Spontaneous thermal waves in a magnetized plasma

Coherent temperature oscillations corresponding to thermal (diffusion) waves are observed to be spontaneously excited in a narrow temperature filament embedded in a large, but colder, magnetized plasma. The parallel and transverse propagation properties of the waves satisfy the predictions of the classical transport theory based on Coulomb collisions. The frequency of the oscillations meets the conditions for a quarter-wave thermal resonator. This is the plasma version of thermal resonators used in the study of other states of matter.     

Use of Video Enhanced Microscopy for Characterization of Solid‐Liquid‐Liquid Mixtures

Although some particle sizing devices have gained a lot of popularity, none of them is able to distinguish between drops and solids and few are able to handle viscous oily samples. This paper discusses the use of the optical microscopy, enhanced by the use of digital video capabilities and image analysis software, to characterize oil production wastes. Several thousands of particles were counted and their projected areas were measured. A discussion is given about the different corrections required in order to extract the most reliable information from the image analysis, overcoming some of the drawbacks of this kind of measurements. The technique developed here offers unique features like the determination of the fraction of droplets that exist in the form of non‐coalesced agglomerates. When cross‐linked with the results obtained by standard ASTM procedures, it allows to infer the amount of disperse phase that exists in the form of submicron droplets or large free water drops that are usually not sampled for microscope slides.     

On the Stability of a Class of Modified Gravitational Models

Motivated by the dark energy issue, a minisuperspace approach to the stability for modified gravitational models in a four dimensional cosmological setting is investigated. Specifically, after revisiting the f(R) case, R being the Ricci curvature, we present a stability condition around a de Sitter solution valid for modified gravitational models of generalized type F(R,G,Q), G and Q being the Gauss-Bonnet and quadratic Riemann invariants respectively. A generalization to higher order invariants is presented.     

The generalized Hulthén potential

A generalization of the Hulthén potential is presented on the basis of an approach that uses the factorization of a general Hamiltonian by means of a specific model of operational equations with the structure ∼β(r)∓(d/dr). To achieve this goal, the treatment of the V _Hs(r) standard Hulthén potential for bound s states is carried out by proposing a particular β_p(r) ansatz to identify V _Hs(r) by means of a particular Riccati-type relationship. Once the identification has been achieved, the generalized Hulthén potential is obtained straightforwardly with the aid of a general Riccati formula. As expected, the Hamiltonian of the generalized Hulthén potential is isospectral when compared with the corresponding standard Hamiltonian. Moreover, according to the Darboux transform there exists a modified Hulthén potential which is also isospectral. We show that the latter is just a particular case of the generalized Hulthén interaction model. Received: 14 September 1999 / Accepted: 3 February 2000 / Published online: 19 April 2000     

1,3,5‐Triiodo‐2,4,6‐trimethylbenzene at 293 K

In the structure of tri­iodo­mesityl­ene (1,3,5‐tri­iodo‐2,4,6‐tri­methyl­benzene), C₉H₉I₃, at 293 K, the benzene ring is found to be significantly distorted from ideal D_6h symmetry; the average endocyclic angles facing the I atoms and the methyl groups are 123.8 (3) and 116.2 (3)°, respectively. The angle between the normal to the molecular plane and the normal to the (100) plane is 5.1°. No disorder was detected at 293 K. The thermal motion was investigated by a rigid‐body motion tensor analysis. Intra‐ and intermolecular contacts are described and topological differences compared with the isomorphous compounds tri­chloro­mesityl­ene and tri­bromo­mesityl­ene are discussed.     

Molecular resistivities in organic polyenic wires: I. A one-dimensional photoconduction charge transfer model

In the present work we have developed a one-dimensional photoconduction charge transfer model, which allows us to determine the resistivities associated to π-conduction molecular channels of organic polyenic wires. In order to calculate the charge transmission coefficient through the conductor molecular wire, our model is based on the scattering process of electrons in metals according to Landauer's one-dimensional conductor approach. This model can be applied to those D–bridge–A molecular systems, constituted by an electron-donor group (D), an electron-acceptor group (A) and a polyenic conductor molecular bridge, which present a photoinduced charge transfer absorption band. According to the present model, molecular resistivities and resistances in the charge transfer excited state of a series of polyenic compounds containing several kinds of electron-donor groups and CO, as an electron-acceptor group, have been calculated by means of the INDO/S-CI method, in the framework of ZINDO semiempirical molecular orbital theory. The calculations allow us to determine the linear and nonlinear contributions to the total molecular resistance of the π-conduction channel in the charge transfer excited state. Linear resistivities for a series of polyenic aldehydes substituted by an electron-donor group (D=–CH₃, –OCH₃, –NH₂ and –N[CH₃]₂) range between 18 and 42 μΩ cm and these results agree with the order of magnitude expected in classic conductors such as metals, nonmetals, and doped polyacetylenic compounds in the ground state.     

Identification of Free and Glycosidically Bound Volatiles and Glycosides by Capillary GC and Capillary GC‐MS in “Lulo del Chocó” (Solanum topiro)

The volatile constituents of lulo del Chocó (Solanum topiro) fruit pulp obtained by liquid‐liquid extraction were analyzed by capillary GC and capillary GC‐MS. In total, 30 components were identified with methyl salicylate, hexadecanoic acid, hexanal, guaiacol, ethyl butanoate, and ethyl acetate being the major components. Chirospecific MDGC analysis revealed the predominance of (R)‐ethyl‐3‐hydroxybutanoate (ee 40%) and the presence of racemic mixtures both of δ‐octalactone and of δ‐decalactone. For γ‐hexalactone, γ‐octalactone, and γ‐decalactone enantiomeric distributions of 22.4 : 77.6, 22.9 : 77.1, and 20.0 : 80.0, (R) : (S), respectively, were determined. Glycosidically bound aroma compounds were identified by capillary GC and capillary GC‐MS after isolation of the glycosidic fraction obtained by Amberlite XAD‐2 adsorption and methanol elution followed by hydrolysis with a commercial pectinase enzyme. In total 13 bound aroma compounds (aglycones) were identified. These aglycones mainly consisted of compounds exhibiting aromatic structures. Additionally, with the aid of capillary GC and capillary GC‐MS (EI and NCI) of trifluoroacetylated derivatives we identified eight glucosides: the novel 3,6‐epoxy‐7‐megastigmen‐5,9‐diol β‐D‐glucopyranoside and the hexyl, benzyl, linalyl oxide (furanic), 2‐phenylethyl, vomifolyl (isomer 1), (6S,9R)‐vomifolyl, and scopoletin β‐D‐glucopyranosides.     

The Effects of Cold Plasma Treatments on LDPE Wettability and Curing Kinetic of a Polyurethane Adhesive

A cold plasma feed with either oxygen or nitrogen was used to modify thin Low Density Polyethylene films. The treatment improved both the surface wettability and the curing kinetic of a polyurethane based adhesive spread on the surface. The chemical functionalities responsible of the observed behaviours were investigated and monitored over time by high resolution x-ray photoelectron spectroscopy. Atomic force microscopy were used to compare the effects of the two feeding gases on the surface morphology of films exposed (to the plasma) for equal times. The effect on the treated films of the atmosphere exposure and of the aging time were also considered.