The connection between non-exponential relaxation and fragility in supercooled liquids

Among the outstanding problems in the theory of supercooled liquids are the reasons for the rapid increase in their viscosity and relaxation times as the temperature is lowered towards the glass transition temperature, the non-exponential time dependence of the relaxation, and the possible connection between these two properties. The ferromagnetic Potts model on a square latice is a simple system that is found to exhibit these properties. Our calculations show that in this system the connection between them is associated with the dependence on temperature and time of the average environment of the sites. Some of the consequences of this for understanding the behavior of supercooled liquids are discussed.Comment: 12 pages, 8 figure

Gaussian density fluctuations, mode coupling theory, and all that

We consider a toy model for glassy dynamics of colloidal suspensions: a single Brownian particle diffusing among immobile obstacles. If Gaussian factorization of static density fluctuations is assumed, this model can be solved without factorization approximation for any dynamic correlation function. The solution differs from that obtained from the ideal mode coupling theory (MCT). The latter is equivalent to including only some, positive definite terms in an expression for the memory function. An approximate re-summation of the complete expression suggests that, under the assumption of Gaussian factorization of static fluctuations, mobile particle's motion is always diffusive. In contrast, MCT predicts that the mobile particle becomes localized at a high enough obstacle density. We discuss the implications of these results for models for glassy dynamics.Comment: to be published in Europhys. Let

Surface diffusion of polymer glasses redux

Measurements of the surface self-diffusion coefficients of glass-forming small molecules, indomethacin (IMC), nifedipine (NIF),2 and o-terphenyl (OTP),3 by Yu and coworkers using the method of surface grating decay have found exceedingly large enhancement of molecular mobility at the surface. The decay of surface grating in all three molecules occurs by viscous flow at high temperatures, but by surface diffusion at lower temperatures starting at about 10 deg above Tg