927 research outputs found
Exact Casimir interaction between eccentric cylinders
The Casimir force is the ultimate background in ongoing searches of
extra-gravitational forces in the micrometer range. Eccentric cylinders offer
favorable experimental conditions for such measurements as spurious
gravitational and electrostatic effects can be minimized. Here we report on the
evaluation of the exact Casimir interaction between perfectly conducting
eccentric cylinders using a mode summation technique, and study different
limiting cases of relevance for Casimir force measurements, with potential
implications for the understanding of mechanical properties of nanotubes.Comment: 5 pages, 4 figure
Fluid structure in the immediate vicinity of an equilibrium three-phase contact line and assessment of disjoining pressure models using density functional theory
We examine the nanoscale behavior of an equilibrium three-phase contact line
in the presence of long-ranged intermolecular forces by employing a statistical
mechanics of fluids approach, namely density functional theory (DFT) together
with fundamental measure theory (FMT). This enables us to evaluate the
predictive quality of effective Hamiltonian models in the vicinity of the
contact line. In particular, we compare the results for mean field effective
Hamiltonians with disjoining pressures defined through (I) the adsorption
isotherm for a planar liquid film, and (II) the normal force balance at the
contact line. We find that the height profile obtained using (I) shows good
agreement with the adsorption film thickness of the DFT-FMT equilibrium density
profile in terms of maximal curvature and the behavior at large film heights.
In contrast, we observe that while the height profile obtained by using (II)
satisfies basic sum rules, it shows little agreement with the adsorption film
thickness of the DFT results. The results are verified for contact angles of
20, 40 and 60 degrees
Constraints on non-Newtonian gravity from measuring the Casimir force in a configuration with nanoscale rectangular corrugations
We report constraints on the parameters of Yukawa-type corrections to
Newtonian gravity from measurements of the gradient of the Casimir force in the
configuration of an Au-coated sphere above a Si plate covered with corrugations
of trapezoidal shape. For this purpose, the exact expression for the gradient
of Yukawa force in the experimental configuration is derived and compared with
that obtained using the proximity force approximation. The reported constraints
are of almost the same strength as those found previously from several
different experiments on the Casimir force and extend over a wide interaction
range from 30 to 1260\,nm. It is discussed how to make them stronger by
replacing the material of the plate.Comment: 14 pages, 2 figures, to appear in Phys. Rev.
Ground state of two unlike charged colloids: An analogy with ionic bonding
In this letter, we study the ground state of two spherical macroions of
identical radius, but asymmetric bare charge ((Q_{A}>Q_{B})). Electroneutrality
of the system is insured by the presence of the surrounding divalent
counterions. Using Molecular Dynamics simulations within the framework of the
primitive model, we show that the ground state of such a system consists of an
overcharged and an undercharged colloid. For a given macroion separation the
stability of these ionized-like states is a function of the difference
((\sqrt{N_{A}}-\sqrt{N_{B}})) of neutralizing counterions (N_{A}) and (N_{B}).
Furthermore the degree of ionization, or equivalently, the degree of
overcharging, is also governed by the distance separation of the macroions. The
natural analogy with ionic bonding is briefly discussed.Comment: published versio
Where the linearized Poisson-Boltzmann cell model fails: (I) spurious phase separation in charged colloidal suspensions
We perform a linearization of the Poisson-Boltzmann (PB) density functional
for spherical Wigner-Seitz cells that yields Debye-H\"uckel-like equations
agreeing asymptotically with the PB results in the weak-coupling
(high-temperature) limit. Both the canonical (fixed number of microions) as
well as the semi-grand-canonical (in contact with an infinite salt reservoir)
cases are considered and discussed in a unified linearized framework. In the
canonical case, for sufficiently large colloidal charges the linearized theory
predicts the occurrence of a thermodynamical instability with an associated
phase separation of the homogeneous suspension into dilute (gas) and dense
(liquid) phases. In the semi-grand-canonical case it is predicted that the
isothermal compressibility and the osmotic-pressure difference between the
colloidal suspension and the salt reservoir become negative in the
low-temperature, high-surface charge or infinite-dilution (of polyions) limits.
As already pointed out in the literature for the latter case, these features
are in disagreement with the exact nonlinear PB solution inside a Wigner-Seitz
cell and are thus artifacts of the linearization. By using explicitly
gauge-invariant forms of the electrostatic potential we show that these
artifacts, although thermodynamically consistent with quadratic expansions of
the nonlinear functional and osmotic pressure, may be traced back to the
non-fulfillment of the underlying assumptions of the linearization.Comment: 32 pages, 3 PostScript figures, submitted to J. Chem. Phy
Hydrodynamic theory of de-wetting
A prototypical problem in the study of wetting phenomena is that of a solid
plunging into or being withdrawn from a liquid bath. In the latter, de-wetting
case, a critical speed exists above which a stationary contact line is no
longer sustainable and a liquid film is being deposited on the solid.
Demonstrating this behavior to be a hydrodynamic instability close to the
contact line, we provide the first theoretical explanation of a classical
prediction due to Derjaguin and Levi: instability occurs when the outer, static
meniscus approaches the shape corresponding to a perfectly wetting fluid
Measurements of the Casimir-Lifshitz force in fluids: the effect of electrostatic forces and Debye screening
In this work, we present detailed measurements of the Casimir-Lifshitz force
between two gold surfaces (a sphere and a plate) immersed in ethanol and study
the effect of residual electrostatic forces, which are dominated by static
fields within the apparatus and can be reduced with proper shielding.
Electrostatic forces are further reduced by Debye screening through the
addition of salt ions to the liquid. Additionally, the salt leads to a
reduction of the Casimir-Lifshitz force by screening the zero-frequency
contribution to the force; however, the effect is small between gold surfaces
at the measured separations and within experimental error. An improved
calibration procedure is described and compared to previous methods. Finally,
the experimental results are compared to Lifshitz's theory and found to be
consistent for the materials used in the experiment.Comment: 11 figures. PRA in pres
Effective interactions in the colloidal suspensions from HNC theory
The HNC Ornstein-Zernike integral equations are used to determine the
properties of simple models of colloidal solutions where the colloids and ions
are immersed in a solvent considered as a dielectric continuum and have a size
ratio equal to 80 and a charge ratio varying between 1 and 4000. At an infinite
dilution of colloids, the effective interactions between colloids and ions are
determined for ionic concentrations ranging from 0.001 to 0.1 mol/l and
compared to those derived from the Poisson-Boltzmann theory. At finite
concentrations, we discuss on the basis of the HNC results the possibility of
an unambiguous definition of the effective interactions between the colloidal
molecules.Comment: 26 pages, 15 figure
Studies on electrostatic interactions of colloidal particles under two-dimensional confinement
We study the effective electrostatic interactions between a pair of charged
colloidal particles without salt ions while the system is confined in two
dimensions. In particular we use a simplified model to elucidate the effects of
rotational fluctuations in counterion distribution. The results exhibit
effective colloidal attractions under appropriate conditions. Meanwhile,
long-range repulsions persist over most of our studied cases. The repulsive
forces arise from the fact that in two dimensions the charged colloids cannot
be perfectly screened by counterions, as the residual quadrupole moments
contribute to the repulsions at longer range. And by applying multiple
expansions we find that the attractive forces observed at short range are
mainly contributed from electrostatic interactions among higher-order electric
moments. We argue that the scenario for attractive interactions discussed in
this work is applicable to systems of charged nanoparticles or colloidal
solutions with macroions.Comment: 23 pages, 11 figures, 1 tabl
Droplet motion driven by surface freezing or melting: A mesoscopic hydrodynamic approach
A fluid droplet may exhibit self-propelled motion by modifying the wetting
properties of the substrate. We propose a novel model for droplet propagation
upon a terraced landscape of ordered layers formed as a result of surface
freezing driven by the contact angle dependence on the terrace thickness.
Simultaneous melting or freezing of the terrace edge results in a joint
droplet-terrace motion. The model is tested numerically and compared to
experimental observations on long-chain alkane system in the vicinity of the
surface melting point.Comment: 4 pages, 3 figure
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