Large Eddy Simulations and Experiments of Flow Around Finite-Height Cylinders

The paper presents experimental and numerical results for the flow around a surface-mounted circular cylinder at the two height-to-diameter ratios of 2.5 and 5. The Reynolds number based on approach flow velocity and cylinder diameter is 43,000 and 22,000 for these two cases and the boundary layer of the approach flow has a thickness of about 10% of the cylinder height. The experiments comprise both flow visualizations with dye and laser Doppler velocimeter measurements of all mean velocity and fluctuation components. The numerical study is performed by an elaborate large eddy simulation on a staggered Cartesian grid using the immersed boundary method. The instantaneous flow behaviour including the shedding is analysed with information based on animations. For the long cylinder alternating shedding is found to occur over most of the height while for the shorter cylinder the shedding is observed mainly near the ground where it is also mostly alternating but intermittently also symmetrical. The mean-flow behaviour is analysed with the aid of streamlines and contour plots of mean-velocity and fluctuation components in various planes and a detailed comparison of LES and LDV results is provided, showing generally good agreement. The LES with very fine resolution near the free end allow a detailed study of the complex flow in this region with owl-face topology on the end wall previously observed in experiments. Behind the cylinder, the longitudinal recirculation region, the downstream development of tip vortices and the emergence of trailing vortices further downstream are analysed. The sum of the results, together with those from previous studies that were reviewed extensively, provides a comprehensive picture of the very complex flow behaviour.     

Disilane Cleavage with Selected Alkali and Alkaline Earth Metal Salts

The industry-scale production of methylchloromonosilanes in the Müller–Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes Me_nSi₂Cl_6-n (n=1–6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono- and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride.     

Effect of cholesterol on diffusion in surfactant bilayers

Biological membranes consist of lipid bilayers with liquid-ordered and liquid-disordered phases. It is believed that cholesterol controls the size of the microdomains in the liquid-ordered phase and thereby affects the mobility as well as the permeability of the membrane. We study this process in a model system consisting of the nonionic surfactant C(12)E(5) and water in the lamellar phase. We measure the diffusion of fluorescent probe molecules (rhodamine B) by fluorescence correlation spectroscopy. For different surfactant to water ratios, we measure how the molecular mobility varies with the amount of cholesterol added. We find that a reduction of the diffusion coefficient is already detectable at a molar ratio of 8 mol % cholesterol.     

Dynamics of hyperbranched polymers and dendrimers: theoretical models

Chemical reactions depend in many ways on the dynamics of the underlying reactants, and an important aspect is the distance covered by the reactants before the reaction act occurs. Hence, even diffusion-limited reactions between point particles in confined geometries and in low-dimensional systems display decay forms which are very different from those obtained from simple chemical kinetics. Clearly, even more complex decay forms hold for macromolecules, given their internal degrees of freedom. Here we discuss how the dynamics of macromolecules in solution relates to their topological structure and focus on the motion of macromolecular segments (monomers) under the influence of external fields. After a general survey of the method of generalized Gaussian structures (GGS) we recall the wealth of forms which are observed, depending on the topology and on the microscopic dynamics involved. Paradigmatic are the findings for the class of hyperbranched macromolecules; to these belong the dendrimers. While the dendrimers do not show a typical scaling behavior, as found, say, for linear chains, the situation is different for particular classes of regular hyperbranched polymers which are fractal. We end by discussing their pattern of motion in the GGS-Rouse-Zimm picture.     

Preparation of submicron-structured alkylsiloxane monolayers using prepatterned silicon substrates by laser direct writing

A new constructive method for the preparation of laterally structured alkylsiloxane monolayers is demonstrated. Laser direct writing has been used to create oxide patterns on H-terminated Si(100) samples under ambient conditions. Depending on the laser power and the writing speed, oxide structures with a lateral resolution below 500 nm are prepared routinely. The patterned samples are suitable as temporary templates for the preparation of laterally structured octadecylsiloxane monolayers. Prior to immersion in an octadecyltrichlorosilane solution, however, hydration of the samples in water is essential to facilitate a selective coating of the oxidized areas. After coating, atomic force microscopy reveals the formation of octadecylsiloxane islands exclusively on top of the oxide lines.     

Reactions of free gold cluster cations with H2O, CH3Cl, and mixtures thereof

The reactions of the mass-selected gas-phase gold cluster cations Au₃⁺ and Au₅⁺ with H₂O, CH₃Cl, and mixtures of these reactants were studied under well-defined multi-collision conditions in an octopole ion trap. The reaction of CH₃Cl with the gold cations was found to proceed fast at room temperature, leading to the adsorption of one CH₃Cl molecule at each ‘corner’ atom of the cluster ion. This strong adsorption hindered the coadsorption of other reactants like H₂O. However, a considerable reduction of the CH₃Cl partial pressure led to distinct patterns of H₂O/CH₃Cl coadsorption products. Furthermore, the mass spectra indicated the loss of hydrogen after the reaction of CH₃Cl with Au₃⁺.     

A Gas‐Phase CanMn4−nO4+ Cluster Model for the Oxygen‐Evolving Complex of Photosystem II

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn₃O₄?MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand‐free tetrameric Ca_nMn_4?nO₄⁺ (n=0–4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas‐phase reactivity experiments with D₂O and H₂¹⁸O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono‐calcium cluster CaMn₃O₄⁺ is observed to oxidize water via elimination of hydrogen peroxide. First‐principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn₃O₄⁺ as well as the role of calcium.     

Synthesis of 3-Alkyl-6-bromo-4,5-E-hexenoate

Two complementary routes for the preparation of 3-alkyl-6-bromo-4,5-E-hexenoate are reported.