15,693 research outputs found
Wetting and capillary nematization of binary hard-platelet and hard-rod fluids
Density-functional theory is used to investigate the phase behavior of
colloidal binary hard-platelet and hard-rod fluids near a single hard wall or
confined in a slit pore. The Zwanzig model, in which the orientations of the
particles of rectangular shape are restricted to three orthogonal orientations,
is analyzed by numerical minimization of the grand potential functional. The
density and orientational profiles as well as the surface contributions to the
grand potential are determined. The calculations exhibit a wall-induced
continuous surface transition from uniaxial to biaxial symmetry for the
hard-rod fluid. Complete wetting of the wall -- isotropic liquid interface by a
biaxial nematic film for rods and a uniaxial nematic film for platelets is
found. For the fluids confined by two parallel hard walls we determine a
first-order capillary nematization transition for large slit widths, which
terminates in a capillary critical point upon decreasing the slit width.Comment: 11 pages, 11 figure
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
Possible multiple evolution of indirect telencephalo-cerebellar pathways in teleosts: studies in Carassius auratus and Pantodon buchholzi
Among vertebrates, telencephalo-pontine systems exist only in birds and mammals. However, three nuclei in the diencephalon and mesencephalon of teleost fishes have been indicated — analogous to the pons — to represent relay stations between telencephalon and cerebellum. Since two of these nuclei (dorsal preglomerular nucleus, dorsal tegmental nucleus) have only been described in the highly derived, electrosensory mormyrids, we investigated telencephalic connections in two nonelectrosensory teleosts, the goldfish Carassius auratus and the freshwater butterflyfish Pantodon buchholzi, and cerebellar connections only in the latter species, since for C. auratus these connections are already established. Horseradish peroxidase tracing reveals that C. auratus has a dorsal tegmental nucleus and a paracommissural nucleus both of which are telencephalo-recipient and project to the cerebellum, and that P. buchholzi has a dorsal preglomerular nucleus with such connections. These results extend our knowlegde of the distribution and, therefore, the phylogeny of telencephalo-cerebellar systems in teleosts. Similar to tetrapods, teleosts appear to have developed telencephalo-cerebellar systems several times independently
Colloidal hard-rod fluids near geometrically structured substrates
Density functional theory is used to study colloidal hard-rod fluids near an
individual right-angled wedge or edge as well as near a hard wall which is
periodically patterned with rectangular barriers. The Zwanzig model, in which
the orientations of the rods are restricted to three orthogonal orientations
but their positions can vary continuously, is analyzed by numerical
minimization of the grand potential. Density and orientational order profiles,
excess adsorptions, as well as surface and line tensions are determined. The
calculations exhibit an enrichment [depletion] of rods lying parallel and close
to the corner of the wedge [edge]. For the fluid near the geometrically
patterned wall, complete wetting of the wall -- isotropic liquid interface by a
nematic film occurs as a two-stage process in which first the nematic phase
fills the space between the barriers until an almost planar isotropic --
nematic liquid interface has formed separating the higher-density nematic fluid
in the space between the barriers from the lower-density isotropic bulk fluid.
In the second stage a nematic film of diverging film thickness develops upon
approaching bulk isotropic -- nematic coexistence.Comment: 9 pages, 9 figure
Fire suppression in human-crew spacecraft
Fire extinguishment agents range from water and foam in early-design spacecraft (Halon 1301 in the present Shuttle) to carbon dioxide proposed for the Space Station Freedom. The major challenge to spacecraft fire extinguishment design and operations is from the micro-gravity environment, which minimizes natural convection and profoundly influences combustion and extinguishing agent effectiveness, dispersal, and post-fire cleanup. Discussed here are extinguishment in microgravity, fire-suppression problems anticipated in future spacecraft, and research needs and opportunities
Moduli decay in the hot early Universe
We consider moduli fields interacting with thermalized relativistic matter.
We determine the temperature dependence of their damping rate and find it is
dominated by thermal effects in the high temperature regime, i.e. for
temperatures larger than their mass. For a simple scalar model the damping rate
is expressed through the known matter bulk viscosity. The high temperature
damping rate is always smaller than the Hubble rate, so that thermal effects
are not sufficient for solving the cosmological moduli problem.Comment: Numerical error in the final result for the damping rate corrected,
conclusions of the paper are not affecte
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