A molecular dynamics simulations study on the relations between dynamical heterogeneity, structural relaxation, and...

Area of Science:

• Condensed matter physics
• Materials science
• Computational chemistry

Background:

• Dynamical heterogeneity is a key feature of viscous liquids near the glass transition.
• Understanding its relationship with structural relaxation and self-diffusion is crucial for materials science.

Purpose of the Study:

• To investigate the interrelations between dynamical heterogeneity, structural (α) relaxation, and self-diffusion in various viscous liquids.
• To characterize the space-time dynamics of heterogeneity using established metrics.

Main Methods:

• Molecular dynamics simulations of supercooled water, polymer melts, and ionic liquids.
• Characterization of dynamical heterogeneity via non-Gaussian parameter (α2), mobile particle cluster size (S(w)), and cooperative string length (L(w)).

Main Results:

• A nearly universal decoupling (τ(α2) ∝ τ(α)(3/4)) was found between the time scales of maximum non-Gaussian parameter and α relaxation.
• No uniform relation was observed between peak times of S(w) or L(w) and τ(α).
• The time scale of maximum string length (τ(L)) inversely correlates with self-diffusion (D) at low temperatures (τ(L) ∝ D(-1)).

Conclusions:

• The findings suggest a breakdown of the Stokes-Einstein relation in the studied viscous liquids.
• The degree of deviation from the Stokes-Einstein relation correlates with the stretching of the α relaxation.