291 research outputs found
Origin of the Kohlrausch exponent
According to a recent numerical finding, the dynamics of a glass former is
exclusively due to the forces within the first coordination shell. This implies
that the Kohlrausch beta should be understandable in terms of the effective
nearest-neighbor potential.
The present paper proposes a relation for the Kohlrausch beta based on the
Adam-Gibbs conjecture of a flow barrier proportional to the number of atoms or
molecules in a cooperatively rearranging region. The conjecture implies that
beta is given by the ratio of the structural entropy increase per particle to
the barrier increase per particle. Making use of a recent numerical
determination of the structural entropy per particle in Lennard-Jones-like
potentials, one can show that the relation leads to values between 0.3 and 0.6.Comment: Third completely changed versio
Irreversible Eshelby description of aging in glasses
The recent description of the cooling through the glass transition in terms
of irreversible structural Eshelby rearrangements with a single average fictive
temperature is extended to a distribution of fictive temperatures around the
average one. The extension has only little influence on the cooling scans, but
turns out to be necessary to understand the heating back to equilibrium.Comment: 3 pages, 3 figure
Modeling the nonlinear dielectric response of glass formers
The recently developed pragmatical model of asymmetric double-well potentials
with a finite lifetime is applied to nonlinear dielectric data in polar
undercooled liquids. The viscous effects from the finite lifetime provide a
crossover from the cooperative jumps of many molecules at short times to the
motion of statistically independent molecules at long times. The model allows
to determine the size of cooperatively rearranging regions from nonlinear
{\omega}-data and throws a new light on a known inconsistency between nonlinear
{\omega} and 3{\omega}-signals for glycerol and propylene carbonate.Comment: Version 4 after hot discussions with Referee
Eshelby description of highly viscous flow -- half model, half theory
A recent description of the highly viscous flow ascribes it to irreversible
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. The description is extended to include
reversible Eshelby transitions, with the Kohlrausch exponent as a free
parameter. The model answers several open questions in the field.Comment: 11 pages, 9 figures, corrected following referee advic
Thermodynamics and dynamics of the inherent states at the glass transition
It has been recently shown that one can understand the Prigogine-Defay ratio
at the glass transition in terms of freezing into one of the many inherent
states of the undercooled liquid. In the present paper, the treatment is
extended to the dynamics at the glass transition to show the connection to
isomorphism and density scaling. In addition, the energy limits for stable
inherent states are discussed.Comment: paper accepted for Proceedings IDMRCS 2013 in Barcelona, special
issue of Journal of Non-Crystalline Solid
Retardation and flow at the glass transition
The crossover from back-and-forth jumps between structural minima to the
no-return jumps of the viscous flow is modeled in terms of an ensemble of
double-well potentials with a finite decay probability. The ensemble is
characterized by the Kohlrausch-exponent of the time dependence
of the response at short times. The model is applied to shear and
dielectric data from the literature.Comment: Second version changed according to advice of Referees: Rewritten to
Regular Article. New data included, in particular monoalcohol data, which
argue for a common crossover to viscous processes in dielectrics and shea
Structural relaxation and highly viscous flow
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.Comment: 6 pages, 3 figures, version published in Journal of Chemical Physic
Supplemental Material: Eshelby ensemble of highly viscous flow out of equilibrium
Supplemental Material to ArXiv:1902.0274
Eshelby description of highly viscous flow III
The recent Eshelby description of the highly viscous flow leads to the
prediction of a factor of two different viscosities in stationary and
alternating flow, in agreement with experimental evidence. The Kohlrausch
barrier density increase with increasing barrier height finds a physical
justification in the Adam-Gibbs increase of the number of structural
alternatives of the Eshelby region with its increasing size. The new Ansatz
allows to determine the number of atoms or molecules in the rearranging Eshelby
domains from a combination of dynamic shear relaxation and calorimetric data.Comment: 9 pages, four figures, continuation of two preceding paper
Gr\"uneisen model for melts
The Gr\"uneisen relation is shown to be important for the thermodynamics of
dense liquids
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