216 research outputs found
Binary mixture of hard disks as a model glass former: Caging and uncaging
I have proposed a measure for the cage effect in glass forming systems. A
binary mixture of hard disks is numerically studied as a model glass former. A
network is constructed on the basis of the colliding pairs of disks. A rigidity
matrix is formed from the isostatic (rigid) sub--network, corresponding to a
cage. The determinant of the matrix changes its sign when an uncaging event
occurs. Time evolution of the number of the uncaging events is determined
numerically. I have found that there is a gap in the uncaging timescales
between the cages involving different numbers of disks. Caging of one disk by
two neighboring disks sustains for a longer time as compared with other cages
involving more than one disk. This gap causes two--step relaxation of this
system
Expansion for -Core Percolation
The physics of -core percolation pertains to those systems whose
constituents require a minimum number of connections to each other in order
to participate in any clustering phenomenon. Examples of such a phenomenon
range from orientational ordering in solid ortho-para mixtures to
the onset of rigidity in bar-joint networks to dynamical arrest in
glass-forming liquids. Unlike ordinary () and biconnected ()
percolation, the mean field -core percolation transition is both
continuous and discontinuous, i.e. there is a jump in the order parameter
accompanied with a diverging length scale. To determine whether or not this
hybrid transition survives in finite dimensions, we present a expansion
for -core percolation on the -dimensional hypercubic lattice. We show
that to order the singularity in the order parameter and in the
susceptibility occur at the same value of the occupation probability. This
result suggests that the unusual hybrid nature of the mean field -core
transition survives in high dimensions.Comment: 47 pages, 26 figures, revtex
-Scale Decoupling of the Mechanical Relaxation and Diverging Shear Wave Propagation Lengthscale in Triphenylphosphite
We have performed depolarized Impulsive Stimulated Scattering experiments to
observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from
10 - 500 MHz. These measurements, in tandem with previously performed
longitudinal and shear measurements, permit further analyses of the relaxation
dynamics of TPP within the framework of the mode coupling theory (MCT). Our
results provide evidence of coupling between the shear and
longitudinal degrees of freedom up to a decoupling temperature = 231 K. A
lower bound length scale of shear wave propagation in liquids verified the
exponent predicted by theory in the vicinity of the decoupling temperature
Resolving long-range spatial correlations in jammed colloidal systems using photon correlation imaging
We introduce a new dynamic light scattering method, termed photon correlation
imaging, which enables us to resolve the dynamics of soft matter in space and
time. We demonstrate photon correlation imaging by investigating the slow
dynamics of a quasi two-dimensional coarsening foam made of highly packed,
deformable bubbles and a rigid gel network formed by dilute, attractive
colloidal particles. We find the dynamics of both systems to be determined by
intermittent rearrangement events. For the foam, the rearrangements extend over
a few bubbles, but a small dynamical correlation is observed up to macroscopic
length scales. For the gel, dynamical correlations extend up to the system
size. These results indicate that dynamical correlations can be extremely
long-ranged in jammed systems and point to the key role of mechanical
properties in determining their nature.Comment: Published version (Phys. Rev. Lett. 102, 085702 (2009)) The Dynamical
Activity Mapsprovided as Supplementary Online Material are also available on
http://w3.lcvn.univ-montp2.fr/~lucacip/dam/movies.ht
Length scale dependence of dynamical heterogeneity in a colloidal fractal gel
We use time-resolved dynamic light scattering to investigate the slow
dynamics of a colloidal gel. The final decay of the average intensity
autocorrelation function is well described by , with and
decreasing from 1.5 to 1 with increasing . We show that the dynamics is not
due to a continuous ballistic process, as proposed in previous works, but
rather to rare, intermittent rearrangements. We quantify the dynamical
fluctuations resulting from intermittency by means of the variance
of the instantaneous autocorrelation function, the analogous of
the dynamical susceptibility studied in glass formers. The amplitude
of is found to grow linearly with . We propose a simple --yet
general-- model of intermittent dynamics that accounts for the dependence
of both the average correlation functions and .Comment: Revised and improved, to appear in Europhys. Let
Ergodicity and Slowing Down in Glass-Forming Systems with Soft Potentials: No Finite-Temperature Singularities
The aim of this paper is to discuss some basic notions regarding generic
glass forming systems composed of particles interacting via soft potentials.
Excluding explicitly hard-core interaction we discuss the so called `glass
transition' in which super-cooled amorphous state is formed, accompanied with a
spectacular slowing down of relaxation to equilibrium, when the temperature is
changed over a relatively small interval. Using the classical example of a
50-50 binary liquid of N particles with different interaction length-scales we
show that (i) the system remains ergodic at all temperatures. (ii) the number
of topologically distinct configurations can be computed, is temperature
independent, and is exponential in N. (iii) Any two configurations in phase
space can be connected using elementary moves whose number is polynomially
bounded in N, showing that the graph of configurations has the `small world'
property. (iv) The entropy of the system can be estimated at any temperature
(or energy), and there is no Kauzmann crisis at any positive temperature. (v)
The mechanism for the super-Arrhenius temperature dependence of the relaxation
time is explained, connecting it to an entropic squeeze at the glass
transition. (vi) There is no Vogel-Fulcher crisis at any finite temperature T>0Comment: 10 pages, 9 figures, submitted to PR
Aging in a Colloidal Glass in Creep Flow: Time-Stress Superposition
In this work, we study ageing behavior of aqueous laponite suspension, a
model soft glassy material, in creep. We observe that viscoelastic behavior is
time dependent and is strongly influenced by the deformation field; the effect
is known to arise due to ageing and rejuvenation. We show that irrespective of
strength of deformation field (shear stress) and age, when imposed time-scale
is normalized with dominating relaxation mode of the system, universal ageing
behavior is obtained demonstrating time-stress superposition; the phenomena
that may be generic in variety of soft materials.Comment: 10 pages, 4 figure
Non-equilibrium thermodynamics. IV: Generalization of Maxwell, Claussius-Clapeyron and Response Functions Relations, and the Prigogine-Defay Ratio for Systems in Internal Equilibrium
We follow the consequences of internal equilibrium in non-equilibrium systems
that has been introduced recently [Phys. Rev. E 81, 051130 (2010)] to obtain
the generalization of Maxwell's relation and the Clausius-Clapeyron relation
that are normally given for equilibrium systems. The use of Jacobians allow for
a more compact way to address the generalized Maxwell relations; the latter are
available for any number of internal variables. The Clausius-Clapeyron relation
in the subspace of observables show not only the non-equilibrium modification
but also the modification due to internal variables that play a dominant role
in glasses. Real systems do not directly turn into glasses (GL) that are frozen
structures from the supercooled liquid state L; there is an intermediate state
(gL) where the internal variables are not frozen. Thus, there is no single
glass transition. A system possess several kinds of glass transitions, some
conventional (L \rightarrow gL; gL\rightarrow GL) in which the state change
continuously and the transition mimics a continuous or second order transition,
and some apparent (L\rightarrow gL; L\rightarrow GL) in which the free energies
are discontinuous so that the transition appears as a zeroth order transition,
as discussed in the text. We evaluate the Prigogine-Defay ratio {\Pi} in the
subspace of the observables at these transitions. We find that it is normally
different from 1, except at the conventional transition L\rightarrow gL, where
{\Pi}=1 regardless of the number of internal variables.Comment: 42 pages, 3 figures, citations correcte
Solidity of viscous liquids. V. Long-wavelength dominance of the dynamics
This paper is the fifth in a series exploring the physical consequences of
the solidity of glass-forming liquids. Paper IV proposed a model where the
density field is described by a time-dependent Ginzburg-Landau equation of the
nonconserved type with rates in space of the form . The
model assumes that where is the average intermolecular
distance; this inequality expresses a long-wavelength dominance of the dynamics
which implies that the Hamiltonian (free energy) to a good approximation may be
taken to be ultralocal. In the present paper we argue that this is the simplest
model consistent with the following three experimental facts: 1) Viscous
liquids approaching the glass transition do not develop long-range order; 2)
The glass has lower compressibility than the liquid; 3) The alpha process
involves several decades of relaxation times shorter than the mean relaxation
time. The paper proceeds to list six further experimental facts characterizing
equilibrium viscous liquid dynamics and shows that these are readily understood
in terms of the model; some are direct consequences, others are quite natural
when viewed in light of the model
Confinement effects on glass forming liquids probed by DMA
Many molecular glass forming liquids show a shift of the glass transition T-g
to lower temperatures when the liquid is confined into mesoporous host
matrices. Two contrary explanations for this effect are given in literature:
First, confinement induced acceleration of the dynamics of the molecules leads
to an effective downshift of T-g increasing with decreasing pore size. Second,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts T-g to lower temperatures. Here we present dynamic mechanical
analysis measurements of the glass forming liquid salol in Vycor and Gelsil
with pore sizes of d=2.6, 5.0 and 7.5 nm. The dynamic complex elastic
susceptibility data can be consistently described with the assumption of two
relaxation processes inside the pores: A surface induced slowed down relaxation
due to interaction with rough pore interfaces and a second relaxation within
the core of the pores. This core relaxation time is reduced with decreasing
pore size d, leading to a downshift of T-g proportional to 1/d in perfect
agreement with recent differential scanning calorimetry (DSC) measurements.
Thermal expansion measurements of empty and salol filled mesoporous samples
revealed that the contribution of negative pressure to the downshift of T-g is
small (<30%) and the main effect is due to the suppression of dynamically
correlated regions of size xi when the pore size xi approaches
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