1,037 research outputs found
Convective instabilities in two superposed horizontal liquid layers heated laterally
This work is devoted to the theoretical study of the stability of two
superposed horizontal liquid layers bounded by two solid planes and subjected
to a horizontal temperature gradient.
The liquids are supposed to be immiscible with a nondeformable interface.
The forces acting on the system are buoyancy and interfacial tension. Four
different flow patterns and temperature profiles are found for the basic state.
A linear perturbative analysis with respect to two and three dimensional
perturbations reveals the existence of three kind of patterns. Depending on the
relative height of both liquids several situations are predicted: either wave
propagation from cold to the hot regions, or waves propagating in the opposite
direction or still stationary longitudinal rolls. The behavior of three
different pairs of liquids which have been used in experiments on bilayers
under vertical gradient by other authors have been examined. The instability
mechanisms are discussed and a qualitative interpretation of the different
behaviors exhibited by the system is provided. In some configurations it is
possible to find a codimension-two point created by the interaction of two Hopf
modes with different frequencies and wavenumbers. These results suggest to
consider two liquid layers as an interesting prototype for the study of
propagation and interaction of waves in the context of the B\'enard-Marangoni
problem.Comment: 21 pages, 9 figures, 2 tables;accepted to be published in PR
Chiral Polyalkylthiophenes for Organic Light Emitting Diodes
Chiral polyalkylthiophenes are noncentrosymmetric organic materials which can be used
both in second harmonic-generation devices and in polarized light emitting diodes. In this work
we present the synthesis and the characterization of a polyalkylthiophene with a chiral center
very close to the conjugated backbone: poly(3-[(S)-2-methylbutyl]thiophene) (PMBT). Circular
dichroism (CD) measurements have been carried out to ascertain the chirality of these materials.
The CD spectra show intense signals both in mixed solvents and in the solid state. The strong
Cotton effect can be associated to a highly ordered aggregated phase whose nature is still under
investigation. We also present the photo and electroluminescence characterization of single layer
light emitting diode (LED) with the following configuration: ITO (Tin Indium Oxide)/PMBT/Al
Vibrational Excitons in CH-Stretching Fundamental and Overtone Vibrational Circular Dichroism Spectra
A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental
region for about twenty compounds belonging to the class of essential oils was empirically analyzed by
the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula
applicable to many coupled dipole oscillators, the rotational strengths of the previously identified
vibrational excitons are evaluated. The results are then critically reviewed by the use of recent ab
initio methodology, as applied to selected molecules of the original set. Further insight is gained by
model calculations adding up the contribution of the coupling between electric dipole moments associated
with normal mode behavior and that of the polarizability from polarizable groups. The former
part is responsible for the excitonic behavior of the VCD spectra. For the same selected molecules we
have also investigated whether some excitonic behavior is taking place in the second overtone region,
and have concluded that this is not the case
Status of QCD
I have been asked to discuss the status of QCD. It seems to me that there are
three main points to be made about the present status of QCD: QCD is
right, and we can do many beautiful things with it. There are several
important concrete problems that lie just beyond the edge of our current
understanding. There are some foundational issues in QCD, and some
recent developments, that may point toward entirely new directions. These
points will, I believe, emerge quite clearly from the following more detailed
discussion. The discussion will be in three parts. I'll first discuss
elementary processes, then more complicated processes, and then finally
foundational issues.Comment: 28 pages, use Phyzzx, figures available by FAX or mail on request,
IASSNS-HEP-93/6
Dynamics of two interacting hydrogen bubbles in liquid aluminium under the influence of a strong acoustic field
Ultrasonic melt processing significantly improves the properties of metallic materials. However, this promising technology has not been successfully transferred to the industry because of difficulties in treating large volumes of melt. To circumvent these difficulties, a fundamental understanding of the efficiency of ultrasonic treatment of liquid metals is required. In this endeavor, the dynamics of two interacting hydrogen bubbles in liquid aluminum are studied to determine the effect of a strong acoustic field on their behavior. It is shown that coalescence readily occurs at low frequencies in the range of 16 to 20 kHz; forcing frequencies at these values are likely to promote degassing. Emitted acoustic pressures from relatively isolated bubbles that resonate with the driving frequency are in the megapascal range and these cavitation shock waves are presumed to promote grain refinement by disrupting the growth of the solidification front
Comparison between low-order and high-order acoustic pressure solvers for bubbly media computations
Numerical modelling is a useful tool for the fundamental study of acoustic cavitation treatment in liquid metals. This treatment, also known as ultrasonic melt processing, significantly improves the properties and quality of metallic materials. However, the mechanisms leading to these observed improvements are still unclear and a fundamental study of cavitation treatment is required to understand this process. In this endeavour, this paper compares the use of high-order discretization schemes for solving acoustic pressures in cavitating liquids with its low-order counterpart. A fourth order scheme is shown to be more stable and accurate than a second order scheme when taking into account the acceleration of bubbles before their collapse, and is recommended for the full cavitation modelling of acoustic treatment of liquid metals.UK Engineering and Physical Sciences Research Council (EPSRC
Molecular dynamics simulation of aqueous solutions of 26-unit segments of p(NIPAAm) and of p(NIPAAm) "doped" with amino acid based comonomers
We have performed 75-ns molecular dynamics (MD) simulations of aqueous solutions of a 26-unit NIPAAm
oligomer at two temperatures, 302 and 315 K, below and above the experimentally determined lower critical
solution temperature (LCST) of p(NIPAAm). We have been able to show that at 315 K the oligomer assumes
a compact form, while it keeps a more extended form at 302 K. A similar behavior has been demonstrated
for a similar NIPAAm oligomer, where two units had been substituted by methacryloyl-l-valine (MAVA)
comonomers, one of them being charged and one neutral. For another analogous oligomer, where the same
units had been substituted by methacryloyl-l-leucine (MALEU) comonomers, no transition from the extended
to the more compact conformation has been found within the same simulation time. Statistical analysis of the
trajectories indicates that this transition is related to the dynamics of the oligomer backbone, and to the formation
of intramolecular hydrogen bonds and water-bridges between distant units of the solute. In the MAVA case,
we have also evidenced an important role of the neutral MAVA comonomer in stabilizing the compact coiled
structure. In the MALEU case, the corresponding comonomer is not equally efficacious and, possibly, is
even hindering the readjustment of the oligomer backbone. Finally the self-diffusion coefficient of water
molecules surrounding the oligomers at the two temperatures for selected relevant times is observed to
characteristically depend on the distance from the solute molecules
A comparative study of overtone CH- stretching vibrational circular dichroism spectra of fenchone and camphor
Near-infrared vibrational circular dichroism (VCD) spectra in the region 1500-850 nm are recorded for both
enantiomers of camphor and fenchone using a home-built dispersive instrument. Two CH-stretching overtone
regions and two CH-stretching/HCH-bending combination regions are investigated. The near-infrared VCD
data are examined and compared with the corresponding previously published data of (3R)-(+)-methylcyclopentanone
and (3R)-(+)-methylcyclohexanone. The absorption spectra of both the fundamental and overtone
regions are also considered, and the fundamental CH-stretching VCD data are taken into account, where
possible. An empirical assignment of the spectra is first considered, with special regard to the second CHstretching
overtone region. This region is then investigated by a heuristic approach based on DFT calculations,
using the hybrid functional B3LYP at the 6-31G** basis set level. Subsequently rotational strengths have
been evaluated by use of a classical model based on the contribution of polarizable groups to the optical
activity. The same models are also used to interpret the VCD spectra between 2000 and 3000 wavenumbers
for (3R)-(+)-methylcyclopentanone-d4
Prenatal Diagnosis of Gómez-López-Hernández Syndrome.
Gómez-López-Hernández syndrome (GLHS), also known as cerebello-trigeminal-dermal dysplasia, is an extremely rare neurocutaneous disease, classically described by the triad of rhombencephalosynapsis (RES), bilateral focal alopecia, and trigeminal anesthesia. The clinical and radiographic spectrum of GLHS is now known to be broader, including craniofacial and supratentorial anomalies, as well as neurodevelopmental issues.
Here, we present a case of antenatally diagnosed GLHS with RES, hydrocephaly, and craniofacial anomalies identified on ultrasound (low-set ears with posterior rotation, hypertelorism, midface hypoplasia, micrognathia, and anteverted nares) which were confirmed by autopsy after termination of pregnancy at 23 weeks of gestation.
As no known genetic causes have been identified and the classical triad is not applicable to prenatal imaging, prenatal diagnosis of GLHS is based on neuroimaging and the identification of supporting features. In presence of an RES associated with craniofacial abnormalities in prenatal (brachycephaly, turricephaly, low-set ears, midface retrusion, micrognathia), GLHS should be considered as "possible" according to postnatal criteria
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