Theory of the highly viscous flow

30 Apr 2020  ·  Buchenau U. ·

In an undercooled liquid close to the glass transition, the flow begins by many thermally activated back-and-forth jumps, structural Eshelby rearrangements of strained regions. Together, they lead at short times t to the Kohlrausch t^beta shear relaxation, with beta around 1/2, before the jumps become irreversible and the viscous flow begins. The Kohlrausch behavior is not yet well understood. Here, a theoretical explanation is given, starting from an exact result for the irreversible jumps, the lifetime distribution of the critical Eshelby region. The barrier for the Eshelby transitions is decomposed into a sum over all vibrational modes which have to find a new minimum. A new close-packing picture is proposed for the numerically found string motion in soft vibrations and low-barrier relaxations in glasses. It enables a quantitative calculation of the Kohlrausch exponent for metallic glasses. A continuity relation between the irreversible and the reversible Kohlrausch relaxation time distribution is derived. The full spectrum can be used in many ways, not only to describe shear relaxation data, but also to relate shear relaxation data to dielectric and bulk relaxation spectra, and to predict aging from shear relaxation data, as demonstrated for a very recent aging experiment.

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Disordered Systems and Neural Networks Soft Condensed Matter