597 research outputs found
Van der Waals interactions across stratified media
Working at the Lifshitz level, we investigate the van der Waals interactions
across a series of layers with a periodic motif. We derive the complete form of
the van der Waals interaction as an explicit function of the number of periodic
layers. We then compare our result with an approximation based on an
anisotropic-continuum representation of the stratified medium. Satisfactory
agreement between discrete-layer and continuum models is reached only for
thicknesses of ten or more layers.Comment: 9 pages and 4 figure
Charge Fluctuation Forces Between Stiff Polyelectrolytes in Salt Solution: Pairwise Summability Re-examined
We formulate low-frequency charge-fluctuation forces between charged
cylinders - parallel or skewed - in salt solution: forces from dipolar van der
Waals fluctuations and those from the correlated monopolar fluctuations of
mobile ions. At high salt concentrations forces are exponentially screened. In
low-salt solutions dipolar energies go as or ; monopolar
energies vary as or , where is the minimal separation
between cylinders. However, pairwise summability of rod-rod forces is easily
violated in low-salt conditions. Perhaps the most important result is not the
derivation of pair potentials but rather the demonstration that some of these
expressions may not be used for the very problems that originally motivated
their derivation.Comment: 8 pages and 1 fig in ps forma
Recursion relations for generalized Fresnel coefficients: Casimir force in a planar cavity
We emphasize and demonstrate that, besides using the usual recursion
relations involving successive layers, generalized Fresnel coefficients of a
multilayer can equivalently be calculated using the recursion relations
involving stacks of layers, as introduced some time ago [M. S. Tomas, Phys.
Rev. A 51, 2545 (1995)]. Moreover, since the definition of the generalized
Fresnel coefficients employed does not imply properties of the stacks, these
nonstandard recursion relations can be used to calculate Fresnel coefficients
not only for local systems but also for a general multilayer consisting of
various types (local, nonlocal, inhomogeneous etc.) of layers. Their utility is
illustrated by deriving a few simple algorithms for calculating the
reflectivity of a Bragg mirror and extending the formula for the Casimir force
in a planar cavity to arbitrary media.Comment: 5 pages, 2 figures, slightly expande
Three-body Casimir effects and non-monotonic forces
Casimir interactions are not pair-wise additive. This property leads to
collective effects that we study for a pair of objects near a conducting wall.
We employ a scattering approach to compute the interaction in terms of
fluctuating multipoles. The wall can lead to a non-monotonic force between the
objects. For two atoms with anisotropic electric and magnetic dipole
polarizabilities we demonstrate that this non-monotonic effect results from a
competition between two- and three body interactions. By including higher order
multipoles we obtain the force between two macroscopic metallic spheres for a
wide range of sphere separations and distances to the wall.Comment: 4 pages, 4 figure
Osmotic properties of polyethyleneglycols: quantitative features of brush and bulk scaling laws
From glycosylated cell surfaces to sterically stabilized liposomes, polymers
attached to membranes attract biological and therapeutic interest. Can the
scaling laws of polymer "brushes" describe the physical properties of these
coats? We delineate conditions where the Alexander - de Gennes theory of
polymer brushes successfully describes the intermembrane distance vs. applied
osmotic stress data of Kenworthy et al. for PEG-grafted multilamellar liposomes
[Biophys. J. (1995) 68:1921]. We establish that the polymer density and size in
the brush must be high enough that, in a bulk solution of equivalent density,
the polymer osmotic pressure is independent of polymer molecular weight (the
des Cloizeaux semi-dilute regime of bulk polymer solutions). The condition that
attached polymers behave as semi-dilute bulk solutions offers a rigorous
criterion for brush scaling-law behavior. There is a deep connection between
the behaviors of polymer solutions in bulk and polymers grafted to a surface at
a density such that neighbors pack to form a uniform brush. In this regime,
two-parameter unconstrained fits of the Alexander - de Gennes brush scaling
laws yield effective monomer lengths of 3.3 to 3.5 AA, which agree with
structural predictions. The fitted distances between grafting sites are larger
than expected from the nominal content of PEG-lipids; the chains apparently
saturate the surface. Osmotic stress measurements can be used to estimate the
actual densities of membrane-grafted polymers.Comment: 26 pages with figure
Polymers pushing Polymers: Polymer Mixtures in Thermodynamic Equilibrium with a Pore
We investigate polymer partitioning from polymer mixtures into nanometer size
cavities by formulating an equation of state for a binary polymer mixture
assuming that only one (smaller) of the two polymer components can penetrate
the cavity. Deriving the partitioning equilibrium equations and solving them
numerically allows us to introduce the concept of "polymers-pushing-polymers"
for the action of non-penetrating polymers on the partitioning of the
penetrating polymers. Polymer partitioning into a pore even within a very
simple model of a binary polymer mixture is shown to depend in a complicated
way on the composition of the polymer mixture and/or the pore-penetration
penalty. This can lead to enhanced as well as diminished partitioning, due to
two separate energy scales that we analyse in detail.Comment: 10 pages, 6 figure
Comparison of the hydrodynamic and Dirac models of the dispersion interaction between graphene and H, He, or Na atoms
The van der Waals and Casimir-Polder interaction of different atoms with
graphene is investigated using the Dirac model which assumes that the energy of
quasiparticles is linear with respect to the momentum. The obtained results for
the van der Waals coefficients of hydrogen atoms and molecules and atoms of
metastable He and Na as a function of separation are compared with
respective results found using the hydrodynamic model of graphene. It is shown
that, regardless of the value of the gap parameter, the Dirac model leads to
much smaller values of the van der Waals coefficients than the hydrodynamic
model. The experiment on quantum reflection of metastable He and Na
atoms on graphene is proposed which is capable to discriminate between the two
models of the electronic structure of graphene. In this respect the parameters
of the phenomenological potential for both these atoms interacting with
graphene described by different models are determined.Comment: 15 pages, 4 figure
Bond Orientational Order, Molecular Motion and Free Energy of High Density DNA Mesophases
By equilibrating condensed DNA arrays against reservoirs of known osmotic
stress and examining them with several structural probes, it has been possible
to achieve a detailed thermodynamic and structural characterization of the
change between two distinct regions on the liquid crystalline phase digram: a
higher-density hexagonally packed region with long-range bond orientational
order in the plane perpendicular to the average molecular direction; and a
lower-density cholesteric region with fluid-like positional order. X-rays
scattering on highly ordered DNA arrays at high density and with the helical
axis oriented parallel to the incoming beam showed a six-fold azimuthal
modulation of the first order diffraction peak that reflects the macroscopic
bond-orientational order. Transition to the less-dense cholesteric phase
through osmotically controlled swelling shows the loss of this bond
orientational order that had been expected from the change in optical
birefringence patterns and that is consistent with a rapid onset of molecular
positional disorder. This change in motion was previously inferred from
intermolecular force measurements and is now confirmed by NMR.
Controlled reversible swelling and compaction under osmotic stress, spanning a
range of densities between mg/ml to mg/ml, allows
measurement of the free energy changes throughout each phase and at the phase
transition, essential information for theories of liquid-crystalline states.Comment: 14 pages, 3 figures in gif format available at
http://abulafia.mgsl.dcrt.nih.gov/pics.html E-mail: [email protected]
Thermal Casimir-Polder interaction of different atoms with graphene
The thermal correction to the energy of Casimir-Polder interaction of atoms
with a suspended graphene membrane described by the Dirac model is
investigated. We show that a major impact on the thermal correction is made by
the size of the gap in the energy spectrum of graphene quasiparticles.
Specifically, if the temperature is much smaller than the gap parameter
(alternatively, larger or of the order of the gap parameter), the thermal
correction is shown to be relatively small (alternatively, large). We have
calculated the free energy of the thermal Casimir-Polder interaction of atoms
of He, Na, Rb, and Cs with graphene described by both the hydrodynamic and
Dirac models. It is shown that in exact computations using the Dirac model, one
should use the polarization operator at nonzero temperature. The computational
results for the Casimir-Polder free energy obtained in the framework of
hydrodynamic model of graphene are several times larger than in the Dirac model
within the separation region below 2m. We conclude that the theoretical
predictions following from the two models can be reliably discriminated in
experiments on quantum reflection of different atoms on graphene.Comment: 19 pages, 6 figures, to appear in Phys. Rev.
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