2,124 research outputs found

### A rapid stress-corrosion test for aluminum alloys

Stressed alloy specimens are immersed in a salt-dichromate solution at 60 degrees C. Because of the minimal general corrosion of these alloys in this solution, stress corrosion failures are detected by low-power microscopic examination

### "Water-free" computer model for fluid bilayer membranes

We use a simple and efficient computer model to investigate the physical
properties of bilayer membranes. The amphiphilic molecules are modeled as short
rigid trimers with finite range pair interactions between them. The pair
potentials have been designed to mimic the hydrophobic interactions, and to
allow the simulation of the membranes without the embedding solvent as if the
membrane is in vacuum. We find that upon decreasing the area density of the
molecules the membrane undergoes a solid-fluid phase transition, where in the
fluid phase the molecules can diffuse within the membrane plane. The surface
tension and the bending modulus of the fluid membranes are extracted from the
analysis of the spectrum of thermal undulations. At low area densities we
observe the formation of pores in the membrane through which molecules can
diffuse from one layer to the other. The appearance of the pores is explained
using a simple model relating it to the area dependence of the free energy.Comment: 12 pages, 8 figures, to appear in J. Chem. Phy

### Decrumpling membranes by quantum effects

The phase diagram of an incompressible fluid membrane subject to quantum and
thermal fluctuations is calculated exactly in a large number of dimensions of
configuration space. At zero temperature, a crumpling transition is found at a
critical bending rigidity $1/\alpha_{\rm c}$. For membranes of fixed lateral
size, a crumpling transition occurs at nonzero temperatures in an auxiliary
mean field approximation. As the lateral size L of the membrane becomes large,
the flat regime shrinks with $1/\ln L$.Comment: 9 pages, 4 figure

### Resonant three-body physics in two spatial dimensions

We discuss the three-body properties of identical bosons exhibiting large
scattering length in two spatial dimensions. Within an effective field theory
for resonant interactions, we calculate the leading non-universal corrections
from the two-body effective range to bound-state and scattering observables. In
particular, we compute the three-body binding energies, the boson-dimer
scattering properties, and the three-body recombination rate for finite
energies. We find significant effective range effects in the vicinity of the
unitary limit. The implications of this result for future experiments are
briefly discussed.Comment: 15 pages, 8 figures, published versio

### Effect of Thermal Undulations on the Bending Elasticity and Spontaneous Curvature of Fluid Membranes

We amplify previous arguments why mean curvature should be used as measure of
integration in calculating the effective bending rigidity of fluid membranes
subjected to a weak background curvature. The stiffening of the membrane by its
fluctuations, recently derived for spherical shapes, is recovered for
cylindrical curvature. Employing curvilinear coordinates, we then discuss
stiffening for arbitrary shapes, confirm that the elastic modulus of Gaussian
curvature is not renormalized in the presence of fluctuations, and show for the
first time that any spontaneous curvature also remains unchanged.Comment: 26 pages, 2 figures, to appear in EPJ

### Dynamics of Vesicle Formation from Lipid Droplet: Mechanism and Controllability

A coarse-grained model developed by Marrink et al. [J. Phys. Chem. B 111,
7812 (2007)] is applied to investigate vesiculation of lipid
[dipalmitoylphosphatidylcholine (DPPC)] droplets in water. Three kinds of
morphologies of micelles are found with increasing lipid droplet size. When the
initial lipid droplet is smaller, the equilibrium structure of the droplet is a
spherical micelle. When the initial lipid droplet is larger, the lipid ball
starts to transform into a disk micelle or vesicle. The mechanism of vesicle
formation from a lipid ball is analyzed from the self-assembly of DPPC on the
molecular level, and the morphological transition from disk to vesicle with
increasing droplet size is demonstrated. Importantly, we discover that the
transition point is not very sharp, and for a fixed-size lipid ball, the disk
and vesicle appear with certain probabilities. The splitting phenomenon, i.e.,
the formation of a disk/vesicle structure from a lipid droplet, is explained by
applying a hybrid model of the Helfrich membrane theory. The elastic module of
the DPPC bilayer and the smallest size of a lipid droplet for certain formation
of a vesicle are successfully predicted.Comment: 22 pages, 11 figures Submitted to J. Chem. Phy

### The unbinding transition of mixed fluid membranes

A phenomenological model for the unbinding transition of multi-component
fluid membranes is proposed, where the unbinding transition is described using
a theory analogous to Flory-Huggins theory for polymers. The coupling between
the lateral phase separation of inclusion molecules and the membrane-substrate
distance explains the phase coexistence between two unbound phases as observed
in recent experiments by Marx et al. [Phys. Rev. Lett. 88, 138102 (2002)].
Bellow a critical end-point temperature, we find that the unbinding transition
becomes first-order for multi-component membranes.Comment: 7 pages, 3 eps figure

### Bulk and wetting phenomena in a colloidal mixture of hard spheres and platelets

Density functional theory is used to study binary colloidal fluids consisting
of hard spheres and thin platelets in their bulk and near a planar hard wall.
This system exhibits liquid-liquid coexistence of a phase that is rich in
spheres (poor in platelets) and a phase that is poor in spheres (rich in
platelets). For the mixture near a planar hard wall, we find that the phase
rich in spheres wets the wall completely upon approaching the liquid demixing
binodal from the sphere-poor phase, provided the concentration of the platelets
is smaller than a threshold value which marks a first-order wetting transition
at coexistence. No layering transitions are found in contrast to recent studies
on binary mixtures of spheres and non-adsorbing polymers or thin hard rods.Comment: 6 pages, 4 figure

### Interaction of Conical Membrane Inclusions: Effect of Lateral Tension

Considering two rigid conical inclusions embedded in a membrane subject to
lateral tension, we study the membrane-mediated interaction between these
inclusions that originates from the hat-shaped membrane deformations associated
with the cones. At non-vanishing lateral tensions, the interaction is found to
depend on the orientation of the cones with respect to the membrane plane. The
interaction of inclusions of equal orientation is repulsive at all distances
between them, while the inclusions of opposite orientation repel each other at
small separations, but attract each other at larger ones. Both the repulsive
and attractive forces become stronger with increasing lateral tension. This is
different from what has been predicted on the basis of the same static model
for the case of vanishing lateral tension. Without tension, the inclusions
repel each other at all distances independently of their relative orientation.
We conclude that lateral tension may induce the aggregation of conical membrane
inclusions.Comment: 10 pages (revtech), 5 figures (postscript

### Three-body problem in heteronuclear mixtures with resonant interspecies interaction

We use the zero-range approximation to study a system of two identical bosons
interacting resonantly with a third particle. The method is derived from
effective field theory. It reduces the three-body problem to an integral
equation which we then solve numerically. We also develop an alternative
approach which gives analytic solutions of the integral equation in coordinate
representation in the limit of vanishing total energy. The atom-dimer
scattering length, the rates of atom-dimer relaxation and three-body
recombination to shallow and to deep molecular states are calculated either
analytically or numerically with a well controlled accuracy for various
energies as functions of the mass ratio, scattering length, and three-body
parameter. We discuss in detail the relative positions of the recombination
loss peaks, which in the universal limit depend only on the mass ratio. Our
results have implications for ongoing and future experiments on Bose-Bose and
Bose-Fermi atomic mixtures.Comment: 13 pages, 8 figures, minor changes, published versio

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