622 research outputs found
Flow patterns generated by oblate medusan jellyfish: field measurements and laboratory analyses
Flow patterns generated by medusan swimmers such as
jellyfish are known to differ according the morphology of
the various animal species. Oblate medusae have been
previously observed to generate vortex ring structures
during the propulsive cycle. Owing to the inherent
physical coupling between locomotor and feeding
structures in these animals, the dynamics of vortex ring
formation must be robustly tuned to facilitate effective
functioning of both systems. To understand how this is
achieved, we employed dye visualization techniques on
scyphomedusae (Aurelia aurita) observed swimming in
their natural marine habitat. The flow created during each
propulsive cycle consists of a toroidal starting vortex
formed during the power swimming stroke, followed by a
stopping vortex of opposite rotational sense generated
during the recovery stroke. These two vortices merge in a
laterally oriented vortex superstructure that induces flow
both toward the subumbrellar feeding surfaces and
downstream. The lateral vortex motif discovered here
appears to be critical to the dual function of the medusa
bell as a flow source for feeding and propulsion.
Furthermore, vortices in the animal wake have a greater
volume and closer spacing than predicted by prevailing
models of medusan swimming. These effects are shown to
be advantageous for feeding and swimming performance,
and are an important consequence of vortex interactions
that have been previously neglected
Time Evolution of Decay Spectrum in K^0, K^0 bar -> pi^+ pi^- e^+ e^-
We consider the decay K^0 (K^0 bar) -> pi^+ pi^- e^+ e^- of a neutral K meson
prepared in a state of strangeness +1 (-1). The time evolution of the state
produces remarkable time-dependent effects in the angular distribution of the
pi^+ pi^- e^+ e^- system. These effects are correlated with the time-dependence
of the photon polarization in the radiative decay K^0 (K^0 bar) -> pi^+ pi^-
gamma. We study, in particular, the CP-odd, T-odd term in the distribution d
Gamma / d phi of the angle between the pi^+ pi^- and the e^+ e^- planes. We
also give the spectrum in the case that the decaying meson is an incoherent
mixture of K^0 and K^0 bar, and discuss the case of K_S regeneration in a K_L
beam.Comment: 16 pages, 8 figures, minor typos corrected, to appear in Phys. Lett.
Two Notions Of Safety
Timothy Williamson (1992, 224–5) and Ernest Sosa (1996) have ar- gued that knowledge requires one to be safe from error. Something is said to be safe from happening iff it does not happen at “close” worlds. I expand here on a puzzle noted by John Hawthorne (2004, 56n) that suggests the need for two notions of closeness. Counterfac- tual closeness is a matter of what could in fact have happened, given the specific circumstances at hand. The notion is involved in the semantics for counterfactuals and is the one epistemologists have typically assumed. Normalized closeness is rather a matter of what could typically have happened, that is, what would go on in a class of normal alternatives to actuality, irrespectively of whether or not they could have happened in the circumstances at hand
A mechanophysical phase transition provides a dramatic example of colour polymorphism: the tribochromism of a substituted tri(methylene)tetrahydrofuran-2-one
BACKGROUND: Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed "tribochromism". We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies. RESULTS: A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb - which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb. CONCLUSIONS: Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice. Graphical abstractGraphical representation of the structural and colour change in the tribochromic compound (III)
Violation of Time Reversal Invariance in the Decays K_L -> pi^+ pi^- gamma and K_L -> pi^+ pi^- e^+ e^-
The origin of the large CP-odd and T-odd asymmetry observed in the decay K_L
-> pi^+ pi^- e^+ e^- is traced to the polarization properties of the photon in
the decay K_L -> pi^+ pi^- gamma. The Stokes vector of the photon S = (S_1,
S_2, S_3) is studied as a function of the photon energy and found to possess
CP-violating components S_1 and S_2 which are sizeable over a large part of the
phase space, despite being proportional to the epsilon parameter of the K_L
wave function. The component S_2 is T-even and manifests itself as a circular
polarization of the photon, while S_1 is T-odd and gives rise to the asymmetry
observed in K_L -> pi^+ pi^- e^+ e^-. The latter is shown to survive in the
``hermitian'' limit in which all unitarity phases are absent, and represents a
genuine example of time reversal symmetry breaking in a CPT invariant theory.Comment: 4 pages including 2 figure
Improved Synthesis of 2,6,6-trimethyl-1-cyclohexene-1-acetaldehyde, a Key Intermediate for Drimane-related Sesquiterpenes.
The titel compound is conveniently prepared in 65% overall yield by a two step synthesis starting from he commercially available β-ionone
Post-depositional subsidence of the Avellino tephra marker bed in the Pontine plain (Lazio, Italy):Implications for Early Bronze Age palaeogeographical, water level and relative sea level reconstruction
Land subsidence has played and is still playing a significant role in coastal wetlands worldwide and in palaeogeographical reconstructions of such wetlands. The varying thickness of compaction-prone sediments over a stable subsurface is a key factor in determining its magnitude and in locating the most affected areas. In the coastal low-lying Agro Pontino (Lazio, Italy), subsidence of the past 90 years has been mapped using historical elevation data. Due to the fortunate preservation of distal Avellino tephra (AV-tephra, ca. 1900 cal. BCE) within its marshy strata, discovered a decade ago, detailed palaeogeographical reconstruction of the landscape in preparation for an assessment of its land use suitability in the Early Bronze Age (EBA) was possible. Current altitude variations of water-lain tephra in lake areas assumed to be connected necessitated a closer look at its original deposition altitude and the role of post-depositional subsidence. Recent subsidence patterns proved very useful for distinguishing stable from subsidence-prone areas. Two different EBA palaeo lake environments are distinguished: an inland and a near-coastal lake. The AV-tephra altitude variation within these lakes partly marks differential post-depositional subsidence within these lakes. Calculation of initial ripening of tephra-bearing lake deposits on top of shallowly buried Pleistocene ridges allowed for an estimation of original tephra deposition altitudes and associated lake levels. For the inland lake, a wide lake edge zone between 0.5 and 2 m above current sea level (m asl) was reconstructed, where EBA habitation or land use was possible. At the near-coastal lake, a water level of −1.5 to −1.3 m asl at the time of AV-tephra deposition was constrained. Because tephra deposition occurred here just after marine influence ceased, this altitude range is proposed to be a Relative Sea Level (RSL) index point at the time of AV-tephra deposition. The altitude range is in agreement with RSL models for tectonically stable areas in this region. The importance of subsidence in palaeogeographical, water level and RSL reconstructions in the region is stressed
Stokes Vector of Photon in the Decays B^0 -> rho^0 gamma and B^0 -> K^* gamma
We consider a model for the decay Bbar^0 -> rho^0 gamma in which the
short-distance amplitude determined by the Hamiltonian describing b -> d gamma
is combined with a typical long-distance contribution Bbar^0 -> D^+ D^- ->
rho^0 gamma. The latter possesses a significant dynamical phase which induces a
CP-violating asymmetry A_CP, as well as an important modification of the Stokes
vector of the photon. The components S_1 and S_3 of the Stokes vector S = (S_1,
S_2, S_3) can be measured in the decay Bbar^0 -> rho^0 gamma^* -> pi^+ pi^- e^+
e^- where they produce a characteristic effect in the angular distribution d
Gamma / d phi, phi being the angle between the pi^+ pi^- and e^+ e^- planes. A
similar analysis is carried out for the decays Bbar^0 -> Kbar^* gamma and
Bbar^0 -> Kbar^* gamma^* -> pi^+ K^- e^+ e^-Comment: 13 pages, 8 figures; v2: Some clarifying remarks added, together with
additional references. Minor typos corrected. No change in result
Large Direct CP Violation in B^0 -> pi^+ pi^- and an Enhanced Branching Ratio for B^0 -> pi^0 pi^0
Recent measurements of B^0 -> pi pi decays reveal two features that are in
conflict with conventional calculations: the channel B^0 (Bbar^0) -> pi^+ pi^-
shows a large direct CP-violating asymmetry, and the channel B^0 (B^0) -> pi^0
pi^0 has an unexpectedly high branching ratio. We show that both features can
be understood in terms of strong-interaction mixing of pi pi and D Dbar
channels in the isospin-zero state, an effect that is important because of the
large experimentally observed ratio Gamma(B^0 / Bbar^0 -> D^+ D^-) / Gamma (B^0
/ Bbar^0 -> pi^+ pi^-) approx. 50. Our dynamical model correlates the branching
ratios and the CP-violating parameters C and S, for the decays B^0 (Bbar^0) ->
pi^+ pi^-, B^0 (Bbar^0) -> pi^0 pi^0, B^0 (Bbar^0) -> D^+ D^- and B^0 (Bbar^0)
-> D^0 Dbar^0.Comment: 10 pages, 2 figures, 1 table; v2: Misprint corrected in Eq. (12),
second line: -a_m replaced by +a_m. To appear in Phys. Lett.
Crystal structure prediction of organic pigments: quinacridone as an example
The structures of the α, β and γ polymorphs of quinacridone were predicted using Polymorph Predictor software in combination with X-ray powder diffraction patterns of limited quality. The present work demonstrates a method to obtain crystal structures of industrially important pigments when only a low-quality powder pattern is available
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