Evidence of surface reconstructions and incorporation of oxygen into the oxide framework on the hydroxylated La2O3(001) surface

By performing first-principles Molecular Dynamics simulations at 300 K, we show that water dissociates on the A-La2O3(001) surface giving rise to one exclusive type of hydroxyl-group, which is associated with a surface reconstruction, incorporating an additional oxygen ion into the oxide subsurface, yielding a surface structure that is oxygen rich.     

Formation of a new Ia-3d cubic meso-structured silica via triblock copolymer-assisted synthesis

A new mesoporous silica structure with cubic Iad symmetry has been synthesized. The structure has very large unit cell dimensions, up to 250 A in the as-synthesized form and 222 A after calcination, and the surface area is around 700 m2/g. The syntheses were done according to well-established synthesis conditions with the triblock copolymers Pluronics P103 or P123, except for the addition of NaI to the synthesis mixture. Small-angle X-ray scattering revealed that the sample has Iad symmetry. According to electron micrographs, the structure is similar to that of MCM-48, and we thus propose that the structure lies on the gyroid minimal surface.     

Velocity statistics and flow structures observed in bypass transition using stereo PTV

It is known from smoke visualizations that in a transitional boundary layer subjected to free-stream turbulence, streaks appear and eventually break down to turbulence after wavy motions. In order to observe the streaky structures directly, a stereo particle-tracking velocimetry system using hydrogen bubbles in a water channel has been developed and validated against laser Doppler velocimetry. Mean flow statistics show good agreement with previous results. With the developed measurement system, the instantaneous spanwise distribution of the streamwise and wall-normal velocities can be measured fast enough to resolve the time development of the streaky structures. Measurements of instantaneous spanwise distributions of the streamwise and wall-normal velocity disturbances show strong negative correlation between the wall-normal and streamwise velocities in the streaks. Published online: 19 November 2002     

Comparison of a calculated and measured XANES spectrum of α-Fe2O3

Comparison and prediction of the experimental XANES spectrum is a good measurement of the quality of the electronic structure calculations employed, and their ability to predict electronic transitions in solids. Here we present a comparison between BLYP + U and hybrid-BLYP calculations regarding the geometric, magnetic and electronic structures of α-Fe(2)O(3) (hematite). Several values of U and different percentages of Fock-exchange have been screened to see how their contributions affect different properties of hematite, paying particular attention to the electronic structure. To estimate the quality of the various methods the calculated density-of-states were compared to the experimentally collected XANES spectrum of the iron K-edge, providing information about the orbitals describing the conduction band. We find that in agreement with previous studies DFT + U and hybrid-functional simulations can correctly predict the character of the valence band, but only Fock-exchange higher than 30% or U-values equal or larger than 6 eV properly reproduce the order between the t(g) and e orbitals in the conduction band, and can, therefore, be used to study and predict XANES spectra and electronic transitions in hematite.     

Anion-exchange separation of hydroxy acids in acetate medium

A great number of hydroxy acids have been separated in an anion-exchange column using sodium acetate as eluant. It is essential that lactones be saponified before loading the column. With hydroxy acids containing only one or two hydroxyl groups this method is superior to separations in borate medium. With complicated mixtures the methods supplement each other.     

Large-Eddy BreakUp Devices – a 40 Years Perspective from a Stockholm Horizon

In the beginning of the 1980’s Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given.     

Nanotubes from a vitamin C-based bolaamphiphile

A bolaform surfactant, 1,12-diascorbyl dodecanedioate (BOLA12), with ascorbic acid units as the polar headgroups was synthesized for the first time. Once dispersed in water above 0.5% w/w, BOLA12 forms hollow nanotubes as revealed by cryo-TEM experiments. These nanostructures transform into clear micellar solutions on heating. X-ray diffraction and SAXS experiments were performed both on the pure solid and on its aqueous dispersions. The critical aggregation concentration and the phase behavior were determined by conductivity and DSC experiments. The latter technique provided also the amount of strongly bound, solvating water molecules that surround the polar headgroups. BOLA12 shows the same reducing properties of ascorbic acid, as indicated by the antioxidant activity evaluated with the DPPH method. This feature was used for the reduction of Pd(II) ions on the surface of the nanoassemblies, which lead to the formation of large bundles homogeneously coated with palladium as observed in SEM micrographs.     

Measurements with a flow direction boundary-layer probe in a two-dimensional laminar separation bubble

 Measurements with a directional sensitive hot-wire probe have been carried out in a two-dimensional laminar separation bubble caused by an adverse pressure gradient. The probe has three parallel, in plane wires and can be traversed in the boundary layer in all spatial directions. The central wire, operated as a conventional hot-wire in CTA mode, and two surrounding resistance wires measure the instantaneous magnitude and direction of the flow, respectively. The probe is calibrated and operated in a similar way as a single hot-wire probe for boundary layer measurements. The frequency response is high enough for measurements of naturally occurring instability waves in the bubble. The flow direction intermittency was measured inside the bubble and regions with reversed flow were mapped out. Prior to reattachment periodical oscillations of the flow direction are found associated with shedding of vortical structures from the bubble. Received: 13 March 1998/Accepted: 22 April 1998