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Supercooled liquids and glasses: dynamics, dynamic heterogeneity, and stability




Staley, Hannah, author
Bradley, R. Mark, advisor
Szamel, Grzegorz, advisor
Gelfand, Martin, committee member
Aristoff, David, committee member

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We used molecular dynamics simulations to study supercooled liquids and glasses. Supercooled liquids are liquids that have been cooled below their freezing temperature. We start the thesis with an introduction on supercooled liquids. We studied several different model glass-formers and compared them by scaling all data to the point where the Stokes-Einstein relation was violated. The Stokes-Einstein relation holds for many liquids, but breaks down at some temperature for most supercooled liquids. In all the systems we studied, we examined dynamic heterogeneity as quantified by the dynamic susceptibility, χ4, and the dynamic correlation length, ξ4. When dynamics are heterogeneous, a liquid breaks up into regions of particles with correlated mobility. The susceptibility is related to the number of particles in such a region, and the dynamic correlation length is related to the size of a region. We broke up our model glass-formers into the categories of strong glass-formers and fragile glass-formers. A strong glass-former has a viscosity, which obeys the Arrhenius relationship, while a fragile glass-former has super-Arrhenius behavior. We compared the systems by relating them at the temperature where the Stokes-Einstein relation was violated. We found that when variables are rescaled to their values at the Stokes-Einstein violation temperature, Ts, the fragile glass-formers all behaved in the same way, and we created plots where the data in all the systems followed the same curve. In the fragile glass-formers, we also found that clusters of correlated particles became compact below Ts. We studied one strong glass-former, and found that it did not match the fragile glass-former curves. However, the Stokes-Einstein violation temperature still appears to be significant in that system, since it appears to mark a change in shape of clusters of correlated particles. However, the clusters did not become compact. We examined the stability of a glass that was created by cooling at different rates. We investigated mechanical stability by measuring the energy and shear modulus of the glass. We also studied the kinetic stability upon heating the glass by examining the average overlap function, a dynamic correlation function. The average overlap function measures how much correlation the positions of particles have with their initial positions after a certain amount of time. We used a stability ratio, S, to probe kinetic stability. Stability is higher in glasses that were prepared by cooling at a slower rate. The different measures of stability have different relationships with initial cooling rate, and we determined that kinetic stability is the best measure of stability.


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