320 research outputs found
Solvent mediated forces in critical fluids
The effective interaction between two planar walls immersed in a fluid is
investigated by use of Density Functional Theory in the super-critical region
of the phase diagram. A hard core Yukawa model of fluid is studied with special
attention to the critical region. To achieve this goal a new formulation of the
Weighted Density Approximation coupled with the Hierarchical Reference Theory,
able to deal with critical long wavelength fluctuations, is put forward and
compared with other approaches. The effective interaction between the walls is
seen to change character on lowering the temperature: The strong oscillations
induced by layering of the molecules, typical of the depletion mechanism in
hard core systems, are gradually smoothed and, close to the critical point, a
long range attractive tail emerges leading to a scaling form which agrees with
the expectations based on the critical Casimir effect. Strong corrections to
scaling are seen to affect the results up to very small reduced temperatures.
By use of Derjaguin approximation, this investigation has natural implications
for the aggregation of colloidal particles in critical solvents.Comment: 15 pages, 16 figure
Depletion interaction between spheres of unequal size and demixing in binary mixtures of colloids
The possibility to induce demixing in a colloidal mixture by adding small
polymers, or other equivalent depletant agents, is theoretically investigated.
By use of Mean Field Theory, suitably generalized to deal with short range
effective interactions, the phase diagram of a binary mixture ofcolloidal
particles (modelled as hard spheres) in a solvent is determined as a function
of the polymer concentration on the basis of the Asakura-Oosawa model.The
topology of the phase diagram changes when the relative size of the colloidal
particles is reduced: the critical line connecting the liquid-vapour critical
points of the two pure fluids breaks and the branch starting from the critical
point of the bigger particles bends to higher volume fractions, where
concentration fluctuations drive the transition. The effects of a softer
colloid-polymer interaction is also investigated: Even the presence of a small
repulsive tail in the potential gives rise to a significant lowering of the
stability threshold. In this case, phase transitions may take place by adding
just a few percent of depletant in volume fraction. These results may be
relevant for the interpretation of recent experiments of solidification
kinetics in colloidal mixtures.Comment: To be published in Molecular Physic
An investigation of the SCOZA for narrow square-well potentials and in the sticky limit
We present a study of the self consistent Ornstein-Zernike approximation
(SCOZA) for square-well (SW) potentials of narrow width delta. The main purpose
of this investigation is to elucidate whether in the limit delta --> 0, the
SCOZA predicts a finite value for the second virial coefficient at the critical
temperature B2(Tc), and whether this theory can lead to an improvement of the
approximate Percus-Yevick solution of the sticky hard-sphere (SHS) model due to
Baxter [R. J. Baxter, J. Chem. Phys. 49, 2770 (1968)]. For SW of non vanishing
delta, the difficulties due to the influence of the boundary condition at high
density already encountered in an earlier investigation [E. Schoell-Paschinger,
A. L. Benavides, and R. Castaneda-Priego, J. Chem. Phys. 123, 234513 (2005)]
prevented us from obtaining reliable results for delta < 0.1. In the sticky
limit this difficulty can be circumvented, but then the SCOZA fails to predict
a liquid-vapor transition. The picture that emerges from this study is that for
delta --> 0, the SCOZA does not fulfill the expected prediction of a constant
B2(Tc) [M. G. Noro and D. Frenkel, J. Chem. Phys. 113, 2941 (2000)], and that
for thermodynamic consistency to be usefully exploited in this regime, one
should probably go beyond the Ornstein-Zernike ansatz.Comment: 40 pages, 13 figures. Previous Sec. 2 on the Yukawa potential has
been removed. Only the square-well potential is considered in this versio
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
Competitive density waves in quasi-one-dimensional electron systems
We investigate the nature of the ground state of the one-dimensional t-J
model coupled to adiabatic phonons by use of the Lanczos technique at quarter
filling. Due to the interplay between electron-electron and electron-phonon
interactions, the model undergoes instabilities toward the formation of lattice
and charge modulations. Moderate on-site and intra-site electron-phonon
couplings lead to a competition of different spin-Peierls and dimerized states.
In the former case two electrons belong to the unit cell and we expect a
paramagnetic band insulator state, while lattice dimerization leads to a Mott
insulating state with quasi long range antiferromagnetic order. The zero
temperature phase diagram is obtained as a function of intra-site and
inter-site electron-phonon couplings, analytically in the limit and
numerically at finite J/t.Comment: 7 pages, 7 figures, to be published in Phys. Rev.
Educación en medios en acción : modelos, herramientas y ejemplos de buenas prácticas
Actualmente la educación en medios (EM) ya no puede ser considerada como un campo de estudio reservado a investigadores de la comunicación, ni como una práctica privilegiada de algunos profesores. Por un lado, la EM ahora es parte de la agenda de las organizaciones internacionales, que consideran el desarrollo de las competencias mediáticas como requisito necesario para ejercer plenamente la ciudadanÃa en la sociedad actual. Por otro lado, las prácticas de EM están cada vez más generalizadas en las escuelas e involucran a un número creciente de profesores. No obstante, la enseñanza de los medios de comunicación aún parece ser una tarea bastante solipsista, donde "todo está bien". De hecho, hay una tremenda falta de investigación sobre la calidad y la eficacia de las prácticas educativas. En su dimensión pedagógica, el proyecto On Air intenta hacer frente a estos problemas proporcionando un conjunto de herramientas para diseñar, desarrollar y evaluar las actividades de EM en las escuelas. El presente trabajo presenta y analiza sus resultados poniendo atención en la estructura general de la investigación y sus conclusiones principales.Currently, Media Education (ME) can no longer be considered as a field of study reserved for communication researchers, or as a privileged practice of some teachers. On the one hand, ME is now part of the agenda of international organizations, which considers the development of media skills as a prerequisite to exercise full citizenship in today's society. On the other hand, ME practices are becoming more widespread in schools and involve an increasing number of teachers. However, the teaching of the media still seems to be a fairly solipsistic, where "everything is fine". In fact, there is a tremendous lack of research on the quality and effectiveness of educational practices. In its educational dimension, the On Air project attempts to address these problems by providing a set of tools to design, develop and evaluate ME activities in schools. This paper presents and analyzes the results paying attention to the overall structure of the research and its main findings
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