2,800 research outputs found
Phase behavior of polydisperse sticky hard spheres: analytical solutions and perturbation theory
We discuss phase coexistence of polydisperse colloidal suspensions in the
presence of adhesion forces. The combined effect of polydispersity and Baxter's
sticky-hard-sphere (SHS) potential, describing hard spheres interacting via
strong and very short-ranged attractive forces, give rise, within the
Percus-Yevick (PY) approximation, to a system of coupled quadratic equations
which, in general, cannot be solved either analytically or numerically. We
review and compare two recent alternative proposals, which we have attempted to
by-pass this difficulty. In the first one, truncating the density expansion of
the direct correlation functions, we have considered approximations simpler
than the PY one. These approximations can be systematically improved.
We have been able to provide a complete analytical description of polydisperse
SHS fluids by using the simplest two orders and , respectively.
Such a simplification comes at the price of a lower accuracy in the phase
diagram, but has the advantage of providing an analytical description of
various new phenomena associated with the onset of polydispersity in phase
equilibria (e.g. fractionation). The second approach is based on a perturbative
expansion of the polydisperse PY solution around its monodisperse counterpart.
This approach provides a sound approximation to the real phase behavior, at the
cost of considering only weak polydispersity. Although a final seattlement on
the soundness of the latter method would require numerical simulations for the
polydisperse Baxter model, we argue that this approach is expected to keep
correctly into account the effects of polydispersity, at least qualitatively.Comment: 12 pages, 4 figures, to appear in Molec. Phys. special issue Liblice
200
Phase diagram of Janus Particles
We deeply investigate a simple model representative of the recently
synthesized Janus particles, i.e. colloidal spherical particles whose surface
is divided into two areas of different chemical composition. When the two
surfaces are solvophilic and solvophobic, these particles constitute the
simplest example of surfactants. The phase diagram includes a colloidal-poor
(gas) colloidal-rich (liquid) de-mixing region, which is progressively
suppressed by the insurgence of micelles, providing the first model where
micellization and phase-separation are simultaneously observed. The coexistence
curve is found to be negatively sloped in the temperature-pressure plane,
suggesting that Janus particles can provide a colloidal system with anomalous
thermodynamic behavior.Comment: 5 pages, 5 figures, Phys. Rev. Lett. in pres
On the compressibility equation of state for multicomponent adhesive hard sphere fluids
The compressibility equation of state for a multicomponent fluid of particles
interacting via an infinitely narrow and deep potential, is considered within
the mean spherical approximation (MSA). It is shown that for a class of models
leading to a particular form of the Baxter functions containing
density-independent stickiness coefficient, the compressibility EOS does not
exist, unlike the one-component case. The reason for this is that a direct
integration of the compressibility at fixed composition, cannot be carried out
due to the lack of a reciprocity relation on the second order partial
derivatives of the pressure with respect to two different densities. This is,
in turn, related to the inadequacy of the MSA. A way out to this drawback is
presented in a particular example, leading to a consistent compressibility
pressure, and a possible generalization of this result is discussed.Comment: 13 pages, no figures, accepted for publication Molec. Physics (2002
Data Workflow in Large Scale Simulations of Blood Flow in Aneurysms
Aneurysms are responsibile for significant morbidity and mortality, and there is a need for an increased understanding of all the aspects of the natural history of these lesions. We are currently working to extend our analyses with the goal of creating models of aneurysmal progression that are able to predict rupture risk through the description of the evolving geometry, structure, properties, and loads.
Realization of patient specific models of the blood circulation necessitates a complex computationally and data intensive procedure that starts from the collection of medical images in a clinical setting and encompasses several stages of data processing on (and transfer to and from) specialized hardware, which include high-performance and visualization clusters as well as consumer workstations and local drives for final storage
Gravitational waves from hyperbolic encounters
The emission of gravitational waves from a system of massive objects
interacting on hyperbolic orbits is studied in the quadrupole approximation.
Analytic expressions are derived for the gravitational radiation luminosity,
the total energy output and the gravitational radiation amplitude. An
estimation of the expected number of events towards different targets (i.e.
globular clusters and the center of the Galaxy) is also given. In particular,
for a dense stellar cluster at the galactic center, a rate up to one event per
year is obtained.Comment: 6 pages, 2 figure
Homozygous mutation in the prokineticin-receptor2 gene (Val274Asp) presenting as reversible Kallmann syndrome and persistent oligozoospermia: case report.
Prokineticin 2 (Prok2) or prokineticin-receptor2 (Prok-R2) gene mutations are associated with Kallmann syndrome
(KS). We describe a new homozygous mutation of Prok-R2 gene in a man displaying KS with an apparent reversal of
hypogonadism. The proband, offspring of consanguineous parents, presented at age 19 years with absent puberty, no
sense of smell, low testosterone and gonadotrophin levels. Magnetic resonance imaging showed olfactory bulb absence.
The patient achieved virilization and spermatogenesis with gonadotrophin administration. Two years after discontinuing
hormonal therapy, he maintained moderate oligozoospermia and normal testosterone levels. Prok2 and Prok-
R2 gene sequence analyses were performed. The proband had a homozygous mutation in Prok-R2 exon 2 that harbours
the c.T820>A base substitution, causing the introduction of an aspartic acid in place of valine at position 274
(Val274Asp). His mother had the same mutation in heterozygous state. This report describes a novel homozygous
mutation of Prok-R2 gene in a man with variant KS, underlying the role of Prok-R2 gene in the olfactory and reproductive
system development in humans. Present findings indicate that markedly delayed activation of gonadotrophin
secretion may occur in some KS cases with definite gene defects, and that oligozoospermia might result from a variant
form of reversible hypogonadotrophic hypogonadism
From rods to helices: evidence of a screw-like nematic phase
Evidence of a special chiral nematic phase is provided using numerical
simulation and Onsager theory for systems of hard helical particles. This phase
appears at the high density end of the nematic phase, when helices are well
aligned, and is characterized by the C symmetry axes of the helices
spiraling around the nematic director with periodicity equal to the particle
pitch. This coupling between translational and rotational degrees of freedom
allows a more efficient packing and hence an increase of translational entropy.
Suitable order parameters and correlation functions are introduced to identify
this screw-like phase, whose main features are then studied as a function of
radius and pitch of the helical particles. Our study highlights the physical
mechanism underlying a similar ordering observed in colloidal helical flagella
[E. Barry et al. \textit{Phys. Rev. Lett.} \textbf{96}, 018305 (2006)] and
raises the question of whether it could be observed in other helical particle
systems, such as DNA, at sufficiently high densities.Comment: List of authors correcte
The penetrable square-well model: extensive versus non-extensive phase
The phase diagram of the penetrable square-well fluid is investigated through
Monte Carlo simulations of various nature. This model was proposed as the
simplest possibility of combining bounded repulsions at short scale and
short-range attractions. We prove that the model is thermodynamically stable
for sufficiently low values of the penetrability parameter, and in this case
the system behaves similarly to the square-well model. For larger penetration,
there exists an intermediate region where the system is metastable, with well
defined fluid-fluid and fluid-solid transitions, at finite size, but eventually
becomes unstable in the thermodynamic limit. We characterize the unstable
non-extensive phase appearing at high penetrability, where the system collapses
into an isolated blob of a few clusters of many ovelapping particles each.Comment: 18 pages, to appear in Molecular Physics, special issue dedicated to
Prof. Luciano Reatt
Osmotic pressure induced coupling between cooperativity and stability of a helix-coil transition
Most helix-coil transition theories can be characterized by a set of three
parameters: energetic, describing the (free) energy cost of forming a helical
state in one repeating unit; entropic, accounting for the decrease of entropy
due to the helical state formation; and geometric, indicating how many
repeating units are affected by the formation of one helical state. Depending
on their effect on the helix-coil transition, solvents or co-solutes can be
classified with respect to their action on these parameters. Solvent
interactions that alter the entropic cost of helix formation by their osmotic
action can affect both the stability (transition temperature) and the
cooperativity (transition interval) of the helix-coil transition. A consistent
inclusion of osmotic pressure effects in a description of helix-coil transition
for poly(L-glutamic acid) in solution with polyethylene glycol can offer an
explanation of the experimentally observed linear dependence of transition
temperature on osmotic pressure as well as the concurrent changes in the
cooperativity of the transition.Comment: 5 pages, 3 figures. To be submitted to Phys.Rev.Let
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