422 research outputs found
What can be learned from the schematic mode-coupling approach to experimental data ?
We propose a detailed investigation of the schematic mode-coupling approach
to experimental data, a method based on the use of simple mode-coupling
equations to analyze the dynamics of supercooled liquids. Our aim here is to
clarify different aspects of this approach that appeared so far uncontrolled or
arbitrary, and to validate the results obtained from previous works. Analyzing
the theoretical foundations of the approach, we first identify the parameters
of the theory playing a key role and obtain simple requirements to be met by a
schematic model for its use in this context. Then we compare the results
obtained from the schematic analysis of a given set of experimental data with a
variety of models and show that they are all perfectly consistent. A number of
potential biases in the method are identified and ruled out by the choice of
appropriate models. Finally, reference spectra computed from the mode-coupling
theory for a model simple liquid are analyzed along the same lines as
experimental data, allowing us to show that, despite the strong simplification
in the description of the dynamics it involves, the method is free from
spurious artifacts and provides accurate estimates of important parameters of
the theory. The only exception is the exponent parameter, the evaluation of
which is hindered, as for other methods, by corrections to the asymptotic laws
of the theory present when the dynamics is known only in a limited time or
frequency range.Comment: 13 pages, 5 figures, revtex4, to appear in J. Chem. Phy
Study of the Depolarized Light Scattering Spectra of Supercooled Liquids by a Simple Mode-Coupling Model
By using simple mode coupling equations, we investigate the depolarized light
scattering spectra of two so-called "fragile" glassforming liquids, salol
(phenylsalicylate) and CKN (Ca_{0.4}K_{0.6}(NO_3)_{1.4}), measured by Cummins
and coworkers. Nonlinear integrodifferential equations for the time evolution
of the density-fluctuations autocorrelation functions are the basic input of
the mode coupling theory. Restricting ourselves to a small set of such
equations, we fit the numerical solution to the experimental spectra. It leads
to a good agreement between model and experiment, which allows us to determine
how a real system explores the parameter space of the model, but it also leads
to unrealistic effective vertices in a temperature range where the theory makes
critical asymptotic predictions. We finally discuss the relevance and the range
of validity of these universal asymptotic predictions when applied to
experimental data on supercooled liquids.Comment: 31 LaTeX pages using overcite.sty, 10 postscript figures, accepted in
J. Chem. Phy
Disentangling density and temperature effects in the viscous slowing down of glassforming liquids
We present a consistent picture of the respective role of density and
temperature in the viscous slowing down of glassforming liquids and polymers.
Specifically, based in part upon a new analysis of simulation and experimental
data on liquid ortho-terphenyl, we conclude that a zeroth-order description of
the approach to the glass transition should be formulated in terms of a
temperature-driven super-Arrhenius activated behavior rather than a
density-driven congestion or jamming phenomenon. The density plays a role at a
quantitative level, but its effect on the viscosity and the structural
relaxation time can be simply described via a single parameter, an effective
interaction energy that is characteristic of the high temperature liquid
regime; as a result, density does not affect the ``fragility'' of the
glassforming system.Comment: RevTeX4, 8 pages, 8 eps figure
On the correlation between fragility and stretching in glassforming liquids
We study the pressure and temperature dependences of the dielectric
relaxation of two molecular glassforming liquids, dibutyl phtalate and
m-toluidine. We focus on two characteristics of the slowing down of relaxation,
the fragility associated with the temperature dependence and the stretching
characterizing the relaxation function. We combine our data with data from the
literature to revisit the proposed correlation between these two quantities. We
do this in light of constraints that we suggest to put on the search for
empirical correlations among properties of glassformers. In particular, argue
that a meaningful correlation is to be looked for between stretching and
isochoric fragility, as both seem to be constant under isochronic conditions
and thereby reflect the intrinsic effect of temperature
Structure of liquid and glassy methanol confined in cylindrical pores
We present a neutron scattering analysis of the density and the static
structure factor of confined methanol at various temperatures. Confinement is
performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24
angstrom and D=35 angstrom. A change of the thermal expansivity of confined
methanol at low temperature is the signature of a glass transition, which
occurs at higher temperature for the smallest pore. This is an evidence of a
surface induced slowing down of the dynamics of the fluid. The structure factor
presents a systematic evolution with the pore diameter, which has been analyzed
in terms of excluded volume effects and fluid-matrix cross-correlation.
Conversely to the case of Van der Waals fluids, it shows that stronger
fluid-matrix correlations must be invoked most probably in relation with the
H-bonding character of both methanol and silicate surface.Comment: version March 12 200
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