Adhesion measurements on patterned elastomeric surfaces

Since the discovery of the gecko\u27s hairy attachment pads, scientists tried to mimic these surface patterns due to the unique adhesion properties. Lately, scientists succeeded to fabricate artificial adhesives, which show similar complexity in geometry and achieved adhesive forces, exceeding the sticking forces of geckos. Due to the increasing commercial interest, a race has started to fabricate more complex surface patterns. However, due to this race some fundamental scientific aspects have fallen into oblivion, e.g. the distinction between real effects and measurement artefacts. In this work, the adhesion of patterned surfaces was investigated using different probe geometries. It was shown that the adhesion changes with the number of contacts due to material transfer between sample and probe. Adhesion measurements with flat and spherical probes on patterned surfaces were compared and the angle dependent adhesion was determined. Flat tip pillars showed a large tilt angle dependency, while pillars with spherical and mushroom shaped tips exhibited angle independent pull-off forces. Due to the angle dependencies, spherical probes tended to underestimate the adhesion of patterned surfaces compared to well-aligned flat probes. Flat probe measurements allowed a closer investigation of contact phenomena and yielded new information on adhesion and mechanical properties of patterned surfaces. These results may help in a more successful design of bioinspired adhesives.Seit Entdeckung der Oberflächenstruktur von Geckozehen haben Wissenschaftler versucht, diese wegen ihrer einzigartigen Hafteigenschaften zu kopieren. Kürzlich ist es gelungen, Haftsysteme mit ähnlich komplexen Strukturen zu entwickeln, die die Haftkräfte des Geckos sogar übertreffen. Aufgrund des wachsenden kommerziellen Interesses werden immer komplexere Strukturen hergestellt. Dabei werden jedoch fundamentale wissenschaftliche Prinzipien vernachlässigt: der Unterschied zwischen Effekt und Messartefakt. In dieser Arbeit wurde die Adhäsion strukturierter Oberflächen mit unterschiedlichen Prüfkörpergeometrien bestimmt. Dabei wurde gezeigt, dass sich die Haftkraft aufgrund von Materialtransfer mit der Anzahl der Kontakte zwischen Prüfkörper und Probe ändert. Adhäsionsmessungen mit sphärischen und flachen Prüfkörpern wurden verglichen. Flache Prüfkörper erlaubten dabei eine Quantifizierung der winkelabhängigen Adhäsion. Strukturen mit flachen Enden zeigen, im Gegensatz zu Strukturen mit sphärischen und pilzförmigen Enden, winkelabhängige Adhäsion. Aus diesem Grund wird bei Adhäsionsmessungen mit sphärischen Prüfkörpern die Haftkraft strukturierter Oberflächen oftmals unterschätzt. Messungen mit flachen Prüfkörpern ermöglichen eine genauere Untersuchung von Kontaktphänomenen und liefern zusätzliche Informationen über Adhäsion und mechanische Eigenschaften von strukturierten Oberflächen. Diese Ergebnisse können dabei helfen, bioinspirierte Haftsysteme weiter zu optimiere

Voltage-controlled electron-hole interaction in a single quantum dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.Comment: Conference Proceedings, Physics and Applications of Spin-Related Phenomena in Semiconductors, Santa Barbara (CA), 2004. 4 pages, 4 figures; content as publishe

Neuroinflammation by cytotoxic T-lymphocytes impairs retrograde axonal transport in an oligodendrocyte mutant mouse

Mice overexpressing proteolipid protein (PLP) develop a leukodystrophy-like disease involving cytotoxic, CD8+ T-lymphocytes. Here we show that these cytotoxic T-lymphocytes perturb retrograde axonal transport. Using fluorogold stereotactically injected into the colliculus superior, we found that PLP overexpression in oligodendrocytes led to significantly reduced retrograde axonal transport in retina ganglion cell axons. We also observed an accumulation of mitochondria in the juxtaparanodal axonal swellings, indicative for a disturbed axonal transport. PLP overexpression in the absence of T-lymphocytes rescued retrograde axonal transport defects and abolished axonal swellings. Bone marrow transfer from wildtype mice, but not from perforin- or granzyme B-deficient mutants, into lymphocyte-deficient PLP mutant mice led again to impaired axonal transport and the formation of axonal swellings, which are predominantly located at the juxtaparanodal region. This demonstrates that the adaptive immune system, including cytotoxic T-lymphocytes which release perforin and granzyme B, are necessary to perturb axonal integrity in the PLP-transgenic disease model. Based on our observations, so far not attended molecular and cellular players belonging to the immune system should be considered to understand pathogenesis in inherited myelin disorders with progressive axonal damage

Absorption and photoluminescence spectroscopy on a single self-assembled charge-tunable quantum dot

We have performed detailed photoluminescence (PL) and absorption spectroscopy on the same single self-assembled quantum dot in a charge-tunable device. The transition from neutral to charged exciton in the PL occurs at a more negative voltage than the corresponding transition in absorption. We have developed a model of the Coulomb blockade to account for this observation. At large negative bias, the absorption broadens as a result of electron and hole tunneling. We observe resonant features in this regime whenever the quantum dot hole level is resonant with two-dimensional hole states located at the capping layer-blocking barrier interface in our structure.Comment: 6 pages, 6 figure

Characterisation of a Coriolis flow meter for fuel consumption measurements in realistic drive cycle tests

When testing light-duty and heavy-duty vehicles on chassis dynamometers, as in the WLTP, or engines on engine test benches, as in the WHDC, it is required to measure the fuel consumption. In the preferable case, the measurement of the fuel consumption is carried out with suitable flow meters. These require high measurement accuracy in a wide flow range, independent of the fuel type, as the flow rate range is often very large and depends on the power range of the vehicle engines. Moreover, the fuel flow rate in the test cycles is very dynamically related to the loads. In the scope of the ongoing EMPIR Joint Research Project 20IND13 SAFEST the dynamic flow behaviour as well as the measurement accuracy of flow meters for different types of fuels are investigated. This paper presents first results from the realisation of dynamic flow profiles, and flow measurements with a Coriolis Flow Meter with different representative fuels in a wide density and viscosity range and a wide flow rate range at different fuel temperatures

Regge Calculus in Teleparallel Gravity

In the context of the teleparallel equivalent of general relativity, the Weitzenbock manifold is considered as the limit of a suitable sequence of discrete lattices composed of an increasing number of smaller an smaller simplices, where the interior of each simplex (Delaunay lattice) is assumed to be flat. The link lengths between any pair of vertices serve as independent variables, so that torsion turns out to be localized in the two dimensional hypersurfaces (dislocation triangle, or hinge) of the lattice. Assuming that a vector undergoes a dislocation in relation to its initial position as it is parallel transported along the perimeter of the dual lattice (Voronoi polygon), we obtain the discrete analogue of the teleparallel action, as well as the corresponding simplicial vacuum field equations.Comment: Latex, 10 pages, 2 eps figures, to appear in Class. Quant. Gra

The impact of news on measures of undiversifiable risk: evidence from the UK stock market

Using UK equity index data, this paper considers the impact of news on time varying measures of beta, the usual measure of undiversifiable risk. The empirical model implies that beta depends on news about the market and news about the sector. The asymmetric response of beta to news about the market is consistent across all sectors considered. Recent research is divided as to whether abnormalities in equity returns arise from changes in expected returns in an efficient market or over-reactions to new information. The evidence suggests that such abnormalities may be due to changes in expected returns caused by time-variation and asymmetry in beta

Contrast in transmission spectroscopy of a single quantum dot

We perform transmission spectroscopy on single quantum dots and examine the effects of a resident carriers spin, the incident laser spot size, polarization, and power on the experimental contrast. We demonstrate a factor of 4 improvement in the maximum contrast by using a solid immersion lens to decrease the spot area. This increase yields a maximum signal to noise ratio of 2000 Hz-1/2, which will allow for MHz detection frequencies. We anticipate that this improvement will allow further investigation of spectral fluctuation and open up the feasibility for an all-optical read-out of an electron spin in a quantum dot