2,588 research outputs found
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
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