490 research outputs found
Tuning the liquid-liquid transition by modulating the hydrogen bond angular flexibility in a model for water
We propose a simple extension of the well known ST2 model for water [F.H.
Stillinger and A. Rahman, J. Chem. Phys. {\bf 60}, 1545 (1974)] that allows for
a continuous modification of the hydrogen bond angular flexibility. We show
that the bond flexibility affects the relative thermodynamic stability of the
liquid and of the hexagonal (or cubic) ice. On increasing flexibility, the
liquid-liquid critical point, which in the original ST2 model is located in the
no-man's land (i. e. the region where ice is the thermodynamically stable
phase) progressively moves to a temperature where the liquid is more stable
than ice. Our study definitively proves that the liquid-liquid transition in
ST2 is a genuine phenomenon, of high relevance in all tetrahedral
network-forming liquids, including water.Comment: Accepted in Phys. Rev. Let
Equilibrium gels of limited valence colloids
Gels are low-packing arrested states of matter which are able to support
stress. On cooling, limited valence colloidal particles form open networks
stabilized by the progressive increase of the interparticle bond lifetime.
These gels, named equilibrium gels, are the focus of this review article.
Differently from other types of colloidal gels, equilibrium gels do not require
an underlying phase separation to form. Oppositely, they form in a region of
densities deprived of thermodynamic instabilities. Limited valence equilibrium
gels neither coarsen nor age with time
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
On the possibility of extending the Nore-Frenkel generalized law of correspondent states to non-isotropic patchy interactions
Colloidal systems (and protein solutions) are often characterized by
attractive interactions whose range is much smaller than the particle size.
When this is the case and the interaction is spherical, systems obey a
generalized law of correspondent states (GLCS), first proposed by Noro and
Frenkel [ J.Chem.Phys. 113, 2941 (2000) ]. The thermodynamic properties become
insensitive to the details of the potential, depending only on the value of the
second virial coefficient B_2 and the density . The GLCS does not
generically hold for the case of non-spherical potentials. In this Letter we
suggest that when particles interact via short-ranged small-angular amplitude
patchy interactions (so that the condition of only one bond per patch is
fulfilled) it is still possible to generalize the GLCS close to the liquid-gas
critical point. Keywords: Colloids, Second Virial Coefficient, Proteins
interactions, Short-ranged attractive attractions.Comment: 11 pages, 3 figures. Accepted for publication on J. Phys. Chem.
Model for Assembly and Gelation of Four-Armed DNA Dendrimers
We introduce and numerically study a model designed to mimic the bulk
behavior of a system composed of single-stranded DNA dendrimers.
Complementarity of the base sequences of different strands results in the
formation of strong cooperative intermolecular links. We find that in an
extremely narrow temperature range the system forms a large-scale, low-density
disordered network via a thermo-reversible gel transition. By controlling the
strand length, the gel transition temperature can be made arbitrarily close to
the percolation transition, in contrast with recent model systems of physical
gelation. This study helps the understanding of self-assembly in this class of
new biomaterials and provides an excellent bridge between physical and chemical
gels
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
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
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