1,620 research outputs found
Electrostatic interaction of neutral semipermeable membranes
We consider an osmotic equilibrium between bulk solutions of polyelectrolyte
bounded by semipermeable membranes and separated by a thin film of salt-free
liquid. Although the membranes are neutral, the counter-ions of the
polyelectrolyte molecules permeate into the gap and lead to a steric charge
separation. This gives rise to a distance-dependent membrane potential, which
translates into a repulsive electrostatic disjoining pressure. From the
solution of the non-linear Poisson-Boltzmann equation we obtain the
distribution of the potential and of ions. We then derive an explicit formula
for the pressure exerted on the membranes and show that it deviates from the
classical van't Hoff expression for the osmotic pressure. This difference is
interpreted in terms of a repulsive electrostatic disjoining pressure
originating from the overlap of counterion clouds inside the gap. We also
develop a simplified theory based on a linearized Poisson-Boltzmann approach. A
comparison with simulation of a primitive model for the electrolyte is provided
and does confirm the validity of the theoretical predictions Beyond the
fundamental result that the neutral surfaces can repel, this mechanism not only
helps to control the adhesion and long-range interactions of living cells,
bacteria, and vesicles, but also allows us to argue that electrostatic
interactions should play enormous role in determining behavior and functions of
systems bounded by semipermeable membranes
Microscopic Derivation of Non-Markovian Thermalization of a Brownian Particle
In this paper, the first microscopic approach to the Brownian motion is
developed in the case where the mass density of the suspending bath is of the
same order of magnitude as that of the Brownian (B) particle. Starting from an
extended Boltzmann equation, which describes correctly the interaction with the
fluid, we derive systematicaly via the multiple time-scale analysis a reduced
equation controlling the thermalization of the B particle, i.e. the relaxation
towards the Maxwell distribution in velocity space. In contradistinction to the
Fokker-Planck equation, the derived new evolution equation is non-local both in
time and in velocity space, owing to correlated recollision events between the
fluid and particle B. In the long-time limit, it describes a non-markovian
generalized Ornstein-Uhlenbeck process. However, in spite of this complex
dynamical behaviour, the Stokes-Einstein law relating the friction and
diffusion coefficients is shown to remain valid. A microscopic expression for
the friction coefficient is derived, which acquires the form of the Stokes law
in the limit where the mean-free in the gas is small compared to the radius of
particle B.Comment: 28 pages, no figure, submitted to Journal of Statistical Physic
Thermally Activated Dynamics of the Capillary Condensation
This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. We present a simple estimation of the
nucleation barrier in slab geometry both in the two dimensional case and in the
three dimensional case. We extend the model in the case of rough surfaces which
is closer to the experimental case and allows comparison with experimental
datas.Comment: 6 pages, 3 figures, Submitted to J. Phys. : Condens. Matter,
Proceedings of the IV Liquid Matter Conference - Grenada(Spain) july 199
Stellar Mass to Halo Mass Scaling Relation for X-ray Selected Low Mass Galaxy Clusters and Groups out to Redshift
We present the stellar mass-halo mass scaling relation for 46 X-ray selected
low-mass clusters or groups detected in the XMM-BCS survey with masses
at
redshift . The cluster binding masses are inferred
from the measured X-ray luminosities \Lx, while the stellar masses
of the galaxy populations are estimated using near-infrared imaging from the
SSDF survey and optical imaging from the BCS survey. With the measured \Lx\ and
stellar mass , we determine the best fit stellar mass-halo mass
relation, accounting for selection effects, measurement uncertainties and the
intrinsic scatter in the scaling relation. The resulting mass trend is
, the intrinsic (log-normal) scatter is
, and there is no
significant redshift trend , although
the uncertainties are still large. We also examine within a fixed
projected radius of ~Mpc, showing that it provides a cluster binding mass
proxy with intrinsic scatter of (1 in ). We
compare our scaling relation from the XMM-BCS
clusters with samples of massive, SZE-selected clusters
() and low mass NIR-selected clusters
() at redshift .
After correcting for the known mass measurement systematics in the compared
samples, we find that the scaling relation is in good agreement with the high
redshift samples, suggesting that for both groups and clusters the stellar
content of the galaxy populations within depends strongly on mass but
only weakly on redshift out to .Comment: 15 pages, 10 figures. Accepted for publication in MNRA
Evolutions of Magnetized and Rotating Neutron Stars
We study the evolution of magnetized and rigidly rotating neutron stars
within a fully general relativistic implementation of ideal
magnetohydrodynamics with no assumed symmetries in three spatial dimensions.
The stars are modeled as rotating, magnetized polytropic stars and we examine
diverse scenarios to study their dynamics and stability properties. In
particular we concentrate on the stability of the stars and possible critical
behavior. In addition to their intrinsic physical significance, we use these
evolutions as further tests of our implementation which incorporates new
developments to handle magnetized systems.Comment: 12 pages, 8 figure
Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an Unconventional Orientation on SiC
We report the use of a surfactant molecule during the epitaxy of graphene on
SiC(0001) that leads to the growth in an unconventional orientation, namely
rotation with respect to the SiC lattice. It yields a very
high-quality single-layer graphene with a uniform orientation with respect to
the substrate, on the wafer scale. We find an increased quality and homogeneity
compared to the approach based on the use of a pre-oriented template to induce
the unconventional orientation. Using spot profile analysis low energy electron
diffraction, angle-resolved photoelectron spectroscopy, and the normal
incidence x-ray standing wave technique, we assess the crystalline quality and
coverage of the graphene layer. Combined with the presence of a
covalently-bound graphene layer in the conventional orientation underneath, our
surfactant-mediated growth offers an ideal platform to prepare epitaxial
twisted bilayer graphene via intercalation.Comment: 7 pages, 3 figure
Innermost stable circular orbits around magnetized rotating massive stars
In 1998, Shibata and Sasaki [Phys. Rev. D 58, 104011 (1998)] presented an
approximate analytical formula for the radius of the innermost stable circular
orbit (ISCO) of a neutral test particle around a massive, rotating and deformed
source. In the present paper, we generalize their expression by including the
magnetic dipole moment. We show that our approximate analytical formulas are
accurate enough by comparing them with the six-parametric exact solution
calculated by Pach\'on et. al. [Phys. Rev. D 73, 104038 (2006)] along with the
numerical data presented by Berti and Stergioulas [MNRAS 350, 1416 (2004)] for
realistic neutron stars. As a main result, we find that in general, the radius
at ISCO exhibits a decreasing behavior with increasing magnetic field. However,
for magnetic fields below 100GT the variation of the radius at ISCO is
negligible and hence the non-magnetized approximate expression can be used. In
addition, we derive approximate analytical formulas for angular velocity,
energy and angular momentum of the test particle at ISCO.Comment: 8 pages, 3 figure
Signal Transduction Pathways in the Pentameric Ligand-Gated Ion Channels
The mechanisms of allosteric action within pentameric ligand-gated ion channels (pLGICs) remain to be determined. Using crystallography, site-directed mutagenesis, and two-electrode voltage clamp measurements, we identified two functionally relevant sites in the extracellular (EC) domain of the bacterial pLGIC from Gloeobacter violaceus (GLIC). One site is at the C-loop region, where the NQN mutation (D91N, E177Q, and D178N) eliminated inter-subunit salt bridges in the open-channel GLIC structure and thereby shifted the channel activation to a higher agonist concentration. The other site is below the C-loop, where binding of the anesthetic ketamine inhibited GLIC currents in a concentration dependent manner. To understand how a perturbation signal in the EC domain, either resulting from the NQN mutation or ketamine binding, is transduced to the channel gate, we have used the Perturbation-based Markovian Transmission (PMT) model to determine dynamic responses of the GLIC channel and signaling pathways upon initial perturbations in the EC domain of GLIC. Despite the existence of many possible routes for the initial perturbation signal to reach the channel gate, the PMT model in combination with Yen's algorithm revealed that perturbation signals with the highest probability flow travel either via the β1-β2 loop or through pre-TM1. The β1-β2 loop occurs in either intra- or inter-subunit pathways, while pre-TM1 occurs exclusively in inter-subunit pathways. Residues involved in both types of pathways are well supported by previous experimental data on nAChR. The direct coupling between pre-TM1 and TM2 of the adjacent subunit adds new insight into the allosteric signaling mechanism in pLGICs. © 2013 Mowrey et al
Possibility to study eta-mesic nuclei and photoproduction of slow eta-mesons at the GRAAL facility
A new experiment is proposed with the aim to study eta-mesic nuclei and
low-energy interactions of eta with nuclei. Two decay modes of eta produced by
a photon beam inside a nucleus will be observed, namely a collisional decay
\eta N \to \pi N inside the nucleus and the radiative decay \eta \to \gamma
\gamma outside. In addition, a collisional decay of stopped S_{11}(1535)
resonance inside the nucleus, S_{11}(1535) N \to N N, will be studied. The
experiment can be performed using the tagged photon beam at ESRF with the
end-point energy 1000 MeV and the GRAAL detector which includes a
high-resolution BGO calorimeter and a large acceptance lead-scintillator
time-of-flight wall. Some results of simulation and estimates of yields are
given.Comment: 20 pages, 19 figure
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