9,830 research outputs found
Percolation study for the capillary ascent of a liquid through a granular soil
Capillary rise plays a crucial role in the construction of road embankments
in flood zones, where hydrophobic compounds are added to the soil to suppress
the rising of water and avoid possible damage of the pavement. Water rises
through liquid bridges, menisci and trimers, whose width and connectivity
depends on the maximal half-length {\lambda} of the capillary bridges among
grains. Low {\lambda} generate a disconnect structure, with small clusters
everywhere. On the contrary, for high {\lambda}, create a percolating cluster
of trimers and enclosed volumes that form a natural path for capillary rise.
Hereby, we study the percolation transition of this geometric structure as a
function of {\lambda} on a granular media of monodisperse spheres in a random
close packing. We determine both the percolating threshold {\lambda}_{c} =
(0.049 \pm 0.004)R (with R the radius of the granular spheres), and the
critical exponent of the correlation length {\nu} = (0.830 \pm 0.051),
suggesting that the percolation transition falls into the universality class of
ordinary percolation
A geometrical analysis of the field equations in field theory
In this review paper we give a geometrical formulation of the field equations
in the Lagrangian and Hamiltonian formalisms of classical field theories (of
first order) in terms of multivector fields. This formulation enables us to
discuss the existence and non-uniqueness of solutions, as well as their
integrability.Comment: 14 pages. LaTeX file. This is a review paper based on previous works
by the same author
Investigation of HNCO isomers formation in ice mantles by UV and thermal processing: an experimental approach
Current gas phase models do not account for the abundances of HNCO isomers
detected in various environments, suggesting a formation in icy grain mantles.
We attempted to study a formation channel of HNCO and its possible isomers by
vacuum-UV photoprocessing of interstellar ice analogues containing HO,
NH, CO, HCN, CHOH, CH, and N followed by warm-up, under
astrophysically relevant conditions. Only the HO:NH:CO and HO:HCN
ice mixtures led to the production of HNCO species. The possible isomerization
of HNCO to its higher energy tautomers following irradiation or due to ice
warm-up has been scrutinized. The photochemistry and thermal chemistry of
HO:NH:CO and HO:HCN ices was simulated using the Interstellar
Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The
ice was monitored in situ by Fourier transform mid-infrared spectroscopy in
transmittance. A quadrupole mass spectrometer (QMS) detected the desorption of
the molecules in the gas phase. UV-photoprocessing of
HO:NH:CO/HO:HCN ices lead to the formation of OCN as main
product in the solid state and a minor amount of HNCO. The second isomer HOCN
has been tentatively identified. Despite its low efficiency, the formation of
HNCO and the HOCN isomers by UV-photoprocessing of realistic simulated ice
mantles, might explain the observed abundances of these species in PDRs, hot
cores, and dark clouds
Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface
The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been
investigated by density functional {\it ab-initio} calculations. An on top
H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be
energetically favorable. The H covered surface is metallic and spin-polarized,
with a noticeable magnetic moment at the surface region. Lower hydrogen
coverages lead to strengthening of the H-Zn bonds, corrugation of the surface
layer and to an insulating surface. Our results explain experimental
observations of hydrogen adsorption on this surface, and not only predict a
metal-insulator transition, but primarily provide a method to reversible switch
surface magnetism by varying the hydrogen density on the surface.Comment: 4 pages, 3 figure
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