398 research outputs found
Capillary instability in a two-component Bose-Einstein condensate
Capillary instability and the resulting dynamics in an immiscible
two-component Bose-Einstein condensate are investigated using the mean-field
and Bogoliubov analyses. A long, cylindrical condensate surrounded by the other
component is dynamically unstable against breakup into droplets due to the
interfacial tension arising from the quantum pressure and interactions. A
heteronuclear system confined in a cigar-shaped trap is proposed for realizing
this phenomenon experimentally.Comment: 7 pages, 6 figure
Stability of viscous long liquid filaments
We study the collapse of an axisymmetric liquid filament both analytically
and by means of a numerical model. The liquid filament, also known as ligament,
may either collapse stably into a single droplet or break up into multiple
droplets. The dynamics of the filament are governed by the viscosity and the
aspect ratio, and the initial perturbations of its surface. We find that the
instability of long viscous filaments can be completely explained by the
Rayleigh-Plateau instability, whereas a low viscous filament can also break up
due to end pinching. We analytically derive the transition between stable
collapse and breakup in the Ohnesorge number versus aspect ratio phase space.
Our result is confirmed by numerical simulations based on the slender jet
approximation and explains recent experimental findings by Castrejon-Pita et
al., PRL 108, 074506 (2012).Comment: 7 page
Recherches physico-chimiques sur les Articulés aquatiques. Première partie. Action des sels en dissolution dans l'eau. Influence de l'eau de mer sur les Articulés aquatiques d'eau douce. Influence de l'eau douce sur les Crustacés marins
Rayleigh and depinning instabilities of forced liquid ridges on heterogeneous substrates
Depinning of two-dimensional liquid ridges and three-dimensional drops on an
inclined substrate is studied within the lubrication approximation. The
structures are pinned to wetting heterogeneities arising from variations of the
strength of the short-range polar contribution to the disjoining pressure. The
case of a periodic array of hydrophobic stripes transverse to the slope is
studied in detail using a combination of direct numerical simulation and
branch-following techniques. Under appropriate conditions the ridges may either
depin and slide downslope as the slope is increased, or first breakup into
drops via a transverse instability, prior to depinning. The different
transition scenarios are examined together with the stability properties of the
different possible states of the system.Comment: Physics synopsis link:
http://physics.aps.org/synopsis-for/10.1103/PhysRevE.83.01630
Solving the brachistochrone and other variational problems with soap films
We show a method to solve the problem of the brachistochrone as well as other
variational problems with the help of the soap films that are formed between
two suitable surfaces. We also show the interesting connection between some
variational problems of dynamics, statics, optics, and elasticity.Comment: 16 pages, 11 figures. This article, except for a small correction,
has been submitted to the American Journal of Physic
An exact algorithm for the constraint satisfaction problem : application to dependance computing in automatic parallelization
Projet CHLOEThe constraint satisfaction problem - denoted by CSP - consists in proving the emptiness of a domain defined by a set of constraints or the existence of a solution. Numerous applications arise in the computer science field (artificial intelligence, vectorization, verification of programs,...). In the case of the study of dependence computing in automatic parallelization, classical methods in literature may break down for some instances of CSP, even with small sizes. By constrast the new method - denoted by FAS3T (Fast Algorithm for the Small Size constraints Satisfaction problem Type) - we propose allows an efficient solution of the CSP concrete instances generated by the VATIL vectorizer. Comparative computational results are reported
Quantum Suppression of the Rayleigh Instability in Nanowires
A linear stability analysis of metallic nanowires is performed in the
free-electron model using quantum chaos techniques. It is found that the
classical instability of a long wire under surface tension can be completely
suppressed by electronic shell effects, leading to stable cylindrical
configurations whose electrical conductance is a magic number 1, 3, 5, 6,...
times the quantum of conductance. Our results are quantitatively consistent
with recent experiments with alkali metal nanowires.Comment: 10 pages, 5 eps figures, updated and expanded, accepted for
publication in "Nonlinearity
Bubbling in a co-flow at high Reynolds numbers
The physical mechanisms underlying bubble formation from a needle in a co-flowing liquid
environment at high Reynolds numbers are studied in detail with the aid of experiments and
boundary-integral numerical simulations. To determine the effect of gas inertia the experiments were
carried out with air and helium. The influence of the injection system is elucidated by performing
experiments using two different facilities, one where the constancy of the gas flow-rate entering the
bubble is ensured, and another one where the gas is injected through a needle directly connected to
a pressurized chamber. In the case of constant flow-rate injection conditions, the bubbling frequency
has been shown to hardly depend on the gas density, with a bubble size given by db / ro
? 6U? K *
U + k2 /? U- 1? 1/3 for U? 2, where U is the gas-to-liquid ratio of the mean velocities, ro is
the radius of the gas injection needle, and
k * = 5,84 and k2 = 4,29, whit db / ro3,3U1 / 3 for U1..
Nevertheless, in this case the effect of gas density is relevant to describe the final instants of bubble
breakup, which take place at a time scale much smaller than the bubbling time, tb. This effect is
evidenced by the liquid jets penetrating the gas bubbles upon their pinch-off. Our measurements
indicate that the velocity of the penetrating jets is considerably larger in air bubbles than in helium
bubbles due to the distinct gas inertia of both situations. However, in the case of constant pressure
supply conditions, the bubble size strongly depends on the density of the gas through the pressure
loss along the gas injection needle. Furthermore, under the operating conditions reported here, the
equivalent diameters of the bubbles are between 10% and 20% larger than their constant flow-rate
counterparts. In addition, the experiments and the numerical results show that, under constant
pressure supply, helium bubbles are approximately 10% larger than air bubbles due to the gas
density effect on the bubbling process
Wetting and Minimal Surfaces
We study minimal surfaces which arise in wetting and capillarity phenomena.
Using conformal coordinates, we reduce the problem to a set of coupled boundary
equations for the contact line of the fluid surface, and then derive simple
diagrammatic rules to calculate the non-linear corrections to the Joanny-de
Gennes energy. We argue that perturbation theory is quasi-local, i.e. that all
geometric length scales of the fluid container decouple from the
short-wavelength deformations of the contact line. This is illustrated by a
calculation of the linearized interaction between contact lines on two opposite
parallel walls. We present a simple algorithm to compute the minimal surface
and its energy based on these ideas. We also point out the intriguing
singularities that arise in the Legendre transformation from the pure Dirichlet
to the mixed Dirichlet-Neumann problem.Comment: 22 page
Minimal surfaces bounded by elastic lines
In mathematics, the classical Plateau problem consists of finding the surface
of least area that spans a given rigid boundary curve. A physical realization
of the problem is obtained by dipping a stiff wire frame of some given shape in
soapy water and then removing it; the shape of the spanning soap film is a
solution to the Plateau problem. But what happens if a soap film spans a loop
of inextensible but flexible wire? We consider this simple query that couples
Plateau's problem to Euler's Elastica: a special class of twist-free curves of
given length that minimize their total squared curvature energy. The natural
marriage of two of the oldest geometrical problems linking physics and
mathematics leads to a quest for the shape of a minimal surface bounded by an
elastic line: the Euler-Plateau problem. We use a combination of simple
physical experiments with soap films that span soft filaments, scaling
concepts, exact and asymptotic analysis combined with numerical simulations to
explore some of the richness of the shapes that result. Our study raises
questions of intrinsic interest in geometry and its natural links to a range of
disciplines including materials science, polymer physics, architecture and even
art.Comment: 14 pages, 4 figures. Supplementary on-line material:
http://www.seas.harvard.edu/softmat/Euler-Plateau-problem
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