282 research outputs found
Stripes of Partially Fluorinated Alkyl Chains: Dipolar Langmuir Monolayers
Stripe-like domains of Langmuir monolayers formed by surfactants with
partially fluorinated lipid anchors (F-alkyl lipids) are observed at the
gas-liquid phase coexistence. The average periodicity of the stripes, measured
by fluorescence microscopy, is in the micrometer range, varying between 2 and 8
microns. The observed stripe-like patterns are stabilized due to dipole-dipole
interactions between terminal -CF3 groups. These interactions are particularly
strong as compared with non-fluorinated lipids due to the low dielectric
constant of the surrounding media (air). These long-range dipolar interactions
tend to elongate the domains, in contrast to the line tension that tends to
minimize the length of the domain boundary. This behavior should be compared
with that of the lipid monolayer having alkyl chains, and which form spherical
micro-domains (bubbles) at the gas-liquid coexistence. The measured stripe
periodicity agrees quantitatively with a theoretical model. Moreover, the
reduction in line tension by adding traces (0.1 mol fraction) of cholesterol
results, as expected, in a decrease in the domain periodicity.Comment: 20 pages, 4 fig
Adsorption of polymers on a fluctuating surface
We study the adsorption of polymer chains on a fluctuating surface. Physical
examples are provided by polymer adsorption at the rough interface between two
non-miscible liquids, or on a membrane. In a mean-field approach, we find that
the self--avoiding chains undergo an adsorption transition, accompanied by a
stiffening of the fluctuating surface. In particular, adsorption of polymers on
a membrane induces a surface tension and leads to a strong suppression of
roughness.Comment: REVTEX, 9 pages, no figure
Electrostatic Interactions of Asymmetrically Charged Membranes
We predict the nature (attractive or repulsive) and range (exponentially
screened or long-range power law) of the electrostatic interactions of
oppositely charged and planar plates as a function of the salt concentration
and surface charge densities (whose absolute magnitudes are not necessarily
equal). An analytical expression for the crossover between attractive and
repulsive pressure is obtained as a function of the salt concentration. This
condition reduces to the high-salt limit of Parsegian and Gingell where the
interaction is exponentially screened and to the zero salt limit of Lau and
Pincus in which the important length scales are the inter-plate separation and
the Gouy-Chapman length. In the regime of low salt and high surface charges we
predict - for any ratio of the charges on the surfaces - that the attractive
pressure is long-ranged as a function of the spacing. The attractive pressure
is related to the decrease in counter-ion concentration as the inter-plate
distance is decreased. Our theory predicts several scaling regimes with
different scaling expressions for the pressure as function of salinity and
surface charge densities. The pressure predictions can be related to surface
force experiments of oppositely charged surfaces that are prepared by coating
one of the mica surfaces with an oppositely charged polyelectrolyte
Polyelectrolyte Persistence Length: Attractive Effect of Counterion Correlations and Fluctuations
The persistence length of a single, strongly charged, stiff polyelectrolyte
chain is investigated theoretically. Path integral formulation is used to
obtain the effective electrostatic interaction between the monomers. We find
significant deviations from the classical Odijk, Skolnick and Fixman (OSF)
result. An induced attraction between monomers is due to thermal fluctuations
and correlations between bound counterions. The electrostatic persistence
length is found to be smaller than the OSF value and indicates a possible
mechanical instability (collapse) for highly charged polyelectrolytes with
multivalent counterions. In addition, we calculate the amount of condensed
counterions on a slightly bent polyelectrolyte. More counterions are found to
be adsorbed as compared to the Manning condensation on a cylinder.Comment: 5 pages, 1 ps figur
Fluctuations of a driven membrane in an electrolyte
We develop a model for a driven cell- or artificial membrane in an
electrolyte. The system is kept far from equilibrium by the application of a DC
electric field or by concentration gradients, which causes ions to flow through
specific ion-conducting units (representing pumps, channels or natural pores).
We consider the case of planar geometry and Debye-H\"{u}ckel regime, and obtain
the membrane equation of motion within Stokes hydrodynamics. At steady state,
the applied field causes an accumulation of charges close to the membrane,
which, similarly to the equilibrium case, can be described with renormalized
membrane tension and bending modulus. However, as opposed to the equilibrium
situation, we find new terms in the membrane equation of motion, which arise
specifically in the out-of-equilibrium case. We show that these terms lead in
certain conditions to instabilities.Comment: 7 pages, 2 figures. submitted to Europhys. Let
Statistical mechanics of budget-constrained auctions
Finding the optimal assignment in budget-constrained auctions is a
combinatorial optimization problem with many important applications, a notable
example being the sale of advertisement space by search engines (in this
context the problem is often referred to as the off-line AdWords problem).
Based on the cavity method of statistical mechanics, we introduce a message
passing algorithm that is capable of solving efficiently random instances of
the problem extracted from a natural distribution, and we derive from its
properties the phase diagram of the problem. As the control parameter (average
value of the budgets) is varied, we find two phase transitions delimiting a
region in which long-range correlations arise.Comment: Minor revisio
Topography and instability of monolayers near domain boundaries
We theoretically study the topography of a biphasic surfactant monolayer in
the vicinity of domain boundaries. The differing elastic properties of the two
phases generally lead to a nonflat topography of ``mesas'', where domains of
one phase are elevated with respect to the other phase. The mesas are steep but
low, having heights of up to 10 nm. As the monolayer is laterally compressed,
the mesas develop overhangs and eventually become unstable at a surface tension
of about K(dc)^2 (dc being the difference in spontaneous curvature and K a
bending modulus). In addition, the boundary is found to undergo a
topography-induced rippling instability upon compression, if its line tension
is smaller than about K(dc). The effect of diffuse boundaries on these features
and the topographic behavior near a critical point are also examined. We
discuss the relevance of our findings to several experimental observations
related to surfactant monolayers: (i) small topographic features recently found
near domain boundaries; (ii) folding behavior observed in mixed phospholipid
monolayers and model lung surfactants; (iii) roughening of domain boundaries
seen under lateral compression; (iv) the absence of biphasic structures in
tensionless surfactant films.Comment: 17 pages, 9 figures, using RevTeX and epsf, submitted to Phys Rev
Self-averaging of random and thermally disordered diluted Ising systems
Self-averaging of singular thermodynamic quantities at criticality for
randomly and thermally diluted three dimensional Ising systems has been studied
by the Monte Carlo approach. Substantially improved self-averaging is obtained
for critically clustered (critically thermally diluted) vacancy distributions
in comparison with the observed self-averaging for purely random diluted
distributions. Critically thermal dilution, leading to maximum relative
self-averaging, corresponds to the case when the characteristic vacancy
ordering temperature is made equal to the magnetic critical temperature for the
pure 3D Ising systems. For the case of a high ordering temperature, the
self-averaging obtained is comparable to that in a randomly diluted system.Comment: 4 pages, 4figures, RevTe
Nonequilibrium Fluctuations, Travelling Waves, and Instabilities in Active Membranes
The stability of a flexible fluid membrane containing a distribution of
mobile, active proteins (e.g. proton pumps) is shown to depend on the structure
and functional asymmetry of the proteins. A stable active membrane is in a
nonequilibrium steady state with height fluctuations whose statistical
properties are governed by the protein activity. Disturbances are predicted to
travel as waves at sufficiently long wavelength, with speed set by the normal
velocity of the pumps. The unstable case involves a spontaneous, pump-driven
undulation of the membrane, with clumping of the proteins in regions of high
activity.Comment: 4 two-column pages, two .eps figures included, revtex, uses eps
Dynamics of orientational ordering in fluid membranes
We study the dynamics of orientational phase ordering in fluid membranes.
Through numerical simulation we find an unusually slow coarsening of
topological texture, which is limited by subdiffusive propagation of membrane
curvature. The growth of the orientational correlation length obeys a
power law with in the late stage. We also discuss
defect profiles and correlation patterns in terms of long-range interaction
mediated by curvature elasticity.Comment: 5 pages, 3 figures (1 in color); Eq.(9) correcte
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