Structural relaxation and highly viscous flow

Abstract

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.

Eshelby description of highly viscous flow—Half model, half theory

“…The finding corroborates the same finding in the vacuum pump oil PPE [15]. Note that the model parameter β is not involved; the explanation of the irreversible part of the decay in terms of Eshelby transitions [15] is not a model, but an exact theory. Section III.…”

“…As derived in the preceding paper [15], τ c is only the average decay time; the precise decay time depends on the shear stress state of the region. Strongly stressed regions decay faster than a stress-free one.…”

“…The other half is shear energy of the surroundings [16]. As shown in the preceding paper [15], a strongly strained region with a high e 2 has much more possibilities to make an irreversible jump than a weakly strained one. Quantitatively…”

“…To see this, one has to remember the derivation of the average squared shear angle ǫ 2 for irreversible processes from reference [15], and then calculate the same quantity for the reversible processes.…”

“…* Electronic address: buchenau-juelich@t-online. de The paper is based on the recent description of the highly viscous flow in terms of irreversible Eshelby processes [15], which showed that the terminal relaxation time τ c is a factor of eight longer than the Maxwell time τ M = η/G (η viscosity, G short time shear modulus), and derived a well-defined relaxation time distribution for the irreversible structural relaxation processes.…”