Supercooled Liquid Dynamics Studied via Shear-Mechanical Spectroscopy

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: Frequency Dependent Viscositysupporting

confidence: 60%

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

Levashov

2014

“…Whether it lies within the time window of experiments would depend on the specific systems under study [57][58][59][60].…”

Section: Visco-elasticitymentioning

confidence: 99%

Replica theory of the rigidity of structural glasses

Yoshino

2012

108

We present a first principle scheme to compute the rigidity, i. e. the shear-modulus of structural glasses at finite temperatures using the cloned liquid theory, which combines the replica theory and the liquid theory. With the aid of the replica method which enables disentanglement of thermal fluctuations in liquids into intra-state and inter-state fluctuations, we extract the rigidity of metastable amorphous solid states in the supercooled liquid and glass phases. The result can be understood intuitively without replicas. As a test case, we apply the scheme to the supercooled and glassy state of a binary mixture of soft-spheres. The result compares well with the shear-modulus obtained by a previous molecular dynamic simulation. The rigidity of metastable states is significantly reduced with respect to the instantaneous rigidity, namely the Born term, due to non-affine responses caused by displacements of particles inside cages at all temperatures down to T = 0. It becomes nearly independent of temperature below the Kauzmann temperature TK. At higher temperatures in the supercooled liquid state, the non-affine correction to the rigidity becomes stronger suggesting melting of the metastable solid state. Inter-state part of the static response implies jerky, intermittent stress-strain curves with static analogue of yielding at mesoscopic scales.

“…To come to the measured values of G ∞ J 0 e of 2 to 3, one needs f N ≈ 0.7..0.9, so there are sixty to eighty inherent states in a single cooperatively rearranging region. The size of the region increases strongly with decreasing temperature [47][48][49] , but the number of inherent states remains fixed by the recoverable compliance G ∞ J 0 e , which according to the results in Table I tends to stay constant. The dramatic increase of the volume needed for a constant number of inherent states with decreasing temperature is consistent with the well-known dramatic entropy loss which leads to the extrapolated Kauzmann catastrophe.…”

Section: Discussionmentioning

confidence: 69%

“…A recently developed four-point correlation method 14 allows to estimate the size of this region from the temperature dependence of, say, the dielectric susceptibility [47][48][49] . The average number of inherent states in such a cooperatively rearranging region can be calculated from the asymmetry model 23 , because the f N for just two inherent states with asymmetry 4k B T in the core is 0.012.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

On the mechanism of the highly viscous flow

Buchenau

2011

The asymmetry model for the highly viscous flow postulates thermally activated jumps from a practically undistorted ground state to strongly distorted, but stable structures, with a pronounced Eshelby backstress from the distorted surroundings. The viscosity is ascribed to those stable distorted structures which do not jump back, but relax by the relaxation of the surrounding viscoelastic matrix. It is shown that this mechanism implies a description in terms of the shear compliance, with a viscosity which can be calculated from the cutoff of the retardation spectrum. Consistency requires that this cutoff lies close to the Maxwell time. The improved asymmetry model compares well with experiment.