154 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
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
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
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
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
How soft repulsion enhances the depletion mechanism
We investigate binary mixtures of large colloids interacting through soft
potentials with small, ideal depletants. We show that softness has a dramatic
effect on the resulting colloid-colloid effective potential when the
depletant-to-colloid size ratio is small, with significant consequences on
the colloidal phase behaviour. We also provide an exact relation that allows us
to obtain the effective pair potential for {\it any} type of colloid-depletant
interactions in the case of ideal depletants, without having to rely on
complicated and expensive full-mixture simulations. We also show that soft
repulsion among depletants further enhances the tendency of colloids to
aggregate. Our theoretical and numerical results demonstrate that --- in the
limit of small --- soft mixtures cannot be mapped onto hard systems and
hence soft depletion is not a mere extension of the widely used Asakura-Oosawa
potential.Comment: Accepted for publication in Soft Matte
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