4,265 research outputs found
Modelling colloids with Baxter's adhesive hard sphere model
The structure of the Baxter adhesive hard sphere fluid is examined using
computer simulation. The radial distribution function (which exhibits unusual
discontinuities due to the particle adhesion) and static structure factor are
calculated with high accuracy over a range of conditions and compared with the
predictions of Percus--Yevick theory. We comment on rigidity in percolating
clusters and discuss the role of the model in the context of experiments on
colloidal systems with short-range attractive forces.Comment: 14 pages, 7 figures. (For proceedings of "Structural arrest in
colloidal systems with short-range attractive forces", Messina, December
2003
Dynamic arrest of colloids in porous environments: disentangling crowding and confinement
Using numerical simulations we study the slow dynamics of a colloidal
hard-sphere fluid adsorbed in a matrix of disordered hard-sphere obstacles. We
calculate separately the contributions to the single-particle dynamic
correlation functions due to free and trapped particles. The separation is
based on a Delaunay tessellation to partition the space accessible to the
centres of fluid particles into percolating and disconnected voids. We find
that the trapping of particles into disconnected voids of the matrix is
responsible for the appearance of a nonzero long-time plateau in the
single-particle intermediate scattering functions of the full fluid. The
subdiffusive exponent , obtained from the logarithmic derivative of the
mean-squared displacement, is observed to be essentially unaffected by the
motion of trapped particles: close to the percolation transition, we determined
for both the full fluid and the particles moving in the
percolating void. Notably, the same value of is found in single-file
diffusion and is also predicted by mode-coupling theory along the
diffusion-localisation line. We also reveal subtle effects of dynamic
heterogeneity in both the free and the trapped component of the fluid
particles, and discuss microscopic mechanisms that contribute to this
phenomenon.Comment: 18 pages, 12 figures, minor change
Dynamical arrest: interplay of the glass and of the gel transitions
The structural arrest of a polymeric suspension might be driven by an
increase of the cross--linker concentration, that drives the gel transition, as
well as by an increase of the polymer density, that induces a glass transition.
These dynamical continuous (gel) and discontinuous (glass) transitions might
interfere, since the glass transition might occur within the gel phase, and the
gel transition might be induced in a polymer suspension with glassy features.
Here we study the interplay of these transitions by investigating via
event--driven molecular dynamics simulation the relaxation dynamics of a
polymeric suspension as a function of the cross--linker concentration and the
monomer volume fraction. We show that the slow dynamics within the gel phase is
characterized by a long sub-diffusive regime, which is due both to the crowding
as well as to the presence of a percolating cluster. In this regime, the
transition of structural arrest is found to occur either along the gel or along
the glass line, depending on the length scale at which the dynamics is probed.
Where the two line meet there is no apparent sign of higher order dynamical
singularity. Logarithmic behavior typical of singularity appear inside
the gel phase along the glass transition line. These findings seem to be
related to the results of the mode coupling theory for the schematic
model
Slicing the Ising model: critical equilibrium and coarsening dynamics
We study the evolution of spin clusters on two dimensional slices of the
Ising model in contact with a heat bath after a sudden quench to a subcritical
temperature. We analyze the evolution of some simple initial configurations,
such as a sphere and a torus, of one phase embedded into the other, to confirm
that their area disappears linearly in time and to establish the temperature
dependence of the prefactor in each case. Two generic kinds of initial states
are later used: equilibrium configurations either at infinite temperature or at
the paramagnetic-ferromagnetic phase transition. We investigate the
morphological domain structure of the coarsening configurations on slices
of the system, comparing with the behavior of the bidimensional model.Comment: 12 page
The restricted primitive model of ionic fluids with nonadditive diameters
The restricted primitive model with nonadditive hard-sphere diameters is
shown to have interesting and peculiar clustering properties. We report
accurate calculations of the cluster concentrations. Implementing efficient and
ad hoc Monte Carlo algorithms we determine the effect of nonadditivity on both
the clustering and the gas-liquid binodal. For negative nonadditivity, tending
to the extreme case of completely overlapping unlike ions, the prevailing
clusters are made of an even number of particles having zero total charge. For
positive nonadditivity, the frustrated tendency to segregation of like
particles and the reduced space available to the ions favors percolating
clusters at high densities.Comment: 6 pages, 3 figure
Classical Liquids in Fractal Dimension
We introduce fractal liquids by generalizing classical liquids of integer
dimensions to a fractal dimension . The particles composing
the liquid are fractal objects and their configuration space is also fractal,
with the same non-integer dimension. Realizations of our generic model system
include microphase separated binary liquids in porous media, and highly
branched liquid droplets confined to a fractal polymer backbone in a gel. Here
we study the thermodynamics and pair correlations of fractal liquids by
computer simulation and semi-analytical statistical mechanics. Our results are
based on a model where fractal hard spheres move on a near-critical percolating
lattice cluster. The predictions of the fractal Percus-Yevick liquid integral
equation compare well with our simulation results.Comment: Changed titl
A numerical study of one-patch colloidal particles: from square-well to Janus
We perform numerical simulations of a simple model of one-patch colloidal
particles to investigate: (i) the behavior of the gas-liquid phase diagram on
moving from a spherical attractive potential to a Janus potential and (ii) the
collective structure of a system of Janus particles. We show that, for the case
where one of the two hemispheres is attractive and one is repulsive, the system
organizes into a dispersion of orientational ordered micelles and vesicles and,
at low , the system can be approximated as a fluid of such clusters,
interacting essentially via excluded volume. The stability of this cluster
phase generates a very peculiar shape of the gas and liquid coexisting
densities, with a gas coexistence density which increases on cooling,
approaching the liquid coexistence density at very low .Comment: 9 pages, 10 figures, Phys. Chem. Chem. Phys. in press (2010
Influence of polydispersity on the critical parameters of an effective potential model for asymmetric hard sphere mixtures
We report a Monte Carlo simulation study of the properties of highly
asymmetric binary hard sphere mixtures. This system is treated within an
effective fluid approximation in which the large particles interact through a
depletion potential (R. Roth {\em et al}, Phys. Rev. E{\bf 62} 5360 (2000))
designed to capture the effects of a virtual sea of small particles. We
generalize this depletion potential to include the effects of explicit size
dispersity in the large particles and consider the case in which the particle
diameters are distributed according to a Schulz form having degree of
polydispersity 14%. The resulting alteration (with respect to the monodisperse
limit) of the metastable fluid-fluid critical point parameters is determined
for two values of the ratio of the diameters of the small and large particles:
and . We find that inclusion of
polydispersity moves the critical point to lower reservoir volume fractions of
the small particles and high volume fractions of the large ones. The estimated
critical point parameters are found to be in good agreement with those
predicted by a generalized corresponding states argument which provides a link
to the known critical adhesion parameter of the adhesive hard sphere model.
Finite-size scaling estimates of the cluster percolation line in the one phase
fluid region indicate that inclusion of polydispersity moves the critical point
deeper into the percolating regime. This suggests that phase separation is more
likely to be preempted by dynamical arrest in polydisperse systems.Comment: 11 pages, 10 figure
Space-resolved dynamics of a tracer in a disordered solid
The dynamics of a tracer particle in a glassy matrix of obstacles displays
slow complex transport as the free volume approaches a critical value and the
void space falls apart. We investigate the emerging subdiffusive motion of the
test particle by extensive molecular dynamics simulations and characterize the
spatio-temporal transport in terms of two-time correlation functions, including
the time-dependent diffusion coefficient as well as the wavenumber-dependent
intermediate scattering function. We rationalize our findings within the
framework of critical phenomena and compare our data to a dynamic scaling
theory.Comment: 10 pages, 7 figures, submitted to Journal of Non-Crystalline Solid
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