1,323 research outputs found
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
Mode-coupling theory predictions for a limited valency attractive square-well model
Recently we have studied, using numerical simulations, a limited valency
model, i.e. an attractive square well model with a constraint on the maximum
number of bonded neighbors. Studying a large region of temperatures and
packing fractions , we have estimated the location of the liquid-gas
phase separation spinodal and the loci of dynamic arrest, where the system is
trapped in a disordered non-ergodic state. Two distinct arrest lines for the
system are present in the system: a {\it (repulsive) glass} line at high
packing fraction, and a {\it gel} line at low and . The former is
essentially vertical (-controlled), while the latter is rather horizontal
(-controlled) in the plane. We here complement the molecular
dynamics results with mode coupling theory calculations, using the numerical
structure factors as input. We find that the theory predicts a repulsive glass
line -- in satisfactory agreement with the simulation results -- and an
attractive glass line which appears to be unrelated to the gel line.Comment: 12 pages, 6 figures. To appear in J. Phys. Condens. Matter, special
issue: "Topics in Application of Scattering Methods for Investigation of
Structure and Dynamics of Soft Condensed Matter", Fiesole, November 200
Colloidal systems with competing interactions: from an arrested repulsive cluster phase to a gel
We report an extensive numerical study of a charged colloidal system with
competing short-range depletion attraction and long-range electrostatic
repulsion. By analizing the cluster properties, we identify two distinct
regions in the phase diagram: a state composed of stable finite-size clusters,
whose relative interactions are dominated by long-range repulsion, and a
percolating network. Both states are found to dynamically arrest at low
temperatures, providing evidence of the existence of two distinct non-ergodic
states in these systems: a Wigner glass of clusters and a gel.Comment: 10 figures, to be published in Soft Matte
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
Internal structure and swelling behaviour of in silico microgel particles
Microgels are soft colloids that, in virtue of their polymeric nature, can
react to external stimuli such as temperature or pH by changing their size. The
resulting swelling/deswelling transition can be exploited in fundamental
research as well as for many diverse practical applications, ranging from art
restoration to medicine. Such an extraordinary versatility stems from the
complex internal structure of the individual microgels, each of which is a
crosslinked polymer network. Here we employ a recently-introduced computational
method to generate realistic microgel configurations and look at their
structural properties, both in real and Fourier space, for several temperatures
across the volume phase transition as a function of the crosslinker
concentration and of the confining radius employed during the `in-silico'
synthesis. We find that the chain-length distribution of the resulting networks
can be analytically predicted by a simple theoretical argument. In addition, we
find that our results are well-fitted to the fuzzy-sphere model, which
correctly reproduces the density profile of the microgels under study
Casimir-like forces at the percolation transition
Percolation and critical phenomena show common features such as scaling and
universality. Colloidal particles, immersed in a solvent close to criticality,
experience long-range effective forces, named critical Casimir forces. %These
originate from the confinement of the solvent critical fluctuations between the
colloids. Building on the analogy between critical phenomena and percolation,
we explore the possibility of observing long-range forces near a percolation
threshold. To this aim we numerically evaluate the effective potential between
two colloidal particles dispersed in a chemical sol and we show that it becomes
attractive and long-ranged on approaching the sol percolation transition. We
develop a theoretical description based on a polydisperse Asakura-Oosawa model
which captures the divergence of the interaction range, allowing us to
interpret such effect in terms of depletion interactions in a structured
solvent. Our results provide the geometric analogue of the critical Casimir
force, suggesting a novel way for tuning colloidal interactions by controlling
the clustering properties of the solvent.Comment: final version of the manuscrip
On the Molecular Origin of the Cooperative Coil-to-globule Transition of Poly(N-isopropylacrylamide) in Water
By means of atomistic molecular dynamics simulations we investigate the
behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures
below and above the lower critical solution temperature (LCST), including the
undercooled regime. The transition between water soluble and insoluble states
at the LCST is described as a cooperative process involving an intramolecular
coil-to-globule transition preceding the aggregation of chains and the polymer
precipitation. In this work we investigate the molecular origin of such
cooperativity and the evolution of the hydration pattern in the undercooled
polymer solution. The solution behaviour of an atactic 30-mer at high dilution
is studied in the temperature interval from 243 to 323 K with a favourable
comparison to available experimental data. In the PNIPAM water soluble states
we detect a correlation between polymer segmental dynamics and diffusion motion
of bound water, occurring with the same activation energy. Simulation results
show that below the coil-to-globule transition temperature PNIPAM is surrounded
by a network of hydrogen bonded water molecules and that the cooperativity
arises from the structuring of water clusters in proximity to hydrophobic
groups. Differently, the perturbation of the hydrogen bond pattern involving
water and amide groups occurs above the transition temperature. Altogether
these findings reveal that even above the LCST PNIPAM remains largely hydrated
and that the coil-to-globule transition is related with a significant
rearrangement of the solvent in proximity of the surface of the polymer. The
comparison between the hydrogen bonding of water in the surrounding of PNIPAM
isopropyl groups and in bulk displays a decreased structuring of solvent at the
hydrophobic polymer-water interface across the transition temperature, as
expected because of the topological extension along the chain of such
interface
On the effect of the thermostat in non-equilibrium molecular dynamics simulations
The numerical investigation of the statics and dynamics of systems in
nonequilibrium in general, and under shear flow in particular, has become more
and more common. However, not all the numerical methods developed to simulate
equilibrium systems can be successfully adapted to out-of-equilibrium cases.
This is especially true for thermostats. Indeed, even though thermostats
developed to work under equilibrium conditions sometimes display good agreement
with rheology experiments, their performance rapidly degrades beyond weak
dissipation and small shear rates. Here we focus on gauging the relative
performances of three thermostats, Langevin, dissipative particle dynamics, and
Bussi-Donadio-Parrinello under varying parameters and external conditions. We
compare their effectiveness by looking at different observables and clearly
demonstrate that choosing the right thermostat (and its parameters) requires a
careful evaluation of, at least, temperature, density and velocity profiles. We
also show that small modifications of the Langevin and DPD thermostats greatly
enhance their performance in a wide range of parameters.Comment: 13 pages, 9 figure
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