Andreas Wierschem

Vortices in film flow over strongly undulated bottom profiles at low Reynolds numbers

We present an experimental study of gravity driven films flowing down sinusoidal bottom profiles of high waviness. We find vortices in the valleys of the undulated bottom profile. They are observed at low Reynolds numbers down to the order of 10−5. The vortices are visualized employing a particle image velocimeter with fluorescent tracers. It turns out that the vortices are generated beyond a critical film thickness. Their size tends to a finite value for thick films. The critical film thickness depends on the waviness of the bottom undulation, the inclination angle, and on the surface tension but not on the Reynolds number. Increasing the waviness, a second vortex can be generated.

Instability of a liquid film flowing down an inclined wavy plane

A linear stability analysis of a Newtonian liquid film flowing down an inclined wavy plane is carried out. It is studied how wavy bottom variations, which are long compared to the film thickness, modify the stability of the steady film flow with respect to that down a flat inclined plane. By allowing for rather moderate bottom variations, it shows the impact of geometric nonlinearities on the instability. In this case, the spatial growth of disturbances becomes dependent on the phase along the bottom wave. Averaging over the bottom variations, it is found that on a large scale the critical Reynolds number for the onset of surface waves is higher than that for a flat bottom. As in the case of a flat bottom, the instability occurs at long wavelength. Locally, however, at the steep slopes the critical Reynolds number is lower than for a flat incline. In a certain range of waves numbers and Reynolds numbers, shorter waves may be excited at the steep slopes and damped at the flat ones.

Influence of inertia on eddies created in films creeping over strongly undulated substrates

We present an experimental study of gravity driven films flowing down a sinusoidal bottom profile of high amplitude. Eddies that are created in the valleys of the undulated bottom profile under creeping flow conditions are modified as inertia becomes significant. We find that the eddies become tilted and their size increases depending on the Froude number. Surface waves, which are generated from natural noise beyond a certain Froude number, induce an oscillation and a breakup of the separatrix by the turnstile-lobe mechanism. This leads to a material exchange between the eddy and the overlying film. We show how the surface waves are coupled to the eddies and discuss the time scales associated with the material exchange of suspended particles.

Hydraulic jumps and standing waves in gravity-driven flows of viscous liquids in wavy open channels

We experimentally study the flow of a viscous liquid down an inclined channel with a sinusoidal bottom profile of moderate waviness. Depending on the film thickness, we find different flow regimes that are usually not observed in the same system. Besides characterizing these regimes, we study the transition from one regime to the other. At low inclination angles, basins form due to nonmonotonously falling bottom slopes. At the inflow of the basins, we observe the formation of stationary hydraulic jumps as shock fronts and surface rollers. We report on a bistable region in which both phenomena can occur. At the low end of the bistable region, an instationary regime of a shock with a fingering lateral modulation is found. The bistability of shocks and surface rollers is traced back to that of the shock front and the fingering. At higher volume flux or inclination angles, standing waves are created in resonance with the bottom contour. At the rising edge of the resonance curves, we observe humps that seem to...

Suppression of eddies in films over topography

We study inertial film flow down undulated inclines. With increasing Reynolds number, eddies are formed in the troughs of the bottom undulation. Further increase leads to a diminution of the eddies until they vanish completely. At even higher Reynolds numbers, they reappear yielding an eddy-free window of Reynolds numbers. Within this window, the free-surface shape changes abruptly. The change comes along with a sudden decrease in the mean film thickness and an abrupt transition of the surface shape type from anharmonic with a strong indentation to harmonic. The anharmonic surface shape shows typical features of a hydraulic jump, which vanishes during the transition. We find that the eddies disappear at Reynolds numbers where the first harmonic of the free-surface contour is sufficiently strong regardless of the exact surface shape. Numerical calculations are in good agreement with the experimental findings.

Pressure-induced crystallization of triacylglycerides

We report on the impact of high pressures on the crystallization of pure triolein and triolein–trilaurin mixtures. We detect growth rates and induction times of single crystals under a high pressure application with a polarization microscope. Statistical studies on the induction time were carried out studying light scattering and transmittance. Similar to supercooling in temperature-induced crystallization, we find a superpressuring in pressure-induced crystallization. In binary mixtures, the inverse induction time indicates a threshold concentration close to the melting point of one component, beyond which it depends linearly on concentration.

Incipient motion of a single particle on regular substrates in laminar shear flow

We study experimentally the critical conditions for incipient motion of a single spherical particle deposited on a regular substrate under laminar flow conditions. The substrates are triangular and quadratic arrangements of identical glass spheres. For the latter configuration, the distance between the substrate spheres is varied, resulting in different partial shielding of the deposited particle to the shear flow. For the studied particle Reynolds numbers range between 3 × 10−4 and 3, the critical Shields number is independent from the particle density and from the particle Reynolds number but it depends significantly on the geometry of the substrate. Depending on the spacing between the substrate beads and thus on the exposure of the particle to the flow, we have observed an increase of about 50 percent in the critical Shields number. Studying the onset of particle motion as a function of the orientation of the substrate to the flow direction we find that the critical Shields number changes by up to a factor of 2, which is mainly due to the fact that the particle travels through the troughs of the substrate and hence the shear force in travel direction diminishes if not in line with the flow direction. Besides the critical Shields number we study the initial stage of particle motion by detecting the minimum time that is necessary for maintaining a certain Shields number to change the position of a single particle on the regular substrates. In the range studied, the initial stage of motion on the scale of the substrates periodicity is mainly governed by the equilibrium particle motion.

Easy and versatile coating approach for long-living white hybrid light-emitting diodes

Herein, we provide a new easy-to-do protocol for preparing luminescent rubber-like materials based on a wide palette of compounds, such as small-molecules, quantum dots, polymers, and coordination complexes. The combination of this new protocol with that for preparing similar rubbers based on fluorescent proteins states the universal character of our approach. This is further assessed by using comprehensive spectroscopic and rheological investigations. Furthermore, the novel luminescent rubbers are applied as down-converting packing systems to develop white hybrid light-emitting diodes (WHLEDs), which are heralded as a solid alternative to achieve energy-saving, solid-state, and white-emitting sources in the coming future. As such, the current work also provides a clear prospect of this emerging lighting technology by means of a direct comparison among WHLEDs fabricated with all the above-mentioned down-converting systems. Here, the use of rubbers based on coordination complexes outperforms the others in terms of both luminous efficiency and colour quality with an unprecedented stability superior to 1000 h under continuous operation conditions. This represents an order of magnitude enhancement compared to the state-of-the-art WHLEDs, while keeping luminous efficiencies of around 100 lm W−1.

Pressure‐induced phase transitions in triacylglycerides

The melting point of triacylglycerides (TAGs) under atmospheric pressure depends on both the fatty acid composition and crystalline structure of the polymorphic state, which are influenced by the temperature treatment history of the TAG. In this contribution, the additional effect of high hydrostatic pressure is described. Samples were placed in a temperature‐controlled cell and pressurized up to 450 MPa. The phase transition was investigated either by perpendicular light scattering and transmission or with a polarized‐light microscope. The high‐pressure polarized‐light microscope allows a precise determination of the melting point. The investigated TAGs showed a significant nonlinear increase of the melting point with pressure. Light scattering and transmission were used to observe the phase change in the high‐pressure cell. Similar to supercooling in temperature‐induced phase transition, we found a dramatic increase of the delay time in our pressure‐induced solidification. Even the dependency of this induction time on the control parameter pressure was similar to that in temperature‐driven crystallization. We propose that different crystalline structures may be obtained by superpressuring instead of supercooling.

Ripple formation in weakly turbulent flow

Abstract.The formation of granular ripples under liquid shear flow in an annular channel is studied experimentally. The erodible granular bed is subject to weakly turbulent flows without a defined sharp boundary layer close to the granular bed. The flow field and the degree of turbulence is characterized quantitatively by using a particle image velocimeter and a laser-Doppler velocimeter, respectively. A new range of particle Reynolds numbers at the lower limit of the Shields diagram were explored. Quantitative measurements of the granular flow on the surface reveal that the threshold for particle motion coincides within the order of one percent with the threshold for ripple formation. In fully developed ripples it was found that on the leeward side of the ripples regions of low-velocity gradients exist where granular motion is scarce, indicating that the coupling between the ripples is mainly caused by the flow field of the liquid.