An asymmetry model for the highly viscous flow

Abstract

The shear flow is modeled in terms of local structural rearrangements. Most of these rearrangements are strongly asymmetric, because the embedding matrix tends to be elastically adapted to the initial state and to have a marked elastic misfit with regard to the final state. As one approaches the Maxwell time, the asymmetry becomes time dependent, thus enabling the system to leave the initial state. The model explains the Kohlrausch behavior at the main peak in terms of the interaction between different local structural rearrangements.

Fragility and elasticity: Description of flow in highly viscous liquids

“…The latter plays a minor role if the density of relaxations at the critical flow barrier is high but can become dominant if it happens to be low. The quantitative agreement supports the validity of the explanation of the asymmetry in terms of an elastic distortion, 6 an Eshelby backstress 7 which tends to stabilize the occupied inherent states.…”

“…There is a second source of fragility in the newly discovered 5 asymmetry of about 4k B T between the actual inherent state and its neighbors, possibly due 6 to the elastic distortion accompanying a structural rearrangement ͑the "Eshelby backstress" 7 ͒. As will be seen, the quantitative explanation of the fragility of six different glass formers requires just this specific explanation of the asymmetry.…”

“…In terms of the definitions of the coupling model, 3 the jumps into neighboring inherent states with barriers below V c are the primitive relaxations. The asymmetry model 6 postulates that their elastic interaction brings the shear modulus down to zero. If this is indeed so, the contribution to the fragility depends crucially on the barrier density of primitive relaxations at V c .…”

“…For this purpose, one can follow the recipe given in the asymmetry paper, 6 describing the dynamical shear data with the three parameters G, V c , and ␥ ͓␥ describes the exponential rise f 0 ͑V͒ϳexp͑␥V / k B T͒ of f 0 ͑V͒ at V c ͔. For G, one takes the measured high-frequency value from Table I.…”

“…In order to quantify this influence within the asymmetry model, 6 an extension of the coupling model, 3 consider its basic definition of the characteristic multiminimum parameter f N , PHYSICAL REVIEW B 80, 172201 ͑2009͒…”