14,641 research outputs found
Diffusive spreading and mixing of fluid monolayers
The use of ultra-thin, i.e., monolayer films plays an important role for the
emerging field of nano-fluidics. Since the dynamics of such films is governed
by the interplay between substrate-fluid and fluid-fluid interactions, the
transport of matter in nanoscale devices may be eventually efficiently
controlled by substrate engineering. For such films, the dynamics is expected
to be captured by two-dimensional lattice-gas models with interacting
particles. Using a lattice gas model and the non-linear diffusion equation
derived from the microscopic dynamics in the continuum limit, we study two
problems of relevance in the context of nano-fluidics. The first one is the
case in which along the spreading direction of a monolayer a mesoscopic-sized
obstacle is present, with a particular focus on the relaxation of the fluid
density profile upon encountering and passing the obstacle. The second one is
the mixing of two monolayers of different particle species which spread side by
side following the merger of two chemical lanes, here defined as domains of
high affinity for fluid adsorption surrounded by domains of low affinity for
fluid adsorption.Comment: 12 pages, 3 figure
Stability of liquid ridges on chemical micro- and nanostripes
We analyze the stability of sessile filaments (ridges) of nonvolatile liquids
versus pearling in the case of externally driven flow along a chemical stripe
within the framework of the thin film approximation. The ridges can be stable
with respect to pearling even if the contact line is not completely pinned. A
generalized stability criterion for moving contact lines is provided. For large
wavelengths and no drive, within perturbation theory, an analytical expression
of the growth rate of pearling instabilities is derived. A numerical analysis
shows that drive further stabilizes the ridge by reducing the growth rate of
unstable perturbations, even though there is no complete stabilization. Hence
the stability criteria established without drive ensure overall stability.Comment: 10 pages, 6 figure
Motion of nanodroplets near edges and wedges
Nanodroplets residing near wedges or edges of solid substrates exhibit a
disjoining pressure induced dynamics. Our nanoscale hydrodynamic calculations
reveal that non-volatile droplets are attracted or repelled from edges or
wedges depending on details of the corresponding laterally varying disjoining
pressure generated, e.g., by a possible surface coating.Comment: 12 pages, 7 figure
High Redshift Quasars and Star Formation in the Early Universe
In order to derive information on the star formation history in the early
universe we observed 6 high-redshift (z=3.4) quasars in the near-infrared to
measure the relative iron and \mgii emission strengths. A detailed comparison
of the resulting spectra with those of low-redshift quasars show essentially
the same FeII/MgII emission ratios and very similar continuum and line spectral
properties, indicating a lack of evolution of the relative iron to magnesium
abundance of the gas since z=3.4 in bright quasars. On the basis of current
chemical evolution scenarios of galaxies, where magnesium is produced in
massive stars ending in type II SNe, while iron is formed predominantly in SNe
of type Ia with a delay of ~1 Gyr and assuming as cosmological parameters H_o =
72 km/s Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7$, we conclude that major
star formation activity in the host galaxies of our z=3.4 quasars must have
started already at an epoch corresponding to z_f ~= 10, when the age of the
universe was less than 0.5 Gyrs.Comment: 29 pages, 5 figures, ApJ in pres
Fluid adsorption near an apex: Covariance between complete and critical wetting
Critical wetting is an elusive phenomenon for solid-fluid interfaces. Using
interfacial models we show that the diverging length scales, which characterize
complete wetting at an apex, precisely mimic critical wetting with the apex
angle behaving as the contact angle. Transfer matrix, renormalization group
(RG) and mean field analysis (MF) shows this covariance is obeyed in 2D, 3D and
for long and short ranged forces. This connection should be experimentally
accesible and provides a means of checking theoretical predictions for critical
wetting.Comment: 4 pages, 1 figure, submitted to Physical Review Letter
Filling transition for a wedge
We study the formation and the shape of a liquid meniscus in a wedge with
opening angle which is exposed to a vapor phase. By applying a suitable
effective interface model, at liquid-vapor coexistence and at a temperature
we find a filling transition at which the height of the meniscus
becomes macroscopically large while the planar walls of the wedge far away from
its center remain nonwet up to the wetting transition occurring at
. Depending on the fluid and the substrate potential the filling
transition can be either continuous or discontinuous. In the latter case it is
accompanied by a prefilling line extending into the vapor phase of the bulk
phase diagram and describing a transition from a small to a large, but finite,
meniscus height. The filling and the prefilling transitions correspond to
nonanalyticities in the surface and line contributions to the free energy of
the fluid, respectively.Comment: 48 pages (RevTex), 14 figures (ps), submitted to PR
Critical Casimir forces between planar and crenellated surfaces
We study critical Casimir forces between planar walls and geometrically
structured substrates within mean-field theory. As substrate structures,
crenellated surfaces consisting of periodic arrays of rectangular crenels and
merlons are considered. Within the widely used proximity force approximation,
both the top surfaces of the merlons and the bottom surfaces of the crenels
contribute to the critical Casimir force. However, for such systems the full,
numerically determined critical Casimir forces deviate significantly fromthe
pairwise addition formalismunderlying the proximity force approximation. A
first-order correction to the proximity force approximation is presented in
terms of a step contribution arising from the critical Casimir interaction
between a planar substrate and the right-angled steps of the merlons consisting
of their upper and lower edges as well as their sidewalls.Comment: 9 pages, 6 figure
Bulk and interfacial properties of binary hard-platelet fluids
Interfaces between demixed fluid phases of binary mixtures of hard platelets
are investigated using density-functional theory. The corresponding excess free
energy functional is calculated within a fundamental measure theory adapted to
the Zwanzig model, in which the orientations of the particles of rectangular
shape are restricted to three orthogonal orientations. Density and
orientational order parameter profiles at interfaces between coexisting phases
as well as the interfacial tension are determined. A density inversion,
oscillatory density profiles, and a Fisher-Widom line have been found in a
mixture of large thin and small thick platelets. The lowest interfacial tension
corresponds to the mean bulk orientation of the platelets being parallel to the
interface. For a mixture of large and small thin platelets, complete wetting of
an isotropic-nematic interface by a second nematic phase is found.Comment: 7 pages, 6 figure
Phase diagram of a model for 3He-4He mixtures in three dimensions
A lattice model of 3He - 4He mixtures which takes into account the continuous
rotational symmetry O(2) of the superfluid degrees of freedom of 4He is studied
in the molecular-field approximation and by Monte Carlo simulations in three
dimensions. In contrast to its two-dimensional version, for reasonable values
of the interaction parameters the resulting phase diagram resembles that
observed experimentally for 3He - 4He mixtures, for which phase separation
occurs as a consequence of the superfluid transition. The corresponding
continuum Ginzburg-Landau model with two order parameters describing 3He- 4He
mixtures near tricriticality is derived from the considered lattice model. All
coupling constants appearing in the continuum model are explicitly expressed in
terms of the mean concentration of 4He, the temperature, and the microscopic
interaction parameters characterizing the lattice system.Comment: 32 pages, 12 figures, submitted to the Phys. Rev.
Motion of nanodroplets near chemical heterogeneities
We investigate the dynamics of nanoscale droplets in the vicinity of chemical
steps which separate parts of a substrate with different wettabilities. Due to
long-ranged dispersion forces, nanodroplets positioned on one side of the step
perceive the different character of the other side even at some distances from
the step, leading to a dynamic response. The direction of the ensuing motion of
such droplets does not only depend on the difference between the equilibrium
contact angles on these two parts but in particular on the difference between
the corresponding Hamaker constants. Therefore the motion is not necessarily
directed towards the more wettable side and can also be different from that of
droplets which span the step.Comment: 6 pages, 6 figure
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