Aug 1 2008
Two Sydney chemists have gone some way towards providing a resolution to one of the most interesting unresolved problems that exist in the field of solid state theory, the theory of the nature of glass and the glass transition.
Professor Peter Harrowell and Dr Asaph Widmer-Cooper, theoretical chemists from the School of Chemistry along with colleagues from Columbia University have been studying the transition of a fluid into a rigid glass in an attempt to understand stress relaxation in a disordered state.
"Glasses are the most viscous of liquids and the most structureless of solids" said Professor Harrowell. "As such, glasses represent the most poorly understood features of both states of matter. That makes them a really profound puzzle."
One of the most important insights to have emerged over the last decade of glass research is that a liquid, on cooling, approaches the glassy state non-uniformly. Some parts of the liquid become stiff while others retain the fluid mobility.
Professor Harrowell, one of the pioneers of the study of these dynamic heterogeneities, likens them to ghostly echoes of the hidden secrets of the atomic disorder of the liquid. "If we could understand what it is was in the arrangement of molecules that rendered one patch rigid and another liquid-like, we would be well on our way to being able to provide a complete account of the glass transition."
Only one problem. Nothing in the structural arrangements of model glasses studied using computer simulations seemed to be able to account for these huge variations in relaxation times. "Asaph and I had looked at everything - density, local structure, energy - nothing matched the spatial pattern of slow and fast particles. It had to be something more subtle."
The breakthough came through a brief visit to Professor David Reichman at Columbia University. "David was interested in looking at the vibrations in these model glasses. The results didn't look very promising at first but, as talked about it, we began to appreciate there were some real similarities between the pattern of slow vibrations and the pattern of mobile particles", said Professor Harrowell.
The result of considerable more work has appeared recently in Nature Physics as a paper entitled Irreversible reorganisation in a supercooled liquid originates from localised soft modes by Dr Widmer-Cooper, Ms Perry and Profesors Reichman and Harrowell. The team concluded that relaxation originates with soft quasi-localised vibrational modes, indicative of local variations in the rigidity of the disordered material.
"This is pretty exciting", said Professor Harrowell. "It means, for the first time, we can look at an arrangement of atoms and predict where motion can occur without actually calculating the dynamics. We still have to understand the origin of these local variations in rigidity but that is a lot easier than trying to explain many body dynamics."
'Improvements in our understanding of this area have potential applications in the world of glassy films development, and could even provide future solutions to the current lifetime limitations of modern data storage devices,' said Professor Harrowell.
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