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An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids
Published on: December 4, 2017
Relaxation processes in liquids: variations on a theme by Stokes and Einstein.
Zane Shi1, Pablo G Debenedetti, Frank H Stillinger
1Department of Physics, Princeton University, Princeton, New Jersey 08544, USA.
The Journal of Chemical Physics
|April 6, 2013
Summary
The Stokes-Einstein ratio (Dη/T) remains constant across temperatures and densities in atomic mixtures and ortho-phenyl. Other variants (Dτ, Dτ/T) show complex behavior, challenging common assumptions.
Area of Science:
- Computational physics and chemistry
- Condensed matter physics
- Materials science
Background:
- The Stokes-Einstein relation connects diffusivity (D), viscosity (η), and temperature (T).
- Deviations from this relation are observed in supercooled liquids, with variants like Dτ and Dτ/T often used.
- Understanding these deviations is crucial for predicting liquid behavior.
Purpose of the Study:
- To numerically investigate the temperature and density dependence of the Stokes-Einstein ratio (Dη/T) and its variants (Dτ, Dτ/T).
- To evaluate the validity of common assumptions relating relaxation time (τ) to viscosity (η) and temperature (T).
- To analyze the behavior of these ratios in atomic binary mixtures and the ortho-terphenyl (OTP) model.
Main Methods:
- Numerical simulations of atomic binary mixtures with softened repulsive interactions.
- Simulations of the Lewis-Wahnström model of ortho-terphenyl (OTP).
- Calculation of diffusivity (D), shear viscosity (η), structural relaxation time (τ), and instantaneous shear modulus.
Main Results:
- The Stokes-Einstein ratio (Dη/T) was found to be constant across a broad range of temperatures and densities for both systems.
- The variants Dτ and Dτ/T significantly increase upon cooling in the supercooled regime.
- Negative violations of Stokes-Einstein behavior (decrease upon cooling) were observed for Dτ in atomic systems at higher temperatures, but not for OTP.
Conclusions:
- The constancy of Dη/T suggests its robustness across different thermodynamic conditions and system types.
- Assumptions like τ ∼ η and τ ∼ η/T require critical evaluation, as they do not universally hold.
- The study highlights the complex dynamics of supercooled liquids and the limitations of simplified models.
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