220 research outputs found
Fluctuations and Pattern Formation in Fluids with Competing Interactions
One of the most interesting phenomena in the soft-matter realm consists in
the spontaneous formation of super-molecular structures (microphases) in
condition of thermodynamic equilibrium. A simple mechanism responsible for this
self-organization or pattern formation is based on the competition between
attractive and repulsive forces with different length scales in the microscopic
potential, typically, a short-range attraction against a longer-range
repulsion.
We analyse this problem by simulations in 2D fluids. We find that, as the
temperature is lowered, liquid-vapor phase separation is inhibited by the
competition between attraction and repulsion, and replaced by a transition to
non-homogeneous phases. The structure of the fluid shows well defined
signatures of the presence of both intra- and inter-cluster correlations.
Even when the competition between attraction and repulsion is not so strong
as to cause microphase formation, it still induces large density fluctuations
in a wide region of the temperature-density plane. In this large-fluctuation
regime, pattern formation can be triggered by a weak external modulating field.Comment: To appear in the proceedings of the "International workshop on
collective phenomena in macroscopic systems", 2006 Villa Olmo (Como), Ital
Force calculation on walls and embedded particles in multiparticle collision dynamics simulations
Colloidal solutions posses a wide range of time and length scales, so that it
is unfeasible to keep track of all of them within a single simulation. As a
consequence some form of coarse-graining must be applied. In this work we use
the Multi-Particle Collision Dynamics scheme. We describe a particular
implementation of no-slip boundary conditions upon a solid surface, capable of
providing correct force s on the solid bypassing the calculation of the
velocity profile or the stre ss tensor in the fluid near the surface. As an
application we measure the friction on a spherical particle, when it is placed
in a bulk fluid and when it is confined in a slit. We show that the
implementation of the no-slip boundary conditions leads to an enhanced Ensko g
friction, which can be understood analytically. Because of the long-range
nature of hydrodynamic interactions, the Stokes friction obtained from the
simulations is sensitive of the simulation box size. We address this topic for
the slit geometry, showing that that the dependence on the system size differs
very much from what is expected in a 3D system, where periodic boundary
conditions are used in all directions.Comment: To appear in Physical Review
Two-dimensional fluid with competing interactions exhibiting microphase separation: theory for bulk and interfacial properties
Colloidal particles that are confined to an interface such as the air-water
interface are an example of a two-dimensional fluid. Such dispersions have been
observed to spontaneously form cluster and stripe morphologies in certain
systems with isotropic pair potentials between the particles, due to the fact
that the pair interaction between the colloids has competing attraction and
repulsion over different length scales. Here we present a simple density
functional theory for a model of such a two-dimensional fluid. The theory
predicts a bulk phase diagram exhibiting cluster, stripe and bubble modulated
phases, in addition to homogeneous fluid phases. Comparing with simulation
results for this model from the literature, we find that the theory is
qualitatively reliable. The model allows for a detailed investigation of the
structure of the fluid and we are able to obtain simple approximate expressions
for the static structure factor and for the length scale characterising the
modulations in the microphase separated phases. We also investigate the
behaviour of the system under confinement between two parallel hard walls. We
find that the confined fluid phase behaviour can be rather complex.Comment: 12 pages, 8 figures. Accepted for publication in Phys. Rev.
Relevance of the socio-cultural perspective in the discussion about critical thinking
This is the final version. Available on open access from the publisher via the DOI in this recordCritical thinking (CT) is considered a key skill for success in the 21st century. Worldwide educational policies advocate the promotion of CT, and scholars across different fields have been involved in a wide debate on its definition, without reaching an agreement. Currently, research has not adequately addressed CT assessment, nor the way in which it should be taught. In the present work, an overview of the topic is provided, as well as an evaluation of the practices, in order to provide researchers or practitioners (particularly those involved in primary school education) a reference for the development of further theories and methods about CT in edu-cation. CT is considered from the perspective of philosophy, cognitive psychology, and education sciences. In addition, we propose the inclusion of a fourth perspective, which could be referred as socio-cultural pedagogic perspective, due to its important implications in teaching and assessment practices
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
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