24 research outputs found
Spiral cracks in drying precipitates
We investigate the formation of spiral crack patterns during the desiccation
of thin layers of precipitates in contact with a substrate. This
symmetry-breaking fracturing mode is found to arise naturally not from torsion
forces, but from a propagating stress front induced by the fold-up of the
fragments. We model their formation mechanism using a coarse-grain model for
fragmentation and successfully reproduce the spiral cracks. Fittings of
experimental and simulation data show that the spirals are logarithmic,
corresponding to constant deviation from a circular crack path. Theoretical
aspects of the logarithmic spirals are discussed. In particular we show that
this occurs generally when the crack speed is proportional to the propagating
speed of stress front.Comment: 4 pages, 5 figures, RevTe
Emission Lines in the Spectrum of the 3He Star 3 Cen A
Emission in the 4d - 4f transitions of MnII (multiplet 13, 6122-6132 Ang), in
the 4f - 6g transitions of PII, and in 6149.5 Ang of HgII has been detected in
the spectrum of the helium weak star 3 Centauri A (B5 III-IVp). Weaker emission
from the same MnII multiplet is also seen in the hot, mild HgMn star 46 Aquila
(B9 III).It is suggested that the emission is of photospheric origin and may be
evidence for the stratification of manganese, phosphorus and mercury in the
photosphere of 3 CenA, and of manganese in 46Aql.Comment: 16 pages, 3 figure
Stable propagation of an ordered array of cracks during directional drying
We study the appearance and evolution of an array of parallel cracks in a
thin slab of material that is directionally dried, and show that the cracks
penetrate the material uniformly if the drying front is sufficiently sharp. We
also show that cracks have a tendency to become evenly spaced during the
penetration. The typical distance between cracks is mainly governed by the
typical distance of the pattern at the surface, and it is not modified during
the penetration. Our results agree with recent experimental work, and can be
extended to three dimensions to describe the properties of columnar polygonal
patterns observed in some geological formations.Comment: 8 pages, 4 figures, to appear in PR
Development and geometry of isotropic and directional shrinkage crack patterns
We have studied shrinkage crack patterns which form when a thin layer of an
alumina/water slurry dries. Both isotropic and directional drying were studied.
The dynamics of the pattern formation process and the geometric properties of
the isotropic crack patterns are similar to what is expected from recent
models, assuming weak disorder. There is some evidence for a gradual increase
in disorder as the drying layer become thinner, but no sudden transition, in
contrast to what has been seen in previous experiments. The morphology of the
crack patterns is influenced by drying gradients and front propagation effects,
with sharp gradients having a strong orienting and ordering effect.Comment: 8 pages, 11 figures, 8 in jpg format, 3 in postscript. See also
http://mobydick.physics.utoronto.ca/mud.htm
Discovery of the magnetic field in the pulsating B star beta Cephei
Although the star itself is not He enriched, the periodicity and the
variability in the UV wind lines of the pulsating B1 IV star beta Cep are
similar to what is observed in magnetic He-peculiar B stars, suggesting that
beta Cep is magnetic. We searched for a magnetic field using
spectropolarimetry. From UV spectroscopy, we analysed the wind variability and
investigated the correlation with the magnetic data. Using 130 time-resolved
circular polarisation spectra, obtained with the MuSiCoS spectropolarimeter at
the 2m TBL from 1998 until 2005, we applied the least-squares deconvolution
method on the Stokes V spectra and derived the longitudinal component of the
integrated magnetic field over the visible hemisphere of the star. We performed
a period analysis on the magnetic data and on EW measurements of UV wind lines
obtained over 17 years. We also analysed the short- and long-term radial
velocity variations, which are due to the pulsations and the 90-year binary
motion. beta Cep hosts a sinusoidally varying magnetic field with an amplitude
97(4) G and an average value -6(3) G. From the UV wind line variability, we
derive a period of 12.00075(11) days, which is the rotation period of the star,
and is compatible with the observed magnetic modulation. Phases of maximum and
minimum field match those of maximum emission in the UV wind lines, strongly
supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour
as a function of pulsation behaviour and UV line variability. This paper
presents the analysis of the first confirmed detection of a dipolar magnetic
field in an upper main-sequence pulsating star. Maximum wind absorption
originates in the magnetic equatorial plane. Maximum emission occurs when the
magnetic north pole points to the Earth. Radial velocities agree with the ~90-y
orbit around its Be-star binary companion.Comment: 14 pages, 10 figures, 5 table
Effect of Interaction on the Formation of Memories in Paste
A densely packed colloidal suspension with plasticity, called paste, is known
to remember directions of vibration and flow. These memories in paste can be
visualized by the morphology of desiccation crack patterns. Here, we find that
paste made of charged colloidal particles cannot remember flow direction. If we
add sodium chloride into such paste to screen the Coulombic repulsive
interaction between particles, the paste comes to remember flow direction. That
is, one drop of salt water changes memory effect in the paste and thereby we
can tune the morphology of desiccation crack patterns more precisely.Comment: 10 pages, 11 figures, Title change
Weak magnetic fields in Ap/Bp stars: evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy
Publisher's version/PDFAims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of
Ap/Bp stars.
Methods. Using the MuSiCoS and NARVAL spectropolarimeters at TĂ©lescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample
stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median [sigma] = 40 G).
Results. For the 28 studied stars, we obtained 27 detections of Stokes V Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to
infer the dipolar field intensity (B[subscript d], measured at the magnetic pole) and the magnetic obliquity ([Beta]). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G.We interpret this “magnetic threshold” as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars
Drying colloidal systems: laboratory models for a wide range of applications
The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes. The scope of this review is the capacity to tune such systems to reproduce and explore specific properties in a physics laboratory. A wide variety of systems are presented, ranging from functional coatings, food science, cosmetology, medical diagnostics and forensics to geophysics and art