1,787 research outputs found
Fragility in p-spin models
We investigate the relation between fragility and phase space properties -
such as the distribution of states - in the mean field p-spin model, a solvable
model that has been frequently used in studies of the glass transition. By
direct computation of all the relevant quantities, we find that: i) the
recently observed correlation between fragility and vibrational properties at
low temperature is present in this model; ii) the total number of states is a
decreasing function of fragility, at variance of what is currently believed. We
explain these findings by taking into account the contribution to fragility
coming from the transition paths between different states. Finally, we propose
a geometric picture of the phase space that explains the correlation between
properties of the transition paths, distribution of states and their
vibrational properties. However, our analysis may not apply to strong systems
where inflection points in the configurational entropy as a function of the
temperature are found
Potential energy topology and relaxation processes in a model glass
We use computer simulation to investigate the topology of the potential
energy and to search for doublewell potential's (DWP) in a
model glass . By a sequence of Newtonian and dissipative dynamics we find
different minima of and the energy profile along the least
action paths joining them. At variance with previous suggestions, we find that
the parameters describing the DWP's are correlated among each others. Moreover,
the trajectory of the system in the 3-d configurational phase space follows
a quasi-1-d manifold. The motion parallel to the path is characterized by jumps
between minima, and is nearly uncorrelated from the orthogonal, harmonic,
dynamics.Comment: 4 pages, RevTex, 4 PostScript figure
More on phase diagram of Laponite
The phase diagram of a charged colloidal system (Laponite) has been
investigated by dynamic light scattering in a previously unexplored range of
salt and clay concentrations. Specifically the clay weight and salt molar
concentrations have been varied in the ranges Cw=0.004- 0.025, Cs=(1x 10^-3- 5x
10^-3) M respectively. As in the case of free salt water samples (Cs= 1x 10^-4
M) an aging dynamics towards two different arrested phases is found in the
whole examined Cw and Cs range. Moreover a transition between these two
different regimes is found for each investigated salt concentration. It is
clear from these measurements that a revision of the phase diagram is necessary
and a new "transition" line between two different arrested states is drawn.Comment: 16 pages, 5 figures, submitted to Langmui
Acoustic attenuation in glasses and its relation with the boson peak
A theory for the vibrational dynamics in disordered solids [W. Schirmacher,
Europhys. Lett. {\bf 73}, 892 (2006)], based on the random spatial variation of
the shear modulus, has been applied to determine the wavevector ()
dependence of the Brillouin peak position ( and width (),
as well as the density of vibrational states (), in disordered
systems. As a result, we give a firm theoretical ground to the ubiquitous
dependence of observed in glasses. Moreover, we derive a
quantitative relation between the excess of the density of states (the boson
peak) and , two quantities that were not considered related before.
The successful comparison of this relation with the outcome of experiments and
numerical simulations gives further support to the theory.Comment: To appear on PR
Routes to gelation in a clay suspension
The gelation of water suspension of a synthetic clay (Laponite) has been
studied by dynamic light scattering in a wide range of clay weight
concentration (Cw = 0.003-0.031). At variance with previous determination,
indicating a stable liquid phase for Cw < Cw*=0.015-0.018, we find that the
gelation takes actually place in the whole examined Cw range. More importantly,
we find that Cw* marks the transition between two different routes to gelation.
We hypothesize that at low concentration Laponite suspension behaves as an
attractive colloid and that the slowing down of the dynamics is attained by the
formation of larger and larger clusters while at high concentration the basic
units of the gel could be the Debye Huckel spheres associated to single
Laponite plates.Comment: 5 pages, 4 figure
Influence of an adsorbing polymer in the aging dynamics of Laponite clay suspensions
Clay-polymer dispersions in aqueous solutions have attracted a great interest
in recent years due to their industrial applications and intriguing physical
properties. Aqueous solutions of bare Laponite particles are known to age
spontaneously from an ergodic state to a non ergodic state in a time varying
from hours to months depending on Laponite concentration. When a polymer
species like Polyethylene Oxide (PEO) is added to the solution, it weakly
adsorbs on clay particle surfaces modifying the effective interaction potential
between Laponite particles. A dynamic light scattering study, varying polymer
concentration at fixed polymer molecular weight (Mw=200.000 g/mol), has been
performed in order to understand the effect of polymer on the aging dynamics of
the system. The results obtained show that arresting phenomena between clay
particles are hindered if PEO is added and consequently the aging dynamics
slows down with increasing PEO concentration. This process is possibly due to
the progressive coverage of the clay surface by polymers that grow with
increasing PEO concentration and may lead to steric stabilization.Comment: 13 pages, 6 figures, manuscript accepted for publication on
Philosophical Magazin
Orientational and induced contributions to the depolarized Rayleigh spectra of liquid and supercooled ortho-terphenyl
The depolarized light scattering spectra of the glass forming liquid
ortho-terphenyl have been calculated in the low frequency region using
molecular dynamics simulation. Realistic system's configurations are produced
by using a recent flexible molecular model and combined with two limiting
polarizability schemes, both of them using the dipole-induced-dipole
contributions at first and second order. The calculated Raman spectral shape
are in good agreement with the experimental results in a large temperature
range. The analysis of the different contributions to the Raman spectra
emphasizes that the orientational and the collision-induced (translational)
terms lie on the same time-scale and are of comparable intensity. Moreover, the
cross terms are always found to be an important contribution to the scattering
intensity.Comment: RevTeX4, 7 pages, 8 eps figure
Heat capacity of liquids: A hydrodynamic approach
We study autocorrelation functions of energy, heat and entropy densities
obtained by molecular dynamics simulations of supercritical Ar and compare them
with the predictions of the hydrodynamic theory. It is shown that the predicted
by the hydrodynamic theory single-exponential shape of the entropy density
autocorrelation functions is perfectly reproduced for small wave numbers by the
molecular dynamics simulations and permits the calculation of the
wavenumber-dependent specific heat at constant pressure. The estimated
wavenumber-dependent specific heats at constant volume and pressure,
and , are shown to be in the long-wavelength limit in good agreement
with the macroscopic experimental values of and for the studied
thermodynamic points of supercritical Ar.Comment: 8 pages, 5 figure
Fluctuations of entropy production in the isokinetic ensemble
We discuss the microscopic definition of entropy production rate in a model
of a dissipative system: a sheared fluid in which the kinetic energy is kept
constant via a Gaussian thermostat. The total phase space contraction rate is
the sum of two statistically independent contributions: the first one is due to
the work of the conservative forces, is independent of the driving force and
does not vanish at zero drive, making the system non-conservative also in
equilibrium. The second is due to the work of the dissipative forces, and is
responsible for the average entropy production; the distribution of its
fluctuations is found to verify the Fluctuation Relation of Gallavotti and
Cohen. The distribution of the fluctuations of the total phase space
contraction rate also verify the Fluctuation Relation. It is compared with the
same quantity calculated in the isoenergetic ensemble: we find that the two
ensembles are equivalent, as conjectured by Gallavotti. Finally, we discuss the
implication of our results for experiments trying to verify the validity of the
FR.Comment: 8 pages, 4 figure
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