1,204 research outputs found
The initial stages of cave formation: Beyond the one-dimensional paradigm
The solutional origin of limestone caves was recognized over a century ago,
but the short penetration length of an undersaturated solution made it seem
impossible for long conduits to develop. This is contradicted by field
observations, where extended conduits, sometimes several kilometers long, are
found in karst environments. However, a sharp drop in the dissolution rate of
CaCO_3 near saturation provides a mechanism for much deeper penetration of
reactant. The notion of a "kinetic trigger" - a sudden change in rate constant
over a narrow concentration range - has become a widely accepted paradigm in
speleogenesis modeling. However, it is based on one-dimensional models for the
fluid and solute transport inside the fracture, assuming that the dissolution
front is planar in the direction perpendicular to the flow. Here we show that
this assumption is incorrect; a planar dissolution front in an entirely uniform
fracture is unstable to infinitesimal perturbations and inevitably breaks up
into highly localized regions of dissolution. This provides an alternative
mechanism for cave formation, even in the absence of a kinetic trigger. Our
results suggest that there is an inherent wavelength to the erosion pattern in
dissolving fractures, which depends on the reaction rate and flow rate, but is
independent of the initial roughness. In contrast to one-dimensional models,
two-dimensional simulations indicate that there is only a weak dependence of
the breakthrough time on kinetic order; localization of the flow tends to keep
the undersaturation in the dissolution front above the threshold for non-linear
kinetics.Comment: to be published in Earth and Planetary Science Letter
Spin Waves in the Ferromagnetic Ground State of the Kagome Staircase System Co3V2O8
Inelastic neutron scattering measurements were performed on single crystal
Co3V2O8 wherein magnetic cobalt ions reside on distinct spine and cross-tie
sites within kagome staircase planes. This system displays a rich magnetic
phase diagram which culminates in a ferromagnetic ground state below Tc~6 K. We
have studied the low-lying magnetic excitations in this phase within the kagome
plane. Despite the complexity of the system at higher temperatures, linear
spin-wave theory describes most of the quantitative detail of the inelastic
neutron measurements. Our results show two spin-wave branches, the higher
energy of which displays finite spin-wave lifetimes well below Tc, and
negligible magnetic exchange coupling between Co moments on the spine sites.Comment: 4 pages and 4 figure
Milliarcsecond structure of water maser emission in two young high-mass stellar objects associated with methanol masers
The 22.2 GHz water masers are often associated with the 6.7 GHz methanol
masers but owing to the different excitation conditions they likely probe
independent spatial and kinematic regions around the powering young massive
star. We compared the emission of these two maser species on milliarcsecond
scales to determine in which structures the masers arise and to test a
disc-outflow scenario where the methanol emission arises in a circumstellar
disc while the water emission comes from an outflow. We obtained high-angular
and spectral resolution 22.2 GHz water maser observations of the two sources
G31.581+00.077 and G33.641-00.228 using the EVN. In both objects the water
maser spots form complex and filamentary structures of sizes 18-160 AU. The
emission towards the source G31.581+00.077 comes from two distinct regions of
which one is related to the methanol maser source of ring-like shape. In both
targets the main axis of methanol distribution is orthogonal to the water maser
distribution. Most of water masers appear to trace shocks on a working surface
between an outflow/jet and a dense envelope. Some spots are possibly related to
the disc-wind interface which is as close as 100-150 AU to the regions of
methanol emission.Comment: 10 pages, accepted to Astronomy and Astrophysic
Magnetic field in Cepheus A as deduced from OH maser polarimetric observations
We present the results of MERLIN polarization mapping of OH masers at 1665
and 1667 MHz towards the Cepheus A star-forming region. The maser emission is
spread over a region of 6 arcsec by 10 arcsec, twice the extent previously
detected. In contrast to the 22 GHz water masers, the OH masers associated with
H II regions show neither clear velocity gradients nor regular structures. We
identified ten Zeeman pairs which imply a magnetic field strength along the
line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the
arcsecond scale, pointing towards us in the west and away from us in the east
side. The linearly polarized components, detected for the first time, show
regularities in the polarization position angles depending on their position.
The electric vectors of OH masers observed towards the outer parts of H II
regions are consistent with the interstellar magnetic field orientation, while
those seen towards the centres of H II regions are parallel to the radio-jets.
A Zeeman quartet inside a southern H II region has now been monitored for 25
years; we confirm that the magnetic field decays monotonically over that
period.Comment: 10 pages, 6 figures,accepted for publication in MNRA
Instabilities in the dissolution of a porous matrix
A reactive fluid dissolving the surrounding rock matrix can trigger an
instability in the dissolution front, leading to spontaneous formation of
pronounced channels or wormholes. Theoretical investigations of this
instability have typically focused on a steadily propagating dissolution front
that separates regions of high and low porosity. In this paper we show that
this is not the only possible dissolutional instability in porous rocks; there
is another instability that operates instantaneously on any initial porosity
field, including an entirely uniform one. The relative importance of the two
mechanisms depends on the ratio of the porosity increase to the initial
porosity. We show that the "inlet" instability is likely to be important in
limestone formations where the initial porosity is small and there is the
possibility of a large increase in permeability. In quartz-rich sandstones,
where the proportion of easily soluble material (e.g. carbonate cements) is
small, the instability in the steady-state equations is dominant.Comment: to be published in Geophysical Research Letter
The effects of the functionality of cross-linkable monomers on selected properties of poly(vinyl-chloride)-monomer mixtures after irradiation with fast electrons
Mixtures containing poly resin, plasticizer, stabilizer and various methacrylate monomers were fluxed on a two roll mill, molded, cut into shapes sutiable for testing, irradiated in air with high-energy electrons from a synchronous transformer accelerator and subjected to several tests. The object of the investigation was to determine the effect of the functionality and/or structure of a series of monomers on certain properties of the ploy-monomer mixtures after electron irradiation
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