Time evolution of clusters of mobile particles in a model glass former

In this work we investigate by means of molecular dynamics simulations the detailed time evolution of string-like cooperative motions in a binary Lennard-Jones system at temperatures close to its mode-coupling temperature, T-c. The strings will be fully geometrically and dynamically characterised, making evident the correlated and concerted nature of the motions of the particles comprised. We shall show that at low temperature each string occurs in a sharp time window within the time interval [0, t*], where t* is a characteristic time related to the lifetime of the global clusters of mobile particles. Moreover, the different strings that comprise a given global cluster will be shown to take place independently and asynchronically, thus providing further support to the heterogeneous scenario of relaxation, but in which different independent string-like sub-regions relax at different timescales. Finally, we shall also demonstrate that as temperature is increased the strings evolve during an increasing fraction of the time interval [0, t*] and that for T similar to 0.55 the lifetimes of the strings and of the global clusters become akin to each other. (C) 2003 Elsevier B.V. All rights reserved.