On the mechanism of the highly viscous flow

We report on a further investigation of a new method that can be used to address vibrational dynamics and propagation of stress waves in liquids. The method is based on the decomposition of the macroscopic Green-Kubo stress correlation function into the atomic level stress correlation functions. This decomposition, as was demonstrated previously for a model liquid studied in molecular dynamics simulations, reveals the presence of stress waves propagating over large distances and a structure that resembles the pair density function. In this paper, by performing the Fourier transforms of the atomic level stress correlation functions, we elucidate how the lifetimes of the stress waves and the ranges of their propagation depend on their frequency, wavevector, and temperature. These results relate frequency and wavevector dependence of the generalized viscosity to the character of propagation of the shear stress waves. In particular, the results suggest that an increase in the value of the frequency dependent viscosity at low frequencies with decrease of temperature is related to the increase in the ranges of propagation of the stress waves of the corresponding low frequencies. We found that the ranges of propagation of the shear stress waves of frequencies less than half of the Einstein frequency, extend well beyond the nearest neighbor shell even above the melting temperature. The results also show that the crossover from quasilocalized to propagating behavior occurs at frequencies usually associated with the Boson peak.

Section: Discussionmentioning

confidence: 99%

Dependence of the atomic level Green-Kubo stress correlation function on wavevector and frequency: Molecular dynamics results from a model liquid

Levashov

2014

“…The equation is based on the assumption [17] of a constant density of stable structural states in distortion space and on the neglect of the difference in structural energy.…”

Section: Viscosity and Viscous Decay Spectrummentioning

confidence: 99%

Structural relaxation and highly viscous flow

Buchenau

2018

Self Cite

The highly viscous flow is due to thermally activated Eshelby transitions which transform a region of the undercooled liquid to a different structure with a different elastic misfit to the viscoelastic surroundings. A self-consistent determination of the viscosity in this picture explains why the average structural relaxation time is a factor of eight longer than the Maxwell time. The physical reason for the short Maxwell time is the very large contribution of strongly strained inherent states to the fluidity (the inverse viscosity). At the Maxwell time, the viscous no-return processes coexist with the back-and-forth jumping retardation processes.

“…These include, for example, Eyring's thermally and stress activated basin hopping shear thinning formula, 2 and Zwanzig's model of self-diffusion in which the liquid is viewed to consist of regions of phase space in which the configuration oscillates before it diffuses into another 'cell' through a saddle point. 3,4 The related shoving, 5,6 and asymmetry, 7 models assume that the activation energy for a flow event is supplied by the reorganizational release of the shear elastic energy arising from deformation of a group of molecules by an applied shear field. More specific descriptions of the evolution of the system through its energy landscape are provided by Stillinger's inherent structures approach, 8 involving a quench to the nearest PEL basin, which has been used to describe supercooled liquids and glassy systems, 9,10 .…”

Section: Introductionmentioning

confidence: 99%

Single trajectory transport coefficients and the energy landscape by molecular dynamics simulations

Heyes

Dini

Smith

2020

The Green-Kubo (GK) method is widely used to calculate the transport coefficients of model liquids by Molecular Dynamics (MD) simulation. A reformulation of GK was proposed in [D.M. Heyes, E.R. Smith and D. Dini J. Chem. Phys., 150, 174504 (2019)] which expressed the shear viscosity in terms of a probability distribution function (PDF) of 'single trajectory viscosities' (ST), called 'viscuits'.This approach is extended here to the bulk viscosity, thermal conductivity and diffusion coefficient.The PDFs of the four ST expressed in terms of their standard deviations (calculated separately for the positive and negative sides) are shown by MD to be statistically the same. This PDF can be represented well by a sum of exponentials, and is independent of system size and state point in the equilibrium fluid regime. The PDF is not well reproduced by a stochastic model. The PDF is statistically the same as that derived from the potential energy, u, and other thermodynamic quantities, indicating that the transport coefficients are determined quantitatively by and follow closely the time evolution of the underlying energy landscape. The PDFs of out-of-equilibrium supercooled high density states are quite different to those of the equilibrium states.