Dynamic wetting of complex liquids

A simple dependency between contact angle θ and velocity or surface tension has been predicted for the wetting and dewetting behavior of simple liquids. According to the hydrodynamic theory, this dependency was described by Cox and Voinov as θ ∼ Ca^(1/3) (Ca: Capillary number). For more complex liquids like surfactant solutions, this prediction is not directly given.rnHere I present a rotating drum setup for studying wetting/dewetting processes of surfactant solutions on the basis of velocity-dependent contact angle measurements. With this new setup I showed that surfactant solutions do not follow the predicted Cox-Voinov relation, but showed a stronger contact angle dependency on surface tension. All surfactants independent of their charge showed this difference from the prediction so that electrostatic interactions as a reason could be excluded. Instead, I propose the formation of a surface tension gradient close to the three-phase contact line as the main reason for the strong contact angle decrease with increasing surfactant concentration. Surface tension gradients are not only formed locally close to the three-phase contact line, but also globally along the air-liquid interface due to the continuous creation/destruction of the interface by the drum moving out of/into the liquid. By systematically hindering the equilibration routes of the global gradient along the interface and/or through the bulk, I was able to show that the setup geometry is also important for the wetting/dewetting of surfactant solutions. Further, surface properties like roughness or chemical homogeneity of the wetted/dewetted substrate influence the wetting/dewetting behavior of the liquid, i. e. the three-phase contact line is differently pinned on rough/smooth or homogeneous/inhomogeneous surfaces. Altogether I showed that the wetting/dewetting of surfactant solutions did not depend on the surfactant type (anionic, cationic, or non-ionic) but on the surfactant concentration and strength, the setup geometry, and the surface properties.rnSurfactants do not only influence the wetting/dewetting behavior of liquids, but also the impact behavior of drops on free-standing films or solutions. In a further part of this work, I dealt with the stability of the air cushion between drop and film/solution. To allow coalescence between drop and substrate, the air cushion has to vanish. In the presence of surfactants, the vanishing of the air is slowed down due to a change in the boundary condition from slip to no-slip, i. e. coalescence is suppressed or slowed down in the presence of surfactant.

Quantum gravity effects in rotating black hole spacetimes

The aim of this work is to explore, within the framework of the presumably asymptotically safe Quantum Einstein Gravity, quantum corrections to black hole spacetimes, in particular in the case of rotating black holes. We have analysed this problem by exploiting the scale dependent Newton s constant implied by the renormalization group equation for the effective average action, and introducing an appropriate "cutoff identification" which relates the renormalization scale to the geometry of the spacetime manifold. We used these two ingredients in order to "renormalization group improve" the classical Kerr metric that describes the spacetime generated by a rotating black hole. We have focused our investigation on four basic subjects of black hole physics. The main results related to these topics can be summarized as follows. Concerning the critical surfaces, i.e. horizons and static limit surfaces, the improvement leads to a smooth deformation of the classical critical surfaces. Their number remains unchanged. In relation to the Penrose process for energy extraction from black holes, we have found that there exists a non-trivial correlation between regions of negative energy states in the phase space of rotating test particles and configurations of critical surfaces of the black hole. As for the vacuum energy-momentum tensor and the energy conditions we have shown that no model with "normal" matter, in the sense of matter fulfilling the usual energy conditions, can simulate the quantum fluctuations described by the improved Kerr spacetime that we have derived. Finally, in the context of black hole thermodynamics, we have performed calculations of the mass and angular momentum of the improved Kerr black hole, applying the standard Komar integrals. The results reflect the antiscreening character of the quantum fluctuations of the gravitational field. Furthermore we calculated approximations to the entropy and the temperature of the improved Kerr black hole to leading order in the angular momentum. More generally we have proven that the temperature can no longer be proportional to the surface gravity if an entropy-like state function is to exist.