An extended analysis of the viscosity kernel for monatomic and diatomic fluids

TL;DR

This paper investigates the wave-vector dependent shear viscosity of monatomic and diatomic fluids, revealing how it varies with density, potential energy, and fitting functions, and emphasizing the importance of generalized hydrodynamics for nonlinear strain rates.

Contribution

It provides a comprehensive analysis of the viscosity kernel for monatomic and diatomic fluids using molecular dynamics simulations, including new insights into fitting functions and real-space kernel widths.

Findings

Viscosity kernel width is 3 to 6 atomic diameters.

Reciprocal space shear viscosity fits n-Gaussian and Lorentzian functions.

Viscosity kernel depends on density, potential, and fitting choice.

Abstract

We present an extended analysis of the wave-vector dependent shear viscosity of monatomic and diatomic (liquid chlorine) fluids over a wide range of wave-vectors and for a variety of state points. The analysis is based on equilibrium molecular dynamics simulations, which involves the evaluation of transverse momentum density and shear stress autocorrelation functions. For liquid chlorine we present the results in both atomic and molecular formalisms. We find that the viscosity kernel of chlorine is statistically indistinguishable with respect to atomic and molecular formalisms. The results further suggest that the real space viscosity kernels of monatomic and diatomic fluids depends sensitively on the density, the potential energy function and the choice of fitting function in reciprocal space. It is also shown that the reciprocal space shear viscosity data can be fitted to two different simple functional forms over the entire density, temperature and wave-vector range: a function composed of n-Gaussian terms and a Lorentzian type function. Overall, the real space viscosity kernel has a width of 3 to 6 atomic diameters which means that the generalized hydrodynamic constitutive relation is required for fluids with strain rates that vary nonlinearly over distances of the order of atomic dimensions.