Viscosity decoupling does not guarantee dynamic heterogeneity: A way out

Generally, dynamic heterogeneity in a media is identified by recognizing the viscosity decoupling of the dynamics (i.e., p not equal 1 in the equation log(tau(x)) = plog(eta/T) + logC). While dynamically heterogeneous media will show a breakdown from the Stokes-Einstein (SE) relationship (p not equal 1), the vice-versa is not automatically true. One should be cautious in relating viscosity decoupling to dynamic heterogeneity. Herein, we developed a novel analysis for identifying dynamic heterogeneity in a better way. The analysis is based on selective probing of different diffussion time-regimes in a heterogeneous system from a single measurement and observing their temperature evolution. To prove our concept, we studied temperature dependent translational diffusion of a fluorophore using fluorescence correlation spectroscopy. We used two molecular liquids and two complex deep eutectic solvents in our study. All four solvents show viscosity decoupling, albeit to a different extent. Following the previous knowledge, this could be explained by dynamic heterogeneity in all these solvents. However, our analysis suggests that two molecular liquids, DMF and glycerol, are dynamically homogenous and the two DESs, acetamide/urea and lauric-acid/menthol, are dynamically heterogeneous. We conclude that difference in the extent of viscosity decoupling for different diffusion time-regimes might be a better criteria to identify dynamic heterogeneity, compared to the traditional way. We believe that the newly developed approach will be a step-forward in determining the dynamic heterogeneity from viscosity decoupling.

Automated summary

This study developed a new analysis method for identifying dynamic heterogeneity in a better way. The analysis selectively probed different diffussion time-regimes in a heterogeneous system from a single measurement and observed their temperature evolution. The temperature dependent translational diffusion of a fluorophore was studied using fluorescence correlation spectroscopy. The results suggest that the difference in the extent of viscosity decoupling for different diffusion time-regimes might be a better criteria to identify dynamic heterogeneity.