508 research outputs found
Extension of the Fluctuation-Dissipation theorem to the physical aging of a model glass-forming liquid
We present evidence in favor of the possibility of treating an
out-of-equilibrium supercooled simple liquid as a system in quasi-equilibrium.
Two different temperatures, one controlled by the external bath and one
internally selected by the system characterize the quasi-equilibrium state. The
value of the internal temperature is explicitly calculated within the inherent
structure thermodynamic formalism. We find that the internal temperature
controls the relation between the response to an external perturbation and the
long-time decay of fluctuations in the liquid.Comment: 5 pages, 3 figure
Role of the unstable directions in the equilibrium and aging dynamics of supercooled liquids
The connectivity of the potential energy landscape in supercooled atomic
liquids is investigated through the calculation of the instantaneous normal
modes spectrum and a detailed analysis of the unstable directions in
configuration space. We confirm the hypothesis that the mode-coupling critical
temperature is the at which the dynamics crosses over from free to
activated exploration of configuration space. We also report the observed
changes in the local connectivity of configuration space sampled during aging,
following a temperature jump from a liquid to a glassy state.Comment: 5 pages, 3 figures. Phys. Rev. Lett., in pres
Aging as dynamics in configuration space
The relaxation dynamics of many disordered systems, such as structural
glasses, proteins, granular materials or spin glasses, is not completely frozen
even at very low temperatures. This residual motion leads to a change of the
properties of the material, a process commonly called aging. Despite recent
advances in the theoretical description of such aging processes, the
microscopic mechanisms leading to the aging dynamics are still a matter of
dispute. In this Letter we investigate the aging dynamics of a simple glass
former by means of molecular dynamics computer simulation. Using the concept of
the inherent structure we give evidence that aging dynamics can be understood
as a decrease of the effective configurational temperature T of the system.
From our results we conclude that the equilibration process is faster when
the system is quenched to T_c, the critical T of mode-coupling theory, and that
thermodynamic concepts are useful to describe the out-of-equilibrium aging
process.Comment: Latex 4 figure
One-dimensional cluster growth and branching gels in colloidal systems with short-range depletion attraction and screened electrostatic repulsion
We report extensive numerical simulations of a simple model for charged
colloidal particles in suspension with small non-adsorbing polymers. The chosen
effective one-component interaction potential is composed of a short-range
attractive part complemented by a Yukawa repulsive tail. We focus on the case
where the screening length is comparable to the particle radius. Under these
conditions, at low temperature, particles locally cluster into quasi
one-dimensional aggregates which, via a branching mechanism, form a macroscopic
percolating gel structure. We discuss gel formation and contrast it with the
case of longer screening lengths, for which previous studies have shown that
arrest is driven by the approach to a Yukawa glass of spherical clusters. We
compare our results with recent experimental work on charged colloidal
suspensions [A. I. Campbell {\it et al.} cond-mat/0412108, Phys. Rev. Lett. in
press].Comment: 14 pages, 25 figure
Aging and Energy Landscapes: Application to Liquids and Glasses
The equation of state for a liquid in equilibrium, written in the potential
energy landscape formalism, is generalized to describe out-of-equilibrium
conditions. The hypothesis that during aging the system explores basins
associated to equilibrium configurations is the key ingredient in the
derivation. Theoretical predictions are successfully compared with data from
molecular dynamics simulations of different aging processes, such as
temperature and pressure jumps.Comment: RevTeX4, 4 pages, 5 eps figure
Evidence of a higher-order singularity in dense short-ranged attractive colloids
We study a model in which particles interact through a hard-core repulsion
complemented by a short-ranged attractive potential, of the kind found in
colloidal suspensions. Combining theoretical and numerical work we locate the
line of higher-order glass transition singularities and its end-point -- named
-- on the fluid-glass line. Close to the point, we detect
logarithmic decay of density correlations and sub linear power-law increase of
the mean square displacement, for time intervals up to four order of
magnitudes. We establish the presence of the singularity by studying how
the range of the potential affects the time-window where anomalous dynamics is
observed.Comment: 4 pages, 4 figures, REVTE
Slow dynamics in a primitive tetrahedral network model
We report extensive Monte Carlo and event-driven molecular dynamics
simulations of the fluid and liquid phase of a primitive model for silica
recently introduced by Ford, Auerbach and Monson [J. Chem. Phys. 17, 8415
(2004)]. We evaluate the iso-diffusivity lines in the temperature-density plane
to provide an indication of the shape of the glass transition line. Except for
large densities, arrest is driven by the onset of the tetrahedral bonding
pattern and the resulting dynamics is strong in the Angell's classification
scheme. We compare structural and dynamic properties with corresponding results
of two recently studied primitive models of network forming liquids -- a
primitive model for water and a angular-constraint free model of
four-coordinated particles -- to pin down the role of the geometric constraints
associated to the bonding. Eventually we discuss the similarities between
"glass" formation in network forming liquids and "gel" formation in colloidal
dispersions of patchy particles.Comment: 9 pages, 10 figure
Effective non-additive pair potential for lock-and-key interacting particles: the role of the limited valence
Theoretical studies of self-assembly processes and condensed phases in
colloidal systems are often based on effective inter-particle potentials. Here
we show that developing an effective potential for particles interacting with a
limited number of ``lock-and-key'' selective bonds (due to the specificity of
bio-molecular interactions) requires -- beside the non-sphericity of the
potential -- a (many body) constraint that prevent multiple bonding on the same
site. We show the importance of retaining both valence and bond-selectivity by
developing, as a case study, a simple effective potential describing the
interaction between colloidal particles coated by four single-strand DNA
chains.Comment: 4 pages, 5 figure
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