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

               

Principal Investigator

Peter Harrowell

School of Chemistry

University of Sydney

Co-Investigators

Donna Perera

School of Chemistry

University of Sydney

Projects

g79 - VPP

We are carrying out molecular dynamics
simulations of a supercooled binary mixture of
soft disks in 2D. This model has been chosen as one of the simplest systems able to generate the full phenomenology of the glass transition while still offering the hope of a detailed resolution of the static correlations and collective motions responsible for this ubiquitous behaviour. In addition to the technological importance of glassy materials, this problem focuses on a range of fundamental problems in the statistical physics of condensed materials.
   
             

     
               

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

These results consist of a) the demonstration that relaxation kinetics and transport in a 2D liquid can be enhanced by the action of solute particles which disrupt solvent structure, and b) an explicit description of the origin of the different temperature dependencies of diffusion and structural relaxation in a supercooled mixture in terms of kinetic heterogeneities and the different length scales which characterise the two kinds of dynamics.

A comprehensive characterisation of the thermodynamic and kinetic properties of the supercooled binary mixture of soft disks in 2D has been completed. This study represents the something of a benchmark in the glass field and is the subject of a completed PhD thesis and a paper currently submitted to Phys.Rev.E. Among the major results of this extensive characterisation are a) the first calculation of the inherent stress distribution in the amorphous groundstate, and b) the demonstration that the elementary excitations of this groundstate exhibit a characteristic temperature and an associated characteristic volume change.

Direct visualization of the time resolved particle displacements has revealed an increasingly intricate picture of the collective motions involved in slow relaxation in the glassy system. Localised and intermittent string-like motions are found to be strongly coupled to long wavelength collective oscillations.

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%).

               
- Appendix B

 
               

       

Publications

D.N.Perera, P.Harrowell, A two dimensional glass: microstructure and dynamics of a 2D binary mixture, J.Non-Cryst.Solids 235-237, 314-319, 1998.

D.N.Perera, P.Harrowell, Solute-enhanced diffusion in a dense two-dimensional liquid, Phys.Rev.Lett. 80, 4446-4449, 1998.

D.N.Perera, P.Harrowell, Origin of the temperature dependences of diffusion and structural relaxation in a supercooled liquid, Phys.Rev.Lett. 81, 120-12, 1998.

       
Appendix B -