12,748 research outputs found
Quantitative study of laterally inhomogeneous wetting films
Based on a microscopic density functional theory we calculate the internal
structure of the three-phase contact line between liquid, vapor, and a
confining wall as well as the morphology of liquid wetting films on a substrate
exhibiting a chemical step. We present a refined numerical analysis of the
nonlocal density functional which describes the interface morphologies and the
corresponding line tensions. These results are compared with those predicted by
a more simple phenomenological interface displacement model. Except for the
case that the interface exhibits large curvatures, we find that the interface
displacement model provides a quantitatively reliable description of the
interfacial structures.Comment: 31 pages, RevTeX, 13 figure
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
Interface Unbinding in Structured Wedges
The unbinding properties of an interface near structured wedges are
investigated by discrete models with short range interactions. The calculations
demonstrate that interface unbinding take place in two stages: ) a
continuous filling--like transition in the pure wedge--like parts of the
structure; ) a conclusive discontinuous unbinding. In 2 an exact
transfer matrix approach allows to extract the whole interface phase diagram
and the precise mechanism at the basis of the phenomenon. The Metropolis Monte
Carlo simulations performed in 3 reveal an analogous behavior. The emerging
scenario allows to shed new light onto the problem of wetting of geometrically
rough walls.Comment: 5 pages, 5 figures, to appear in Phys. Rev.
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
Bulk and wetting phenomena in a colloidal mixture of hard spheres and platelets
Density functional theory is used to study binary colloidal fluids consisting
of hard spheres and thin platelets in their bulk and near a planar hard wall.
This system exhibits liquid-liquid coexistence of a phase that is rich in
spheres (poor in platelets) and a phase that is poor in spheres (rich in
platelets). For the mixture near a planar hard wall, we find that the phase
rich in spheres wets the wall completely upon approaching the liquid demixing
binodal from the sphere-poor phase, provided the concentration of the platelets
is smaller than a threshold value which marks a first-order wetting transition
at coexistence. No layering transitions are found in contrast to recent studies
on binary mixtures of spheres and non-adsorbing polymers or thin hard rods.Comment: 6 pages, 4 figure
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Magnetization and susceptibility of ferrofluids
A second-order Taylor series expansion of the free energy functional provides
analytical expressions for the magnetic field dependence of the free energy and
of the magnetization of ferrofluids, here modelled by dipolar Yukawa
interaction potentials. The corresponding hard core dipolar Yukawa reference
fluid is studied within the framework of the mean spherical approximation. Our
findings for the magnetic and phase equilibrium properties are in quantitative
agreement with previously published and new Monte Carlo simulation data.Comment: 8 pages including 4 figure
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The development of the passé composé in lower-intermediate learners of French as a second language
In this study we tracked the development of the passe compose in second-language learners of French whose first language is English. Although the passe compose is a highly used tense among native speakers of French and it appears to present particular difficulty for first-language English speakers, its second-language development has been surprisingly under-researched. In order to trace developmental patterns of the passe compose we obtained a corpus of obligatory context use of this tense by 30 Year-12 (lower-intermediate) students at two time points six months apart and analysed the data both quantitatively and qualitatively. Our findings suggest that these students used remarkably few memorized formulas, that they passed through five distinct stages in their acquisition of the passe compose, that those early stages were characterized by transfer errors, and that the presence of the auxiliary, whether correct or incorrect, formed a crucial stage in the development of the tense. Theoretical explanations for the findings are presented together with some tentative pedagogical implications
Phase diagram for morphological transitions of wetting films on chemically structured substrates
Using an interface displacement model we calculate the shapes of thin
liquidlike films adsorbed on flat substrates containing a chemical stripe. We
determine the entire phase diagram of morphological phase transitions in these
films as function of temperature, undersaturation, and stripe widthComment: 15 pages, RevTeX, 7 Figure
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