Large and Long-Range Dynamic Correlations in Supercooled Fluids Revealed via Four-Point Correlation Functions.
Dynamic heterogeneity in supercooled fluids is often monitored using four-point structure factors. We show that a four-point structure factor slightly different from the one commonly used reveals the existence of large and long-range dynamic correlations, which dominate the correlations identified with dynamic heterogeneity. Specifically, we compare two four-point structure factors that differ by the mobility field used in their definition. The mobility field used in our alternative four-point structure factor, which we denote as S4(F)(q, k; t), is the full microscopic self-intermediate scattering function, whereas the mobility field in the structure factor commonly used to examine dynamic heterogeneity, which we denote as S4(cos)(q, k; t), is is the real part of the microscopic self-intermediate scattering function. The susceptibility χ4(F)(t) and the correlation length ξ4(F)(t) associated with S4(F)(q, k; t) are both much larger than the susceptibility χ4(cos)(t) and the correlation length ξ4(cos)(t) associated with S4(cos)(q, k; t). In particular, we find that ξ4(F)(τα) grows proportionally with the relaxation time, ξ4(F)(τα) ∼ τα, while we previously established that ξ4(cos)(τα) has a much weaker dependence on the relaxation time. We argue that the growth of ξ4(F)(τα) is primarily due to the growth of a transient elastic response, while the growth of ξ4(cos)(τα) is due to spatially correlated domains of slow and fast particles, usually referred to as dynamic heterogeneity.
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States.
Elijah
E
Flenner
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States.
Grzegorz
G
Szamel