Anomalous diffusion in heterogeneous glass-forming liquids: Temperature-dependent behavior

Abstract

In a preceding paper [1], Mukhopadhyay and I studied the diffusive motion of a tagged molecule in an heterogeneous glass-forming liquid at temperatures just above a glass transition. Among other features of this system, we postulated a relation between heterogeneity and stretched-exponential decay of correlations, and we also confirmed that systems of this kind generally exhibit non-Gaussian diffusion on intermediate length and time scales. Here I extend this analysis to higher temperatures approaching the point where the heterogeneities disappear and thermal activation barriers become small. I start by modifying the continuous-time random-walk theory proposed in [1], and supplement this analysis with an extension of the excitation-chain theory of glass dynamics. I also use a key result from the shear-transformation-zone theory of viscous deformation of amorphous materials. Elements of each of these theories are then used to interpret experimental data for ortho-terphenyl, specifially, the diffusion and viscosity coefficients and neutron scattering measurements of the self intermediate scattering function. Reconciling the theory with these data sets provides insights about the crossover between super-Arrhenius and Arrhenius dynamics, length scales of spatial heterogeneities, violation of the Stokes-Einstein relation in glass-forming liquids, and the origin of stretched exponential decay of correlations.

Dynamic and thermodynamic characteristics associated with the glass transition of amorphous trehalose–water mixtures

“…Many of the important questions in the physics of glassy materials have to do with spatial heterogeneities, 47 and the spatial heterogeneity can, in part, be attributed to the self-aggregation behavior of molecules. The current MD simulation results suggest that the trehalose-water solution is highly heterogenous in the glassy state, with trehalose forming large clusters that exclude water.…”

Thermodynamic theory of dislocation-mediated plasticity

“…This form of the rate factor emerges from the excitation-chain theory of the glass transition [30,31]. A more physically motivated expression for α(χ ss ) has been introduced in [32]. The dashed line in Fig.6 shows what happens when we set Q(χ ss ) =q, solve Eq.…”

Atomistic mechanism of physical ageing in glassy materials

“…As time progresses, a Gaussian caged peak at small displacements loses particles to widening exponential tails. The origin of the exponential tails has recently generated intense interest, as they appear to be a universal feature of structural glasses [22,[28][29][30][31]. In our calculation, they arise naturally from the wide distribution of persistence times.…”