1,068 research outputs found
Universal reduction of pressure between charged surfaces by long-wavelength surface charge modulation
We predict theoretically that long-wavelength surface charge modulations
universally reduce the pressure between the charged surfaces with counterions
compared with the case of uniformly charged surfaces with the same average
surface charge density. The physical origin of this effect is the fact that
surface charge modulations always lead to enhanced counterion localization near
the surfaces, and hence, fewer charges at the midplane. We confirm the last
prediction with Monte Carlo simulations.Comment: 8 pages 1 figure, Europhys. Lett., in pres
Swelling of particle-encapsulating random manifolds
We study the statistical mechanics of a closed random manifold of fixed area
and fluctuating volume, encapsulating a fixed number of noninteracting
particles. Scaling analysis yields a unified description of such swollen
manifolds, according to which the mean volume gradually increases with particle
number, following a single scaling law. This is markedly different from the
swelling under fixed pressure difference, where certain models exhibit
criticality. We thereby indicate when the swelling due to encapsulated
particles is thermodynamically inequivalent to that caused by fixed pressure.
The general predictions are supported by Monte Carlo simulations of two
particle-encapsulating model systems -- a two-dimensional self-avoiding ring
and a three-dimensional self-avoiding fluid vesicle. In the former the
particle-induced swelling is thermodynamically equivalent to the
pressure-induced one whereas in the latter it is not.Comment: 8 pages, 6 figure
The lamellar-to-isotropic transition in ternary amphiphilic systems
We study the dependence of the phase behavior of ternary amphiphilic systems
on composition and temperature. Our analysis is based on a curvature elastic
model of the surfactant film with sufficiently large spontaneous curvature and
sufficiently negative saddle-splay modulus that the stable phases are the
lamellar phase and a droplet microemulsion. In addition to the curvature
energy, we consider the contributions to the free energy of the long-ranged van
der Waals interaction and of the undulation modes. We find that for bending
rigidities of order k_B T, the lamellar phase extends further and further into
the water apex of the phase diagram as the phase inversion temperature is
approached, in good agreement with experimental results.Comment: LaTeX2e, 11 pages with references and 2 eps figures included,
submitted to Europhys. Let
Title Stabilization of Membrane Pores by Packing
We present a model for pore stabilization in membranes without surface
tension. Whereas an isolated pore is always unstable (since it either shrinks
tending to re-seal or grows without bound til to membrane disintegration), it
is shown that excluded volume interactions in a system of many pores can
stabilize individual pores of a given size in a certain range of model
parameters. For such a multipore membrane system, the distribution of pore size
and associated pore lifetime are calculated within the mean field
approximation. We predict that, above certain temperature when the effective
line tension becomes negative, the membrane exhibits a dynamic sieve-like
porous structure.Comment: 4 pages, 4 figure
Monovalent counterion distributions at highly charged water interfaces: Proton-transfer and Poisson-Boltzmann theory
Surface sensitive synchrotron-X-ray scattering studies reveal the
distributions of monovalent ions next to highly charged interfaces. A lipid
phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the
air-water interface, containing CsI at various concentrations. Using anomalous
reflectivity off and at the Cs resonance, we provide, for the first
time, spatial counterion distributions (Cs) next to the negatively charged
interface over a wide range of ionic concentrations. We argue that at low salt
concentrations and for pure water the enhanced concentration of hydroniums
HO at the interface leads to proton-transfer back to the phosphate
group by a high contact-potential, whereas high salt concentrations lower the
contact-potential resulting in proton-release and increased surface
charge-density. The experimental ionic distributions are in excellent agreement
with a renormalized-surface-charge Poisson-Boltzmann theory without fitting
parameters or additional assumptions
WhichGenes: a web-based tool for gathering, building, storing and exporting gene sets with application in gene set enrichment analysis
WhichGenes is a web-based interactive gene set building tool offering a very simple interface to extract always-updated gene lists from multiple databases and unstructured biological data sources. While the user can specify new gene sets of interest by following a simple four-step wizard, the tool is able to run several queries in parallel. Every time a new set is generated, it is automatically added to the private gene-set cart and the user is notified by an e-mail containing a direct link to the new set stored in the server. WhichGenes provides functionalities to edit, delete and rename existing sets as well as the capability of generating new ones by combining previous existing sets (intersection, union and difference operators). The user can export his sets configuring the output format and selecting among multiple gene identifiers. In addition to the user-friendly environment, WhichGenes allows programmers to access its functionalities in a programmatic way through a Representational State Transfer web service. WhichGenes front-end is freely available at http://www.whichgenes.org/, WhichGenes API is accessible at http://www.whichgenes.org/api/
Thermal fluctuations of an interface near a contact line
The effect of thermal fluctuations near a contact line of a liquid interface
partially wetting an impenetrable substrate is studied analytically and
numerically. Promoting both the interface profile and the contact line position
to random variables, we explore the equilibrium properties of the corresponding
fluctuating contact line problem based on an interfacial Hamiltonian involving
a "contact" binding potential. To facilitate an analytical treatment we
consider the case of a one-dimensional interface. The effective boundary
condition at the contact line is determined by a dimensionless parameter that
encodes the relative importance of thermal energy and substrate energy at the
microscopic scale. We find that this parameter controls the transition from a
partially wetting to a pseudo-partial wetting state, the latter being
characterized by a thin prewetting film of fixed thickness. In the partial
wetting regime, instead, the profile typically approaches the substrate via an
exponentially thinning prewetting film. We show that, independently of the
physics at the microscopic scale, Young's angle is recovered sufficiently far
from the substrate. The fluctuations of the interface and of the contact line
give rise to an effective disjoining pressure, exponentially decreasing with
height. Fluctuations therefore provide a regularization of the singular contact
forces occurring in the corresponding deterministic problem.Comment: 40 Pages, 12 Figure
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
Excess length of stay due to methicillin-resistant Staphylococcus aureus (MRSA) infection at a large Swiss hospital estimated by multi-state modelling
Theory of continuum percolation II. Mean field theory
I use a previously introduced mapping between the continuum percolation model
and the Potts fluid to derive a mean field theory of continuum percolation
systems. This is done by introducing a new variational principle, the basis of
which has to be taken, for now, as heuristic. The critical exponents obtained
are , and , which are identical with the mean
field exponents of lattice percolation. The critical density in this
approximation is \rho_c = 1/\ve where \ve = \int d \x \, p(\x) \{ \exp [-
v(\x)/kT] - 1 \}. p(\x) is the binding probability of two particles
separated by \x and v(\x) is their interaction potential.Comment: 25 pages, Late
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