47 research outputs found
Liquid-vapor coexistence in square-well fluids: an RHNC study
We investigate the ability of the reference hypernetted-chain integral
equation to describe the phase diagram of square-well fluids with four
different ranges of attraction. Comparison of our results with simulation data
shows that the theory is able to reproduce with fairly good accuracy a
significant part of the coexistence curve, provided an extrapolation procedure
is used to circumvent the well-known pathologies of the pseudo-spinodal line,
which are more severe at reduced width of the attractive well. The method
provides a useful approach for a quick assessment of the location of the
liquid-vapor coexistence curve in this kind of fluid and serves as a check for
the more complex problem of anisotropic "patchy" square-well molecules
Phase diagram and structural properties of a simple model for one-patch particles
We study the thermodynamic and structural properties of a simple, one-patch
fluid model using the reference hypernetted-chain (RHNC) integral equation and
specialized Monte Carlo simulations. In this model, the interacting particles
are hard spheres, each of which carries a single identical,
arbitrarily-oriented, attractive circular patch on its surface; two spheres
attract via a simple square-well potential only if the two patches on the
spheres face each other within a specific angular range dictated by the size of
the patch. For a ratio of attractive to repulsive surface of 0.8, we construct
the RHNC fluid-fluid separation curve and compare with that obtained by Gibbs
ensemble and grand canonical Monte Carlo simulations. We find that RHNC
provides a quick and highly reliable estimate for the position of the
fluid-fluid critical line. In addition, it gives a detailed (though
approximate) description of all structural properties and their dependence on
patch size.Comment: 27 pages, 10 figures, J. Chem. Phys. in pres
Effects of patch size and number within a simple model of patchy colloids
We report on a computer simulation and integral equation study of a simple model of patchy spheres, each of whose surfaces is decorated with two opposite attractive caps, as a function of the fraction of covered attractive surface. The simple model explored?the two-patch Kern?Frenkel model?interpolates between a square-well and a hard-sphere potential on changing the coverage . We show that integral equation theory provides quantitative predictions in the entire explored region of temperatures and densities from the square-well limit =1.0 down to 0.6. For smaller , good numerical convergence of the equations is achieved only at temperatures larger than the gas-liquid critical point, where integral equation theory provides a complete description of the angular dependence. These results are contrasted with those for the one-patch case. We investigate the remaining region of coverage via numerical simulation and show how the gas-liquid critical point moves to smaller densities and temperatures on decreasing . Below 0.3, crystallization prevents the possibility of observing the evolution of the line of critical points, providing the angular analog of the disappearance of the liquid as an equilibrium phase on decreasing the range for spherical potentials. Finally, we show that the stable ordered phase evolves on decreasing from a three-dimensional crystal of interconnected planes to a two-dimensional independent-planes structure to a one-dimensional fluid of chains when the one-bond-per-patch limit is eventually reache
The Insula and Its Epilepsies
Insular seizures are great mimickers of seizures originating elsewhere in the brain. The insula is a highly connected brain structure. Seizures may only become clinically evident after ictal activity propagates out of the insula with semiology that reflects the propagation pattern. Insular seizures with perisylvian spread, for example, manifest first as throat constriction, followed next by perioral and hemisensory symptoms, and then by unilateral motor symptoms. On the other hand, insular seizures may spread instead to the temporal and frontal lobes and present like seizures originating from these regions. Due to the location of the insula deep in the brain, interictal and ictal scalp electroencephalogram (EEG) changes can be variable and misleading. Magnetic reso- nance imaging, magnetic resonance spectroscopy, magnetoencephalography, positron emission tomography, and single-photon computed tomography imaging may assist in establishing a diagnosis of insular epilepsy. Intracranial EEG recordings from within the insula, using stereo-EEG or depth electrode techniques, can prove insular seizure origin. Seizure onset, most commonly seen as low-voltage, fast gamma activity, however, can be highly localized and easily missed if the insula is only sparsely sampled. Moreover, seizure spread to the contralateral insula and other brain regions may occur rapidly. Extensive sampling of the insula with multiple electrode trajectories is necessary to avoid these pitfalls. Understanding the functional organization of the insula is helpful when interpreting the semiology produced by insular seizures. Electrical stimulation mapping around the central sulcus of the insula results in paresthesias, while stimulation of the posterior insula typically produces painful sensations. Visceral sensations are the next most common result of insular stimulation. Treatment of insular epilepsy is evolving, but poses challenges. Surgical resections of the insula are effective but risk significant morbidity if not carefully planned. Neurostimulation is an emerging option for treatment, especially for seizures with onset in the posterior insula. The close association of the insula with marked autonomic changes has led to interest in the role of the insula in sudden unexpected death in epilepsy and warrants additional study with larger patient cohorts
Effects of patch size and number within a simple model of patchy colloids
We report on a computer simulation and integral equation study of a simple
model of patchy spheres, each of whose surfaces is decorated with two opposite
attractive caps, as a function of the fraction of covered attractive
surface. The simple model explored --- the two-patch Kern-Frenkel model ---
interpolates between a square-well and a hard-sphere potential on changing the
coverage . We show that integral equation theory provides quantitative
predictions in the entire explored region of temperatures and densities from
the square-well limit down to . For smaller
, good numerical convergence of the equations is achieved only at
temperatures larger than the gas-liquid critical point, where however integral
equation theory provides a complete description of the angular dependence.
These results are contrasted with those for the one-patch case. We investigate
the remaining region of coverage via numerical simulation and show how the
gas-liquid critical point moves to smaller densities and temperatures on
decreasing . Below , crystallization prevents the
possibility of observing the evolution of the line of critical points,
providing the angular analog of the disappearance of the liquid as an
equilibrium phase on decreasing the range for spherical potentials. Finally, we
show that the stable ordered phase evolves on decreasing from a
three-dimensional crystal of interconnected planes to a two-dimensional
independent-planes structure to a one-dimensional fluid of chains when the
one-bond-per-patch limit is eventually reached.Comment: 26 pages, 11 figures, J. Chem. Phys. in pres
Percolation of clusters with a residence time in the bond definition: Integral equation theory
We consider the clustering and percolation of continuum systems whose particles interact via the Lennard-Jones pair potential. A cluster definition is used according to which two particles are considered directly connected (bonded) at time t if they remain within a distance d, the connectivity distance, during at least a time of duration tau, the residence time. An integral equation for the corresponding pair connectedness function, recently proposed by two of the authors [Phys. Rev. E 61, R6067 (2000)], is solved using the orthogonal polynomial approach developed by another of the authors [Phys. Rev. E 55, 426 (1997)]. We compare our results with those obtained by molecular dynamics simulations.Instituto de Física de Líquidos y Sistemas BiológicosGrupo de Aplicaciones Matemáticas y Estadísticas de la Facultad de Ingenierí
Corticocortical evoked potentials reveal projectors and integrators in human brain networks.
The cerebral cortex is composed of subregions whose
functional specialization is largely determined by their
incoming and outgoing connections with each other. In the
present study, we asked which cortical regions can exert the
greatest influence over other regions and the cortical
network as a whole. Previous research on this question has
relied on coarse anatomy (mapping large fiber pathways) or
functional connectivity (mapping inter-regional statistical
dependencies in ongoing activity). Here we combined direct
electrical stimulation with recordings from the cortical
surface to provide a novel insight into directed, inter-
regional influence within the cerebral cortex of awake
humans. These networks of directed interaction were
reproducible across strength thresholds and across subjects.
Directed network properties included (1) a decrease in the
reciprocity of connections with distance; (2) major projector
nodes (sources of influence) were found in peri-Rolandic
cortex and posterior, basal and polar regions of the temporal
lobe; and (3) major receiver nodes (receivers of influence)
were found in anterolateral frontal, superior parietal, and
superior temporal regions. Connectivity maps derived from
electrical stimulation and from resting electrocorticography
(ECoG) correlations showed similar spatial distributions for
the same source node. However, higher-level network topology
analysis revealed differences between electrical stimulation
and ECoG that were partially related to the reciprocity of
connections. Together, these findings inform our
understanding of large-scale corticocortical influence as
well as the interpretation of functional connectivity
networks
The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III
The third generation of the Sloan Digital Sky Survey (SDSS-III) took data from 2008 to 2014 using the original SDSS wide-field imager, the original and an upgraded multi-object fiber-fed optical spectrograph, a new near-infrared high-resolution spectrograph, and a novel optical interferometer. All of the data from SDSS-III are now made public. In particular, this paper describes Data Release 11 (DR11) including all data acquired through 2013 July, and Data Release 12 (DR12) adding data acquired through 2014 July (including all data included in previous data releases), marking the end of SDSS-III observing. Relative to our previous public release (DR10), DR12 adds one million new spectra of galaxies and quasars from the Baryon Oscillation Spectroscopic Survey (BOSS) over an additional 3000 deg2 of sky, more than triples the number of H-band spectra of stars as part of the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE), and includes repeated accurate radial velocity measurements of 5500 stars from the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS). The APOGEE outputs now include the measured abundances of 15 different elements for each star. In total, SDSS-III added 5200 deg2 of ugriz imaging; 155,520 spectra of 138,099 stars as part of the Sloan Exploration of Galactic Understanding and Evolution 2 (SEGUE-2) survey; 2,497,484 BOSS spectra of 1,372,737 galaxies, 294,512 quasars, and 247,216 stars over 9376 deg2; 618,080 APOGEE spectra of 156,593 stars; and 197,040 MARVELS spectra of 5513 stars. Since its first light in 1998, SDSS has imaged over 1/3 of the Celestial sphere in five bands and obtained over five million astronomical spectra. \ua9 2015. The American Astronomical Society
Simulation study of the phase behavior of a planar Maier-Saupe nematogenic liquid
Using extensive Monte Carlo simulations and a simple approximation in density functional theory, we study the phase behavior of a fluid of nematogenic molecules with centers of mass constrained to lie in a plane but with axes free to rotate in any direction, both with and without an external disorienting field perpendicular to the plane. We find that simulation predicts the existence of an order-disorder phase transition belonging to the Berezinskii-Kosterlitz-Thouless type, along with a low temperature gas-liquid transition. In contrast to the simulation results, density functional theory predicts a first-order orientational phase transition coupled continuously with a first-order gas-liquid transition. The approximate theoretical approach qualitatively reproduces the field dependence of the order-disorder and gas-liquid transitions but is far from quantitative.
© 2005 The American Physical Societythe Dirección General de Investigación
Científica is acknowledged by E.L, C.M., and
N.G.A. under Grant No. FIS2004-02954-C03-01.Peer reviewe
Phase behavior of a hard sphere Maier-Saupe nematogenic system in three dimensions
We present a detailed computer simulation and integral equation study of the phase behavior of a nematogenic system composed of hard spheres with embedded three-dimensional Maier-Saupe spins. For this well-known system, we map the gas-liquid equilibrium, which is coupled to a first-order isotropic-nematic transition. The anisotropic integral equation theory is found to yield excellent agreement with the simulation data within the fluid regime. Additionally, we determine the fluid-solid equilibrium transition by means of computer simulation.
© 2006 The American Physical SocietyDirección General de Investigación Científica
y Técnica under Grant No. FIS2004-02954-C03-01 and the
Dirección General de Universidades e Investigación of the
Comunidad de Madrid under Grant No. S0505/ESP/0299-1.Peer reviewe