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The origin of viscosity as seen through atomic level stress correlation function.

V A Levashov¹, J R Morris, T Egami

¹Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA.

The Journal of Chemical Physics

|February 8, 2013

Summary

Understanding atomic-level stress dynamics in supercooled liquids reveals key insights into viscosity and relaxation times. This study details how stress correlation functions relate to shear waves and relaxation, crucial for the glass transition.

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Area of Science:

Condensed matter physics
Materials science
Physical chemistry

Background:

The atomic-level origins of viscosity and relaxation times are critical for understanding supercooled liquids and the glass transition.
Previous work linked shear stress waves to viscosity via Green-Kubo formalism and atomic-level stress correlation functions.

Purpose of the Study:

To investigate the behavior of atomic-level stress correlation functions at various temperatures.
To elucidate the relationship between different timescales within the stress correlation function and the Maxwell relaxation time.
To explore the influence of periodic boundary conditions on these dynamics.

Main Methods:

Analysis of the Green-Kubo expression for viscosity.
Decomposition into correlation functions of local atomic-level stresses.
Detailed examination of atomic-level stress correlation function behavior across different temperatures.
Comparison of characteristic timescales: stress correlation decay (τ(S)), intermediate self-scattering function decay (τ(α)), and Maxwell relaxation time (τ(M)).

Main Results:

The long-time decay of the stress correlation function (τ(S)) is approximately three times shorter than the intermediate self-scattering function decay (τ(α)).
The Maxwell relaxation time (τ(M)) is approximately five times shorter than τ(α).
Demonstration of how different timescales within the stress correlation function collectively determine the Maxwell relaxation time.

Conclusions:

Atomic-level stress dynamics are intrinsically linked to macroscopic viscosity and relaxation phenomena in supercooled liquids.
The timescales derived from stress correlation functions provide a detailed mechanism for understanding the Maxwell relaxation time.
Periodic boundary conditions play a significant, non-trivial role in the observed dynamics.