Fragmentation Tendency Research Articles

High-resolution imaging of nanoparticle bimetallic catalysts supported on mesoporous silica

Although conventional high‐resolution transmission electron microscopy is a powerful method for the elucidation of the structure of mesoporous solids (diameter of pores from 1.5 to 20 nm), it is far less capable than high‐resolution scanning transmission electron microscopy in identifying the spatial distribution of nanocrystals of catalysts encapsulated within the mesopores. Using high‐angle annular dark‐field imaging (either in a 100 or 300 keV STEM system), it is possible to locate precisely individual bimetallic nanoparticles (Ag3Ru10, Cu4Ru12 and Pd6Ru6 hydrogenation catalysts) supported on mesoporous silica, to determine their size distribution, and to record their characteristic X‐ray emission maps. It is also established that there is little tendency for elemental fragmentation of the bimetallic catalysts, all of which were prepared by decarbonylating, by thermolysis, precursor cluster carbonylate anions: [Ag3Ru10C2(CO)28Cl]−, [Ru6C(CO)16Cu2Cl]2− and [Ru6Pd6(CO)24]2−.

Detection of sequence variation in parasite ribosomal DNA by electrophoresis in agarose gels supplemented with a DNA-intercalating agent.

This study evaluated the use of a commercially available DNA intercalating agent (Resolver Gold) in agarose gels for the direct detection of sequence variation in ribosomal DNA (rDNA). This agent binds preferentially to AT sequence motifs in DNA. Regions of nuclear rDNA, known to provide genetic markers for the identification of species of parasitic ascarid nematodes (order Ascaridida), were amplified by polymerase chain reaction (PCR) and subjected to electrophoresis in standard agarose gels versus gels supplemented with Resolver Gold. Individual taxa examined could not be distinguished reliably based on the size of their amplicons in standard agarose gels, whereas they could be readily delineated based on mobility using Resolver Gold-supplemented gels. The latter was achieved because of differences (approximately 0.1-8.2%) in the AT content of the fragments among different taxa, which were associated with significant interspecific differences (approximately 11-39%) in the rDNA sequences employed. There was a tendency for fragments with higher AT content to migrate slower in supplemented agarose gels compared with those of lower AT content. The results indicate the usefulness of this electrophoretic approach to rapidly screen for sequence variability within or among PCR-amplified rDNA fragments of similar sizes but differing AT contents. Although evaluated on rDNA of parasites, the approach has potential to be applied to a range of genes of different groups of infectious organisms.

The Magnetohydrodynamics of Supersonic Gas Clouds: MHD Cosmic Bullets and Wind‐swept Clumps

We report an extensive set of two-dimensional MHD simulations exploring the role and evolution of magnetic fields in the dynamics of supersonic plasma clumps. We examine the influence of both ambient field strength and orientation on the problem. Of those two characteristics, field orientation is far more important in the cases we have considered with β0 = pg/pb ≥ 1. That is due to the geometry-sensitivity of field stretching/amplification from large-scale shearing motions around the bullet. When the ambient magnetic field is transverse to the bullet motion, even a very modest field, well below equipartition strength, can be amplified by field line stretching around the bullet within a couple of bullet crushing times so that Maxwell stresses become comparable to the ram pressure associated with the bullet motion. The possibility is discussed that those situations might lead to large, induced electric potentials capable of accelerating charged particles. When the ambient field is aligned to the bullet motion, on the other hand, reconnection-prone topologies develop that shorten the stretched field and release much of the excess energy it contains. In this geometry, the Maxwell stresses on the bullet never approach the ram pressure level. In both cases, however, the presence of a field with even moderate initial strength acts to help the flow realign itself around the bullet into a smoother, more laminar form. That reduces bullet fragmentation tendencies caused by destructive instabilities. Eddies seem less effective at field amplification than flows around the bullet, because fields within eddies tend to be expelled to the eddy perimeters. Similar effects cause the magnetic field within the bullet itself to be reduced below its initial value over time.For oblique fields, we expect that the transverse field cases modeled here are more generally relevant. What counts is whether field lines threading the face of the bullet are swept around it in a fashion that folds them (leading to reconnection) or that keeps them unidirectional one each side of the bullet. In the second instance, behaviors should resemble those of the transverse field cases. We estimate that this quasi-transverse behavior is appropriate whenever the angle, θ, between the motion and the field satisfies tan θ 1/M, where M is the bullet Mach number.From these simulations, we find support in either field geometry for the conclusions reached in previous studies that nonthermal radio emission associated with supersonic clumps is likely to be controlled largely by the generation of strong magnetic fields around the perimeters of the clumps, rather than local particle acceleration and field compression within the bow shock. In addition, since the magnetic pressure on the nose of the bullet likely becomes comparable to the ram pressure and hence the total pressure behind the bow shock, the gas pressure there could be substantially lower than that in a gasdynamical bullet. That means, as well, that the temperature in the region on the nose of the bullet would be lower than that predicted in the gasdynamical case. That detail could alter expectations of the thermal emission, including X-rays and UV-IR lines.

Compression behavior of spray dried rice starch

Compression behaviors of spray dried rice starch (SDRS), as well as pregelatinized starch (PS), and microcrystalline cellulose (MCC) were characterized using Heckel analysis. SDRS was found to undergo plastic deformation with lower elasticity compared with PS. SDRS showed very low fragmentation tendency due to the fact that the difference between extrapolated and actual densification of its Heckel plot was low. Having aggregate sphere in shape, its densification could be initiated by deaggregation and tight packing without requiring high pressure. The above evidence explains why the compactibility of SDRS is excellent. MCC on the other hand, showed some fragmentation before undergoing plastic deformation. The fragmentation might have increased the contacts among particles which resulted in higher crushing strength of the tablets compared with that of SDRS. The slope of the Heckel plot of a mixture of each excipient and hydrochlorothiazide fairly agreed with the summation of the weight fraction of each component. The deformation of the mixture tested could be easily predicted. Since SDRS possesses both good compactibility and flowability, this new direct compression excipient has a high potential for successful tablet formulation.

Protostellar hydrodynamics: Constructing and testing a spatially and temporally second-order-accurate method. I - Spherical coordinates

Two related numerical schemes for calculating the 3D collapse of protostellar clouds are defined, developed, and checked on a wide variety of test problems in spherical symmetry and multiple dimensions. One scheme is first-order accurate in time (code S), and the other second-order accurate in time (code ST). Through convergence testing, the codes are shown to be second-order accurate in spatial differences. Compared with the previous 3D code, the combination of reduced numerical dissipation through second-order accuracy and of removing the systematic bias toward central concentrations implies that the tendency for fragmentation into binary or multiple protostars should increase. A reinvestigation of fragmentation as a mechanism for forming binary stars is expected to yield an even more favorable evaluation.

Yehuda Amichai: A Modern Metaphysical Poet*

A comparative study of Amichai, one of Israel's leading poets (and the most translated one), and the English metaphysical poets of the 17th century, notably Donne, may help us to appreciate and to better understand Amichai's modernistic sensibilities. In my essay I focus on five levels of the poetic text shared by Amichai and Donne: (1) the extensive use of striking similes (the “metaphysical conceit”); (2) the tendency to elaborate on the similarities between the two dissimilar concepts introduced by the similes (the internal “rhetoric” of the simile); (3) the use of pseudo‐logical and pseudo‐causal formulations; (4) the creation of dramatic situations in which a specific speaker addresses a specific addressee (usually a woman) in a relatively specific type of communicative situation; and (5) the treatment of the tension between religious concepts and secular experience. These close affinities between Amichai and the metaphysical poets should not, however, cloud the significant differences between them. Instead, these shared poetic characteristics ultimately highlight the modernist nature of Amichai's poetry ‐ with its tendency for fragmentation, exploring man's internal world, and expressing an existential point of view.

Physicochemical aspects of drug release. XI. Tableting properties of solid dispersions, using xylitol as carrier material

A set of tests, including characterization of carrier fragmentation tendency, radial and axial tablet tensile strength, friability, disintegration time and dissolution rate, is presented for the evaluation of the suitability of substances to be used as carriers in solid dispersions prepared by the melting method. The carrier used as a model substance was the sugar alcohol xylitol. Solid dispersions of the sparingly soluble drug griseofulvin were prepared at three concentrations (2, 10 and 20% w/w) by the melting method. Tablets were made of pure untreated xylitol, of fused/recrystallized xylitol and of solid dispersions with external lubrication or with lubricant included in the composition. In some of the compositions the binding/disintegrating agents Avicel and Ac-Di-Sol were included. It was observed that xylitol fragments extensively. The tableting properties were somewhat improved if xylitol was fused/recrystallized. The incorporation of griseofulvin increased the tablet tensile strength. This indicates that conclusions regarding tableting properties cannot be made directly from studies of an untreated carrier. Thus, the set of tests can be a valuable tool in tablet formulation. The fastest drug dissolution was obtained with solid dispersions of the lowest concentration of drug in both particulate and tablet form. An increase in drug content and compression pressure and a decrease in rotational paddle speed decreased the dissolution rate. Internal lubrication with the admixing of 0.5% magnesium stearate prior to tablet compression had a limited retarding effect on the dissolution rate.

Effects of various neuropeptide Y/peptide YY fragments on electrically‐evoked contractions of the rat vas deferens

1. The effects of various neuropeptide Y (NPY) and peptide YY (PYY) fragments on electrically-evoked twitches in the rat isolated vas deferens were studied and compared with the effects of full length NPY and PYY. The aim was to identify the shortest NPY/PYY fragments that are capable of suppressing the contractions. 2. NPY (1-36) and C-terminal fragments of NPY (from 11-36 to 22-36) suppressed the electrically-evoked twitches in a concentration-dependent manner. On the whole there seemed to be a gradual lowering of the pIC50 values with progressive shortening of the NPY fragments (except for fragments 16-36 and 22-36 that had rather high pIC50 values). NPY 23-36, 24-36 and 25-36 suppressed the twitches at high concentrations (3 microM). NPY 26-36 was without effect as were C-terminal carboxy-deaminated NPY and glycine extended NPY (NPY-Gly-Lys-Arg). 3. PYY (1-36) and C-terminal fragments of PYY (from 11-36 to 23-36) suppressed the electrically-evoked twitches in a concentration-dependent manner. PYY 1-36 was more potent than any of the fragments. There was a tendency for shorter fragments to have lower pIC50 values. PYY 24-36 and 25-36 suppressed the twitches at high concentrations (3 microM). PYY 26-36 was without effect. 4. The findings suggest that the 12 C-terminal amino acid residues of NPY and PYY are the minimum length required to activate the Y2-receptor.

Density-functional calculation of the fragmentation of doubly ionized spherical jelliumlike metallic microparticles.

The fragmentation of doubly charged ${X}_{N}$${}^{2+}$ (X=Na,Mg) model microparticles into two singly charged fragments ${X}_{N}$${}^{2+}$\ensuremath{\rightarrow}${X}_{P}$${}^{+}$+${X}_{N\mathrm{\ensuremath{-}}P}$${}^{+}$ has been studied as a function of N and P, by using the density-functional formalism. In contrast to naive expectations which would predict the most asymmetric channel (P=1) to be the most favorable one, we find that the main factor controlling the most favorable channel is the tendency for fragments to have a ``magic'' number of electrons: 2, 8, 18, etc. These magic numbers, which correspond to filled electronic shells in a spherically symmetric potential, are already familiar from the analysis of experiments of the relative abundance of alkaline-atom clusters produced by supersonic expansion. Our results suggest the existence of a critical number (larger than 100) for Coulomb explosion of ${\mathrm{Na}}_{\mathrm{N}}$${\mathrm{}}^{2+}$ clusters.

Intraocular grafts of fresh and freeze-stored rat hippocampal tissue: a comparison of survivability and histological and connective organization.

Intraocular grafts of rat hippocampal tissue, grafted either directly from the immature donor brains (fresh) or after storage in liquid nitrogen at -196 degrees C (freeze-stored), were compared with regard to survivability and histological and connective organization. For direct grafting, pieces of hippocampal tissue from rat embryos (embryonic day 19, E21) and newborn rats (PO) were placed in the anterior eye chamber of adult rats immediately after dissection. For grafting after deep-freeze storage, pieces of hippocampal tissue were taken from rat embryos (E16-E21) and newborn rats (PO), frozen at a cooling rate of 1 degrees C/min in CO2 or N2 vapours after addition of the cryoprotective agent dimethylsolfoxide (DMSO), and stored in liquid nitrogen for 1 to 33 days before thawing and intraocular grafting. From 20 to 68 days after grafting, the recipient rats were sacrificed, their eyes sectioned, and the sections stained with thionine for cell bodies, Timm's sulphide silver method for hippocampal fiber systems and terminal fields, and acetylcholinesterase (AChE) for cholinergic fibers and AChE-positive neurons. When examining the 101 grafted eyes (34 grafted with fresh and 67 with freeze-stored tissue) a significantly lower survival rate of the freeze-stored tissue was found (28 vs. 88%). The survivability of the freeze-stored tissue was age-dependent with no survival at donor ages E16 and PO, while tissue from E18-E21 had a 50% survival rate. The grafts of the freeze-stored tissue were also smaller and showed an increased tendency for fragmentation. When evaluating the structure of the grafts, the deep-freeze storage was found primarily to have been harmful to the dentate granule cells and their precursors. The organization of intrinsic fiber connections followed the pattern known from lesion and intracerebral transplant studies. While demonstrating that immature brain tissue can survive deep-freeze storage and subsequent intraocular grafting, the study also indicates that different schemes may have to be used to get optimal survival of different neuronal populations.

Mechanisms of compaction of powdered materials

The compaction of powdered sodium chloride, sucrose and a coal have been studied by observing changes in specific surface area, pore volume and extent of compaction with pressure. Observations have also been made of appropriate sections using a scanning electron microscope. It has been shown that the initial compaction of the three materials is not governed by the sliding of particles into new equilibrium positions. The compaction of sodium chloride is achieved by plastic deformation of the individual particles, without their fracture. This process continues into the region in which coherent compacts are produced, but work hardening of the material makes it progressively more difficult to compact as higher pressures are reached. For sodium chloride the area of contact, A , is related to the compacting pressure, P , by the equation In P = k A + K , where k and K are constants. The compaction of both sucrose and coal is dominated by particle fracture. At lower pressures, in particular, the movement of particle fragments into positions of closer proximity is an important secondary mechanism. This movement of fragments into the available pore space is particularly important in the case of sucrose. With coal there is, in addition, a tendency for fragments to remain wedged in the form of chain bridges which maintain a more open structure in the compact. At high pressures, for both materials, plastic deformation of small particle fragments may well be important.

Pactos educativos: difíciles pero necesarios

En el documento se abordan uno de los temas más importantes de la agenda educativa contemporánea: los pactos educativos. Estos pueden considerar el producto deliberado de la negociación entre los diferentes actores sociales para enfrentar las tendencias a la fragmentación, la ruptura y la exclusión, tanto de los sectores más pobres de la población como de los sectores que ocupan la cúpula de la estructura social. El documento es dividido en función de tres palabras-clave: la necesidad de construir pactos que garanticen la continuidad de las políticas educativas; la dificultad para hacerlo, pero existe la posibilidad de lograr el objetivo: plantear la construcción de sociedades justas e cohesionadas.

Asteroid collisions and evolution of asteroidal mass distribution and meteoritic flux

In Part I, several early papers are reviewed. In Part II, a theory of fragmentation is developed, based not on statistical treatment of many collisions as has been done earlier, but rather on the physics of a single collision. In Part III, the fragmentation theory is applied to the original distribution of asteroid masses found by Anders, to show that this distribution will migrate toward that presently observed. In Part IV, we consider the tendency for fragmentation to produce a steady state mass distribution, and conclude that B, the slope in a log-log mass distribution, will tend toward a value 2 3 except at masses less than roughly 10 8 gm, where certain physical effects produce a steeper slope. In Part V, we estimate the total number of parent bodies (i.e., those belonging to Anders' initial mass distribution) required to provide the present number of asteroidal fragments, and conclude that in the zone 2.15 to 2.6 a.u., 5 to 30 such bodies, each of mass greater than several times 10 20 gm, have been broken. In Part VI, we attempt to relate the buildup of particles in the asteroid belt to the time history of meteoritic flux observed on the Earth. Anders' model of meteorite origin among the “Mars asteroids” is used. A critical factor is the half-life against perturbation by Mars into Earth-crossing orbits, found to be about 1 × 10 9 years. This produces an exponential depletion of the meteorite pool. The short cosmic ray exposure ages of meteorites is attributed to (1) this depletion effect, (2) grinding and refragmentation, especially of stones, and (3) a possibly statistical excess of recent major collisions. The flux near the Earth has increased nearly linearly but with a superimposed sawtooth effect caused by discrete collisions and the relatively short perturbation half-life. The total cratering flux, composed of large fragments and a possible nonsteroidal component but excluding anomalous early effects, is now roughly a factor 2 above its average value over solar system history unless recent major cillisions have occurred, a conclusion of importance in interpreting planetary surfaces. In Part VII, these results are contrasted with other literature, in particular an earlier suggestion that the flux has increased exponentially by a factor 50 over its average value. Part VIII applies the results of this paper in an attempt to explain properties of meteorites as observed on Earth, and predicts the terrestrial flux of asteroidal meteorites.

CYTOTAXONOMIC STUDIES IN CHRYSOTHAMNUS (ASTEREAE, COMPOSITAE)

The basic chromosome number for Chrysothamnus is x = 9 as determined by 234 original counts representing all species. Polyploidy is frequent only in C. viscidiflorus where diploids, tetraploids, and hexaploids are found. Pollen size in the genus is basically correlated with style length rather than ploidy level. Most aspects of meiosis appear normal except for the tendency for nucleolar fragmentation. To reduce chromosome clumping during meiosis, an ice‐water pretreatment was utilized. Karyotypes in Chrysothamnus reflect similar trends of specialization noted in chromosome structure for other Compositae. The taxa are predominantly self‐fertilized; however, one instance of interspecific hybridization is noted. Taxonomic significance of the chromosome numbers, karyotypes, and breeding system in Chrysothamnus is discussed.