Refraktometrie

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Hans Hahn

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Hans Hahn

     

Buchbesprechungen

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Buchbesprechungen

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Differential interferometry with adjustable spatial carrier fringes for turbine blade cascade flow investigations

 Digital evaluated differential interferometry using adjustable spatial carrier fringes was applied to flow measurements in a transonic turbine blade cascade. The interferograms were evaluated using a two-dimensional Fourier Analysis. This evaluation provided density gradient maps of the flow field in a digital form. A specially designed interferometer was used allowing adjustment of sensitivity and superposition of carrier-fringe system separately. This type of interferometer is also highly insensitive to vibration noise. For a turbine blade cooling film a comparison of this type of differential interferometry with holographic interferometry and a Schlieren visualisation is also given. As a result differential interferometry using adjustable spatial carrier fringes and digital fringe analysis gave density-gradient maps similar to Schlieren recordings but of quantitative nature, thus enabling compensation of wave front distortions. Integration of these density gradient maps resulted in density maps which were then compared to recordings done with pulsed holographic interferometry. Received: 27 December 1996/Accepted: 30 April 1997     

Dynamic Supramolecular Ruthenium-Based Gels Responsive to Visible/NIR Light and Heat

A simple supramolecular crosslinked gel is reported with a photosensitive ruthenium bipyridine complex functioning as a crosslinker and poly(4-vinylpyridine) (P4VP) as a macromolecular ligand. Irradiation of the organogels in H₂O/MeOH with visible and NIR light (in a multiphoton process) leads to cleavage of pyridine moieties from the ruthenium complex breaking the cross-links and causing degelation and hence solubilization of the P4VP chains. Real-time (RT) photorheology experiments of thin films showed a rapid degelation in several seconds, whereas larger bulk samples could also be photocleaved. Furthermore, the gels could be reformed or healed by simple heating of the system and restoration of the metal–ligand crosslinks. The relatively simple dynamic system with a high sensitivity towards light in the visible and NIR region make them interesting positive photoresists for nano/micropatterning applications, as was demonstrated by writing, erasing, and rewriting of the gels by single- and multiphoton lithography.     

Near-surface ion distribution and buffer effects during electrochemical reactions

The near-surface ion distribution at the solid-liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H(+) in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.