On a Moving Boundary Problem Aristing in Fluidized Bed Combustion

We consider a model describing the combustion of a coal particlein a fluidized bed, in which attrition plays a dominant role.The model consists of (1) a quasi-linear elliptic equation forthe oxygen concentration, supplemented by boundary conditionson the moving surface representing the burning particle interface,(2) an evolution equation for the carbon consumption, and (3)an equation governing the motion of the interface in terms ofa specified function of the carbon consumption at the interface.We prove a global existence and uniqueness result, togetherwith a priori bounds for the solution; the existence of travellingwaves will also be established.     

Preparation of peralkylchromium(IV) compounds

Tetra-t-butylchromium was prepared from CrCl₃·3THF and t-butyllithium in pentane by a disproportionation reaction or from tetra-t-butoxychromium and t-butyllithium. It was isolated by sublimation and characterized by ESR, UV, visible and IR spectra. The following compounds were not isolated but characterized in solution by their ESR spectra: tetramethylchromium, tetraisopropylchromium, tetra-sec-butylchromium, tetra-n-butylchromium, tetra-3-pentylchromium and tetrakis(cyclohexylmethyl)chromium.     

Free volume in C60 modified PPO polymer membranes by positron annihilation lifetime spectroscopy

PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) is a well-known membrane material showing good gas separation properties. The incorporation of nanoparticles can enhance or deteriorate the performance of composite membranes, sometimes depending only on the way of the composite preparation. We have modified the PPO polymer with C60 fullerenes up to a content of 2 wt %. Previous investigations showed a strong dependence of permeability on whether the C60 is simply dispersed in the polymer or chemically bonded to the polymer chains. Free volume effects were suggested as an explanation but not experimentally confirmed. Here, we present free volume studies by positron annihilation lifetime spectroscopy. An additional long positron lifetime shows the increased free volume of composite samples, while the high electron affinity of C60 helps to indicate the homogeneity of the samples. Combining the presented results with permeability measurements refines the understanding of this promising membrane material.     

80th anniversary of Anatoly M. Kharitonov

Anniversary Dates

80th anniversary of Anatoly M. Kharitonov     

Characterization of bistability for stochastic multistep methods

The focus of this article lies on the bistability of multistep methods applied to stochastic ordinary differential equations. Here bistability is understood in the sense of F. Stummel and leads to two-sided estimates of the strong error of convergence. It is shown that bistability can be characterized by Dahlquist’s strong root condition. The main ingredient of the stability analysis is a stochastic version of Spijker’s norm.     

Direct Prebiotic Pathway to DNA Nucleosides

It is assumed that RNA played a key role in the origin of life, and the transition to more complex but more stable DNA for continuous information storage and replication requires the development of a ribonucleotide reductase to obtain the deoxyribonucleotides from ribonucleotides. This step, as well as an alternative path from abiotic molecules to DNA‐based life is completely unknown. Shown here is the formation of deoxyribonucleosides under relevant prebiotic conditions in water in high regio‐ and stereoselectivity, from all canonical purine and pyrimidine bases, by condensation with acetaldehyde and sugar‐forming precursors. Thus, a continuous path to deoxyribonucleosides, starting from simple, prebiotically available molecules has been discovered. Furthermore, the deoxyapionucleosides (DApiNA) were identified as a potential DNA progenitor. The results suggest that the DNA world evolved much earlier than previously assumed.     

ABSOLUTELY UNSTABLE WAVES IN INVISCID HYDROELASTIC SYSTEMS

The effect of inviscid plug flow on the stability of several hydroelastic systems is investigated by determining the absolute or convective nature of the instability from the linear dispersion relation. The fluid-structure systems consist of plates and membranes with bounded and unbounded flow. A method is proposed to derive systematically in parameter space the boundary between convective and absolute instability, based on the particular symmetries of the dispersion relation as originally noted by Crighton and Oswell. This method is then applied to the case of plates with superimposed tension, thick plates with rotary inertia and walls made of plates or membranes bounding channel flow, oscillating in a sinuous or varicose mode of deformation. A relation is drawn with solutions by previous authors for plates, for pipes and for the Kelvin-Helmholtz instability with surface tension. To illustrate these results some temporal evolutions are calculated by using an integration in the wavenumber space. Based on the large set of new cases solved in the paper some general trends are discussed as to the influence of flow velocity, confinement and structural stiffness on the existence of absolutely unstable waves in inviscid hydroelastic systems.     

Actively contracting bundles of polar filaments

We introduce a phenomenological model to study the properties of bundles of polar filaments which interact via active elements. The stability of the homogeneous state, the attractors of the dynamics in the unstable regime, and the tensile stress generated in the bundle are discussed. We find that the interaction of parallel filaments can induce unstable behavior and is responsible for active contraction and tension in the bundle. The interaction between antiparallel filaments leads to filament sorting. Our model could apply to simple contractile structures in cells such as stress fibers.