of liquids that are close to a surface (free space, confining wall, etc.) is even more complex in that the stretching is increased and in that there is evidence for the presence of multiple time scales. In this paper we review some results of recent molecular dynamics computer simulations in which we investigated the dynamics of a simple glass forming liquid in the vicinity of a wall. Two types of walls axe studied: A rough one and a smooth one. We find that at low temperatures the relaxation dynamics close to the rough wall is orders of magnitude slower than the one in the bulk, whereas close to a smooth wall a significant acceleration is found. We discuss how the relaxation dynamics of the liquid depends on the distance from the wall and propose some simple functional forms to describe this dependence. Using these dependencies it is possible to extract various dynamical length scales and we find that these scales show an Arrhenius dependence on temperature. Finally we show how these type of investigations can be used to rationalized some experimental findings on the dynamics of confined simple liquids.