576 research outputs found

### Interatomic van der Waals potential in the presence of a magneto-electric sphere

On the basis of a general formula obtained earlier via fourth-order
erturbation theory within the framework of macroscopic quantum electrodynamics,
the van der Waals potential between two neutral, unpolarized, ground-state
atoms in the presence of a homogeneous, dispersing and absorbing
magnetoelectric sphere is studied. When the radius of the sphere becomes
sufficiently large, the result asymptotically agrees with that for two atoms
near a planar interface. In the opposite limit of a very small sphere, the
sphere can effectively be regarded as being a third ground-state atom, and the
nonadditive three-atom van der Waals potential is recovered. To illustrate the
effect of a sphere of arbitrary radius, numerical results are presented for the
triangular arrangement where the atoms are at equidistance from the sphere, and
for the linear arrangement where the atoms and the sphere are aligned along a
straight line. As demonstrated, the enhancement or reduction of the interaction
potential in the presence of purely electric or magnetic spheres can be
physically understood in terms of image charges.Comment: 13 pages, 7 figure

### Attraction between Neutral Dielectrics Mediated by Multivalent Ions in an Asymmetric Ionic Fluid

We study the interaction between two neutral plane-parallel dielectric bodies
in the presence of a highly asymmetric ionic fluid, containing multivalent as
well as monovalent (salt) ions. Image charge interactions, due to dielectric
discontinuities at the boundaries, as well as effects from ion confinement in
the slit region between the surfaces are taken fully into account, leading to
image-generated depletion attraction, ion correlation attraction and
steric-like repulsive interactions. We investigate these effects by employing a
combination of methods including explicit-ion and implicit-ion Monte-Carlo
simulations, as well as an effective interaction potential analytical theory.
The latter incorporates strong ion-image charge correlations, which develop in
the presence of high valency ions in the mixture. We show that the implicit-ion
simulations and the proposed analytical theory can describe the explicit
simulation results on a qualitative level, while excellent quantitative
agreement can be obtained for sufficiently large monovalent salt
concentrations. The resultant attractive interaction between the neutral
surfaces is shown to be significant, as compared with the usual van der Waals
interactions between semi-infinite dielectrics, and can thus play a significant
role at the nano scale.Comment: 9 pages, 4 figure

### Global consequences of a local Casimir force: Adhered cantilever

Although stiction is a cumbersome problem for microsystems, it stimulates
investigations of surface adhesion. In fact, the shape of an adhered cantilever
carries information of the adhesion energy that locks one end to the substrate.
We demonstrate here that the system is also sensitive to the dispersion forces
that are operative very close to the point of contact, but their contribution
to the shape is maximum at about one third of the unadhered length. When the
force exceeds a critical value the cantilever does not lose stability but it
settles at smaller unadhered length, whose relation to adhesion energy is only
slightly affected by the force. Our calculations suggest to use adhered
cantilevers to measure the dispersion forces at short separations, where other
methods suffer from jump-to-contact instability. Simultaneous measurement of
the force and adhesion energy allows the separation of the dispersion
contribution to the surface adhesion.Comment: 5 pages, 3 figure

### Electromagnetic fluctuation-induced interactions in randomly charged slabs

Randomly charged net-neutral dielectric slabs are shown to interact across a
featureless dielectric continuum with long-range electrostatic forces that
scale with the statistical variance of their quenched random charge
distribution and inversely with the distance between their bounding surfaces.
By accounting for the whole spectrum of electromagnetic field fluctuations, we
show that this long-range disorder-generated interaction extends well into the
retarded regime where higher-order Matsubara frequencies contribute
significantly. This occurs even for highly clean samples with only a trace
amount of charge disorder and shows that disorder effects can be important down
to the nano scale. As a result, the previously predicted non-monotonic behavior
for the total force between dissimilar slabs as a function of their separation
distance is substantially modified by higher-order contributions, and in almost
all cases of interest, we find that the equilibrium inter-surface separation is
shifted to substantially larger values compared to predictions based solely on
the zero-frequency component. This suggests that the ensuing non-monotonic
interaction is more easily amenable to experimental detection. The presence of
charge disorder in the intervening dielectric medium between the two slabs is
shown to lead to an additional force that can be repulsive or attractive
depending on the system parameters and can, for instance, wash out the
non-monotonic behavior of the total force when the intervening slab contains a
sufficiently large amount of disorder charges.Comment: 9 pages, 5 figure

### Electrostatic attraction between cationic-anionic assemblies with surface compositional heterogeneities

Electrostatics plays a key role in biomolecular assembly. Oppositely charged
biomolecules, for instance, can co-assembled into functional units, such as DNA
and histone proteins into nucleosomes and actin-binding protein complexes into
cytoskeleton components, at appropriate ionic conditions. These
cationic-anionic co-assemblies often have surface charge heterogeneities that
result from the delicate balance between electrostatics and packing
constraints. Despite their importance, the precise role of surface charge
heterogeneities in the organization of cationic-anionic co-assemblies is not
well understood. We show here that co-assemblies with charge heterogeneities
strongly interact through polarization of the domains. We find that this leads
to symmetry breaking, which is important for functional capabilities, and
structural changes, which is crucial in the organization of co-assemblies. We
determine the range and strength of the attraction as a function of the
competition between the steric and hydrophobic constraints and electrostatic
interactions.Comment: JCP June/200

### Precision measurement of the Casimir-Lifshitz force in a fluid

The Casimir force, which results from the confinement of the quantum
mechanical zero-point fluctuations of the electromagnetic fields, has received
significant attention in recent years for its effect on micro- and nano-scale
mechanical systems. With few exceptions, experimental observations have been
limited to conductive bodies interacting separated by vacuum or air. However,
interesting phenomena including repulsive forces are expected to exist in
certain circumstances between metals and dielectrics when the intervening
medium is not vacuum. In order to better understand the effect of the Casimir
force in such situations and to test the robustness of the generalized
Casimir-Lifshitz theory, we have performed the first precision measurements of
the Casimir force between two metals immersed in a fluid. For this situation,
the measured force is attractive and is approximately 80% smaller than the
force predicted by Casimir for ideal metals in vacuum. We present experimental
results and find them to be consistent with Lifshitz's theory.Comment: 6 pages, 3 figures. (version before final publication

### Nonuniversality of the dispersion interaction: analytic benchmarks for van der Waals energy functionals

We highlight the non-universality of the asymptotic behavior of dispersion
forces, such that a sum of inverse sixth power contributions is often
inadequate. We analytically evaluate the cross-correlation energy Ec between
two pi-conjugated layers separated by a large distance D within the
electromagnetically non-retarded Random Phase Approximation, via a
tight-binding model. For two perfect semimetallic graphene sheets at T=0K we
find Ec = C D^{-3}, in contrast to the "insulating" D^{-4} dependence predicted
by currently accepted approximations. We also treat the case where one graphene
layer is replaced by a thin metal, a model relevant to the exfoliation of
graphite. Our general considerations also apply to nanotubes, nanowires and
layered metals.Comment: 4 pages, 0 fig

### Some observations on the renormalization of membrane rigidity by long-range interactions

We consider the renormalization of the bending and Gaussian rigidity of model
membranes induced by long-range interactions between the components making up
the membrane. In particular we analyze the effect of a finite membrane
thickness on the renormalization of the bending and Gaussian rigidity by
long-range interactions. Particular attention is paid to the case where the
interactions are of a van der Waals type.Comment: 11 pages RexTex, no figure

### Body-assisted van der Waals interaction between two atoms

Using fourth-order perturbation theory, a general formula for the van der
Waals potential of two neutral, unpolarized, ground-state atoms in the presence
of an arbitrary arrangement of dispersing and absorbing magnetodielectric
bodies is derived. The theory is applied to two atoms in bulk material and in
front of a planar multilayer system, with special emphasis on the cases of a
perfectly reflecting plate and a semi-infinite half space. It is demonstrated
that the enhancement and reduction of the two-atom interaction due to the
presence of a perfectly reflecting plate can be understood, at least in the
nonretarded limit, by using the method of image charges. For the semi-infinite
half space, both analytical and numerical results are presented.Comment: 17 pages, 9 figure

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