226 research outputs found
Relaxation of a dewetting contact line Part 2: Experiments
The dynamics of receding contact lines is investigated experimentally through
controlled perturbations of a meniscus in a dip coating experiment. We first
characterize stationary menisci and their breakdown at the coating transition.
It is then shown that the dynamics of both liquid deposition and
long-wavelength perturbations adiabatically follow these stationary states.
This provides a first experimental access to the entire bifurcation diagram of
dynamical wetting, confirming the hydrodynamic theory developed in Part 1. In
contrast to quasi-static theories based on a dynamic contact angle, we
demonstrate that the transition strongly depends on the large scale flow
geometry. We then establish the dispersion relation for large wavenumbers, for
which we find that sigma is linear in q. The speed dependence of sigma is well
described by hydrodynamic theory, in particular the absence of diverging
time-scales at the critical point. Finally, we highlight some open problems
related to contact angle hysteresis that lead beyond the current description.Comment: 20 pages, 11 figures Part 1 is stored as Arxiv 0705.357
Relaxation of a dewetting contact line Part 1: A full-scale hydrodynamic calculation
The relaxation of a dewetting contact line is investigated theoretically in
the so-called "Landau-Levich" geometry in which a vertical solid plate is
withdrawn from a bath of partially wetting liquid. The study is performed in
the framework of lubrication theory, in which the hydrodynamics is resolved at
all length scales (from molecular to macroscopic). We investigate the
bifurcation diagram for unperturbed contact lines, which turns out to be more
complex than expected from simplified 'quasi-static' theories based upon an
apparent contact angle. Linear stability analysis reveals that below the
critical capillary number of entrainment, Ca_c, the contact line is linearly
stable at all wavenumbers. Away from the critical point the dispersion relation
has an asymptotic behaviour sigma~|q| and compares well to a quasi-static
approach. Approaching Ca_c, however, a different mechanism takes over and the
dispersion evolves from |q| to the more common q^2. These findings imply that
contact lines can not be treated as universal objects governed by some
effective law for the macroscopic contact angle, but viscous effects have to be
treated explicitly.Comment: 21 pages, 9 figure
Casimir Dispersion Forces and Orientational Pairwise Additivity
A path integral formulation is used to study the fluctuation-induced
interactions between manifolds of arbitrary shape at large separations. It is
shown that the form of the interactions crucially depends on the choice of the
boundary condition. In particular, whether or not the Casimir interaction is
pairwise additive is shown to depend on whether the ``metallic'' boundary
condition corresponds to a ``grounded'' or an ``isolated'' manifold.Comment: 6 pages, RevTe
Coherent Hydrodynamic Coupling for Stochastic Swimmers
A recently developed theory of stochastic swimming is used to study the
notion of coherence in active systems that couple via hydrodynamic
interactions. It is shown that correlations between various modes of
deformation in stochastic systems play the same role as the relative internal
phase in deterministic systems. An example is presented where a simple swimmer
can use these correlations to hunt a non-swimmer by forming a hydrodynamic
bound state of tunable velocity and equilibrium separation. These results
highlight the significance of coherence in the collective behavior of
nano-scale stochastic swimmers.Comment: 6 pages, 3 figure
Casimir Torques between Anisotropic Boundaries in Nematic Liquid Crystals
Fluctuation-induced interactions between anisotropic objects immersed in a
nematic liquid crystal are shown to depend on the relative orientation of these
objects. The resulting long-range ``Casimir'' torques are explicitely
calculated for a simple geometry where elastic effects are absent. Our study
generalizes previous discussions restricted to the case of isotropic walls, and
leads to new proposals for experimental tests of Casimir forces and torques in
nematics.Comment: 4 pages, 1 figur
Lifshitz Interaction between Dielectric Bodies of Arbitrary Geometry
A formulation is developed for the calculation of the
electromagnetic--fluctuation forces for dielectric objects of arbitrary
geometry at small separations, as a perturbative expansion in the dielectric
contrast. The resulting Lifshitz energy automatically takes on the form of a
series expansion of the different many-body contributions. The formulation has
the advantage that the divergent contributions can be readily determined and
subtracted off, and thus makes a convenient scheme for realistic numerical
calculations, which could be useful in designing nano-scale mechanical devices
Small object limit of Casimir effect and the sign of the Casimir force
We show a simple way of deriving the Casimir Polder interaction, present some
general arguments on the finiteness and sign of mutual Casimir interactions and
finally we derive a simple expression for Casimir radiation from small
accelerated objects.Comment: 13 pages, late
Effect of the Heterogeneity of Metamaterials on Casimir-Lifshitz Interaction
The Casimir-Lifshitz interaction between metamaterials is studied using a
model that takes into account the structural heterogeneity of the dielectric
and magnetic properties of the bodies. A recently developed perturbation theory
for the Casimir-Lifshitz interaction between arbitrary material bodies is
generalized to include non-uniform magnetic permeability profiles, and used to
study the interaction between the magneto-dielectric heterostructures within
the leading order. The metamaterials are modeled as two dimensional arrays of
domains with varying permittivity and permeability. In the case of two
semi-infinite bodies with flat boundaries, the patterned structure of the
material properties is found to cause the normal Casimir-Lifshitz force to
develop an oscillatory behavior when the distance between the two bodies is
comparable to the wavelength of the patterned features in the metamaterials.
The non-uniformity also leads to the emergence of lateral Casimir-Lifshitz
forces, which tend to strengthen as the gap size becomes smaller. Our results
suggest that the recent studies on Casimir-Lifshitz forces between
metamaterials, which have been performed with the aim of examining the
possibility of observing the repulsive force, should be revisited to include
the effect of the patterned structure at the wavelength of several hundred
nanometers that coincides with the relevant gap size in the experiments.Comment: 9 pages, 13 figures. Rewriting equations (10) and (12) and increasing
the size of the lettering/numeral in figure
Quantum energies with worldline numerics
We present new results for Casimir forces between rigid bodies which impose
Dirichlet boundary conditions on a fluctuating scalar field. As a universal
computational tool, we employ worldline numerics which builds on a combination
of the string-inspired worldline approach with Monte-Carlo techniques.
Worldline numerics is not only particularly powerful for inhomogeneous
background configurations such as involved Casimir geometries, it also provides
for an intuitive picture of quantum-fluctuation-induced phenomena. Results for
the Casimir geometries of a sphere above a plate and a new perpendicular-plates
configuration are presented.Comment: 8 pages, 2 figures, Submitted to the Proceedings of the Seventh
Workshop QFEXT'05 (Barcelona, September 5-9, 2005), Refs updated, version to
appear in JPhys
A class of anisotropic (Finsler-) space-time geometries
A particular Finsler-metric proposed in [1,2] and describing a geometry with
a preferred null direction is characterized here as belonging to a subclass
contained in a larger class of Finsler-metrics with one or more preferred
directions (null, space- or timelike). The metrics are classified according to
their group of isometries. These turn out to be isomorphic to subgroups of the
Poincar\'e (Lorentz-) group complemented by the generator of a dilatation. The
arising Finsler geometries may be used for the construction of relativistic
theories testing the isotropy of space. It is shown that the Finsler space with
the only preferred null direction is the anisotropic space closest to isotropic
Minkowski-space of the full class discussed.Comment: 12 pages, latex, no figure
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