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(390c) Connection between Local Dynamic Environments & Relaxation Mechanism

Badilla, K. - Presenter, University of South Alabama
Bommarius, A., Georgia Institute of Technology
Cicerone, M. T., National Institute of Standards and Technology
Since the 1950s, there has been an interest in deeply exploring transport in amorphous condensed matter phases, but in these seventy years, an in-depth understanding of these phenomena has yet to be attained. As time progresses, however, the importance of the role of βJG relaxation becomes increasingly apparent; the molecular motion tied to βJG relaxation is molecules breaking out from their local cage structure by “hopping”. This hopping motion is facilitated by excitations which are local density fluctuations that reduce local energy barriers to motion. In this presentation, I will demonstrate that quantifying these excitations can provide information about the shift in relaxation mechanism of liquids with respect to temperature.

As shown in the figure1, in the liquid and glass phases, there are temperatures TA and TB at which mechanistic changes in relaxation occur2. At relatively high temperatures, T > TA, relaxation is purely collisional and there is no evidence of cooperative motion, while at intermediate temperatures TB > T > TA, dynamics follow super-Arrhenius behavior. At low temperatures, T < TB, a secondary relaxation process that bifurcates from primary relaxation emerges. Using data collected from neutron scattering of five simple liquids3, our group has seen that at TA and TB, there is a shift in the average number of excitations in the first shell of the molecule. Further, we can categorize local dynamic environments based on the number of excitations in the first shell; these environments are titled Piwhere i denotes the number of excitations in the first shell. By assuming a random distribution of these environments, we have seen that at temperatures TA and TB, environments P1 and P≥2 both have populations of approximately 24%, respectively. These thresholds for excitations paint a fundamental picture of the local processes lead to the mechanistic changes in liquid relaxation. With this information, we reach closer a level of predictive frameworks for liquids comparable to that we have for solids and gases.

  1. Schneider et al., Phys Rev E59 (1999)
  2. Stickel et al., J Chem Phys 104, 2043 (1996)
  3. Cicerone et al., Phys Rev Lett 113, 117801 (2014)