Correlation of Nonexponentiality with Dynamic Heterogeneity from Four-Point Dynamic Susceptibility X4(t) and Its Approximation XT(t)
NAVAL RESEARCH LAB WASHINGTON DC
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Various properties of vitrifying liquids are correlated with the dispersity of the dynamics, the latter reflected in the magnitude of the nonexponentiality parameter, Beta K, describing the distribution of relaxation times. These properties include the mean relaxation time, , the fragility, and the dynamic crossover. The correlations with Beta K are observed in both experimental data and the results from molecular dynamics simulations on Lennard-Jones LJ type systems. Another, rather obvious property to correlate with Beta K is the dynamic heterogeneity, which can be quantified from the number of molecules, Nc, dynamically correlated over a time span. For a given LJ system, Nc can be rigorously calculated and we find that it does indeed correlate with Beta K over a range of thermodynamic conditions. However, the analysis of experimental data for a broad range of real materials, wherein an approximation is required to obtain Nc, reveals the absence of any relationship between Nc and K among different materials.
- Fluid Mechanics