The Characteristic Length of Slow Dynamics in a Glass-Forming Binary Mixture

                 

Principal Investigator

Peter Harrowell

School of Chemistry,

University of Sydney

Molecular dynamics simulations of a glass- forming binary mixture have been carried out with the principal goal of establishing the nature of the collective motions responsible for the slow relaxation as the mixture is supercooled. In the course of addressing this question we are also carrying out an extensive study of the static and dynamic properties of the model liquid. This kind of extensive study of a single model glass-former is unique and is allowing us to make real progress in developing a complete and coherent microscopic picture of the whole range of anomalous properties of glassy materials.  

Co-Investigators

     

Donna Perera

School of Chemistry,

University of Sydney

     
             

Projects

g79 - VPP

           

   
                 

What are the results to date and the future of the work?

The results to date are as follows:

1) We have established that the 2D mixture used in the study exhibits well defined glassy behaviour, reproducing much of the standard phenomenology found experimentally. These features include two step structural relaxation functions with stretched exponential behaviour at long times, nonArrhenius temperature dependence of the associated time scales, an abrupt drop in the heat capacity at large supercooling, a breakdown in the scaling between structural relaxation times and the diffusion constant and evidence of a heterogeneous distribution of local relaxation times.

2) We have uncovered an unusual effect in which small concentrations of solute can enhance the diffusion of the solvent particles. This has been shown to arise through the disruption of the solvent structure by the solute particles due to their size difference

3) The origin of the breakdown in temperature scaling between the structural relaxation time and the diffusion constant in the binary mixture has been established. Two separate mechanisms have been identified, the transient confinement ('caging') of the surrounding particles and the heterogeneous distribution of local mobilities.

4) The collective motion of the faster particles has been shown to consist of 'strings' of particles. We have established that the numbers of such mobile particles fluctuate greatly in time at low temperatures with intermittent and extended 'convulsions' playing an important role in the slow component of the relaxation process.

                 
Appendix B -

                 

       

What computational techniques are used?

We integrate the Newtonian equations of motion of 1048 disks using a 4th order Gear predictor-corrector algorithm. Constant pressure and temperature are maintained by way of Gaussian constraints. As is typical with molecular dynamics, the code can be highly vectorized (up to 95%).

Publications

D. N. Perera and P. Harrowell, J. Non-Crystal. Solids., accepted for publication (1998)

       
- Appendix B