427 research outputs found
Lattice-Boltzmann simulations of the dynamics of liquid barrels
We study the relaxation towards equilibrium of a liquid barrel—a partially wetting droplet in a wedge geometry—using a diffuse-interface approach. We formulate a hydrodynamic model of the motion of the barrel in the framework of the Navier-Stokes and Cahn-Hilliard equations of motion. We present a lattice-Boltzmann method to integrate the diffuse-interface equations, where we introduce an algorithm to model the dynamic wetting of the liquid on smooth solid boundaries. We present simulation results of the over-damped dynamics of the liquid barrel. We find that the relaxation of the droplets is driven by capillary forces and damped by friction forces. We show that the friction is determined by the contribution of the bulk flow, the corner flow near the contact lines and the motion of the contact lines by comparing simulation results for the relaxation time of the barrel. Our results are in broad agreement with previous analytical predictions based on a sharp interface model
A sublimation heat engine
Heat engines are based on the physical realization of a thermodynamic cycle, most famously the liquid–vapour Rankine cycle used for steam engines. Here we present a sublimation heat engine, which can convert temperature differences into mechanical work via the Leidenfrost effect. Through controlled experiments, quantified by a hydrodynamic model, we show that levitating dry-ice blocks rotate on hot turbine-like surfaces at a rate controlled by the turbine geometry, temperature difference and solid material properties. The rotational motion of the dry-ice loads is converted into electric power by coupling to a magnetic coil system. We extend our concept to liquid loads, generalizing the realization of the new engine to both sublimation and the instantaneous vapourization of liquids. Our results support the feasibility of low-friction in situ energy harvesting from both liquids and ices. Our concept is potentially relevant in challenging situations such as deep drilling, outer space exploration or micro-mechanical manipulation
Growth saturation of unstable thin films on transverse-striped hydrophilic-hydrophobic micropatterns
Using three-dimensional numerical simulations, we demonstrate the growth
saturation of an unstable thin liquid film on micropatterned
hydrophilic-hydrophobic substrates. We consider different transverse-striped
micropatterns, characterized by the total fraction of hydrophilic coverage and
the width of the hydrophilic stripes. We compare the growth of the film on the
micropatterns to the steady states observed on homogeneous substrates, which
correspond to a saturated sawtooth and growing finger configurations for
hydrophilic and hydrophobic substrates, respectively. The proposed
micropatterns trigger an alternating fingering-spreading dynamics of the film,
which leads to a complete suppression of the contact line growth above a
critical fraction of hydrophilic stripes. Furthermore, we find that increasing
the width of the hydrophilic stripes slows down the advancing front, giving
smaller critical fractions the wider the hydrophilic stripes are. Using
analytical approximations, we quantitatively predict the growth rate of the
contact line as a function of the covering fraction, and predict the threshold
fraction for saturation as a function of the stripe width.Comment: 11 pages, 5 figure
Passing to an effective 4D phantom cosmology from 5D vacuum theory of gravity
Starting from a five-dimensional (5D) vacuum theory of gravity where the
extra coordinate is considered as noncompact, we investigate the possibility of
inducing four-dimensional (4D) phantom scenarios by applying form-invariance
symmetry transformations. In particular we obtain phantom scenarios for two
cosmological frameworks. In the first framework we deal with an induced 4D
de-Sitter expansion and in the second one a 4D induced model where the
expansion of the universe is dominated by a decreasing cosmological parameter
is discussed.Comment: version accepted in Physics Letters
Super exponential inflation from a dynamical foliation of a 5D vacuum state
We introduce super exponential inflation () from a 5D
Riemann-flat canonical metric on which we make a dynamical foliation. The
resulting metric describes a super accelerated expansion for the early universe
well-known as super exponential inflation that, for very large times, tends to
an asymptotic de Sitter (vacuum dominated) expansion. The scalar field
fluctuations are analyzed. The important result here obtained is that the
spectral index for energy density fluctuations is not scale invariant, and for
cosmological scales becomes . However, for astrophysical
scales this spectrum changes to negative values .Comment: Final version, to be published in Phys. Lett.
Extra force and extra mass from noncompact Kaluza-Klein theory in a cosmological model
Using the Hamilton-Jacobi formalism, we study extra force and extra mass in a
recently introduced noncompact Kaluza-Klein cosmological model. We examine the
inertial 4D mass of the inflaton field on a 4D FRW bulk in two examples.
We find that has a geometrical origin and antigravitational effects on a
non inertial 4D bulk should be a consequence of the motion of the fifth
coordinate with respect to the 4D bulk.Comment: final version to be published in EPJ
Cosmological expansion governed by a scalar field from a 5D vacuum
We consider a single field governed expansion of the universe from a five
dimensional (5D) vacuum state. Under an appropiate change of variables the
universe can be viewed in a effective manner as expanding in 4D with an
effective equation of state which describes different epochs of its evolution.
In the example here worked the universe fistly describes an inflationary phase,
followed by a decelerated expansion. Thereafter, the universe is accelerated
and describes a quintessential expansion to finally, in the future, be vacuum
dominated.Comment: version accepted in Phys. Lett.
Noncompact KK theory of gravity: stochastic treatment for a nonperturbative inflaton field in a de Sitter expansion
We study a stochastic formalism for a nonperturbative treatment of the
inflaton field in the framework of a noncompact Kaluza-Klein (KK) theory during
an inflationary (de Sitter) expansion, without the slow-roll approximation.Comment: version to be published in Phys. Lett.
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