Viscosity ratio: A key factor for control of oil drop size distribution in effervescent atomization of oil-in-water emulsions

The present study focused on the effervescent atomization of oil-in-water emulsions and its influence on the internal emulsion structure. The impact of atomization conditions and nozzle geometry on an oil-in-water emulsion with varied viscosity ratio of disperse phase to emulsion was determined. The viscosity ratio is known as a key factor for the drop breakup in emulsification processes. The results depict that oil drops are broken up according to emulsification mechanisms during atomization. A maximum breakup was found for a viscosity ratio between 0.5 and 1. The expected stress dependency of the drop size change was observed for high viscosity ratios only. Relevance of elongational stress on the drop breakup was proven with significant drop size change at high viscosity ratios. The viscosity ratio thus allows for an efficient control of oil drop size change in effervescent atomization as the spray characteristics are invariant to viscosity changes within certain limits.     

Highly defective graphene: A key prototype of two-dimensional Anderson insulators

Electronic structure and transport properties of highly defective two-dimensional (2D) sp² graphene are investigated theoretically. Classical molecular dynamics are used to generate large graphene planes containing a considerable amount of defects. Then, a tight-binding Hamiltonian validated by ab initio calculations is constructed in order to compute quantum transport within a real-space order-N Kubo-Greenwood approach. In contrast to pristine graphene, the highly defective sp² carbon sheets exhibit a high density of states at the charge neutrality point raising challenging questions concerning the electronic transport of associated charge carriers. The analysis of the electronic wavepacket dynamics actually reveals extremely strong multiple scattering effects giving rise to mean free paths as low as 1 nm and localization phenomena. Consequently, highly defective graphene is envisioned as a remarkable prototype of 2D Anderson insulating materials.      

Coalescence of InP Epitaxial Lateral Overgrowth by MOVPE with V/III Ratio Variation

The effects of V/III ratio and seed window orientation on the coalescence of epitaxial lateral overgrowth InP over SiO₂ using metal organic vapor-phase epitaxy with tertiary butyl phosphine were investigated. Parallel lines having θ = 60° and 30° off [0[`1] \bar{1} 1] were coalesced, and their lateral growth rate variation with V/III was measured. Coalescence of lines separated by narrow angles in a star-like pattern was also studied. We find the greatest extent of coalescence to occur when the window stripe is oriented just off of the ⟨010⟩ directions. V/III ratio strongly affects the extent of coalescence, showing an alternating enhancement or inhibition depending on which side of the ⟨010⟩ direction the stripes are oriented. The variation in quality of coalesced material between stripes separated by narrow angles is examined with cross-sectional transmission electron microscopy, illustrating the most problematic growth directions under two V/III ratio conditions.     

Scalable Microstructured Photoconductive Terahertz Emitters

The development of scalable emitters for pulsed broadband terahertz (THz) radiation is reviewed. Their large active area in the 1 – 100 mm² range allows for using the full power of state-of-the-art femtosecond lasers for excitation of charge carriers. Large fields for acceleration of the photogenerated carriers are achieved at moderate voltages by interdigitated electrodes. This results in efficient emission of single-cycle THz waves. THz field amplitudes in the range of 300 V/cm and 17 kV/cm are reached for excitation with 10 nJ pulses from Ti:sapphire oscillators and for excitation with 5 μJ pulses from amplified lasers, respectively. The corresponding efficiencies for conversion of near-infrared to THz radiation are 2.5 × 10^-4 (oscillator excitation) and 2 × 10^-3 (amplifier excitation). In this article the principle of operation of scalable emitters is explained and different technical realizations are described. We demonstrate that the scalable concept provides freedom for designing optimized antenna patterns for different polarization modes. In particular emitters for linearly, radially and azimuthally polarized radiation are discussed. The success story of photoconductive THz emitters is closely linked to the development of mode-locked Ti:sapphire lasers. GaAs is an ideal photoconductive material for THz emitters excited with Ti:sapphire lasers, which are widely used in research laboratories. For many applications, especially in industrial environments, however, fiber-based lasers are strongly preferred due to their lower cost, compactness and extremely stable operation. Designing photoconductive emitters on InGaAs materials, which have a low enough energy gap for excitation with fiber lasers, is challenging due to the electrical properties of the materials. We discuss why the challenges are even larger for microstructured THz emitters as compared to conventional photoconductive antennas and present first results of emitters suitable for excitation with ytterbium-based fiber lasers. Furthermore an alternative concept, namely the lateral photo-Dember emitter, is presented. Due to the strong THz output scalable emitters are well suited for THz systems with fast data acquisition. Here the application of scalable emitters in THz spectrometers without mechanical delay stages, providing THz spectra with 1 GHz spectral resolution and a signal-to-noise ratio of 37 dB within 1 s, is presented. Finally a few highlight experiments with radiation from scalable THz emitters are reviewed. This includes a brief discussion of near-field microscopy experiments as well as an overview over gain studies of quantum-cascade lasers.     

Experimental analysis of a degraded open-cathode PEM fuel cell stack

The well-known challenges to overcome in PEM fuel cell research are their relatively low durability and the high costs for the platinum catalysts. This work focuses on degradation mechanisms that are present in open-cathode PEM fuel cell systems and their links to the decaying fuel cell performance. Therefore a degraded, open-cathode, 20 cell, PEM fuel cell stack was analyzed by means of in-situ and ex-situ techniques. Voltage transients during external perturbations, such as changing temperature, humidity and stoichiometry show that degradation affects individual cells quite differently towards the end of life of the stack. Cells located close to the endplates of the stack show the biggest performance decay. Electrochemical impedance spectroscopy (EIS) data present non-reversible catalyst layer degradation but negligible membrane degradation of several cells. Post-mortem, ex-situ experiments, such as cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) show a significant active area loss of the first cells within the stack due to Pt dissolution, oxidation and agglomeration. Scanning electron microscope (SEM) images of the degraded cells in comparison with the normally working cells in the stack show severe carbon corrosion of the cathode catalyst layers.     

Short‐term spatio‐temporal prediction of wind speed and direction

This paper aims to produce a low‐complexity predictor for the hourly mean wind speed and direction from 1 to 6 h ahead at multiple sites distributed around the UK. The wind speed and direction are modelled via the magnitude and phase of a complex‐valued time series. A multichannel adaptive filter is set to predict this signal on the basis of its past values and the spatio‐temporal correlation between wind signals measured at numerous geographical locations. The filter coefficients are determined by minimizing the mean square prediction error. To account for the time‐varying nature of the wind data and the underlying system, we propose a cyclo‐stationary Wiener solution, which is shown to produce an accurate predictor. An iterative solution, which provides lower computational complexity, increased robustness towards ill‐conditioning of the data covariance matrices and the ability to track time‐variations in the underlying system, is also presented. The approaches are tested on wind speed and direction data measured at various sites across the UK. Results show that the proposed techniques are able to predict wind speed as accurately as state‐of‐the‐art wind speed forecasting benchmarks while simultaneously providing valuable directional information. Copyright © 2013 John Wiley & Sons, Ltd.     

Strain-induced crystallization around a crack tip in natural rubber under dynamic load

The local degree of crystallinity around a crack tip in natural rubber under dynamic load was measured by time-resolved scanning wide-angle X-ray diffraction (WAXD). Using a high-flux synchrotron microbeam, WAXD patterns with less than 20 ms exposure time were acquired while the notched rubber sample was subjected to cyclic dynamic stretching at a frequency of 1 Hz, similar to the loading conditions in tear fatigue experiments. By scanning the continuously cycling sample, a complete crystallinity map at any given strain phase angle was obtained. The crystallinity at the crack tip is considerably reduced compared to static crack tip scans. Further investigations revealed the underlying structural reasons for the well-known relation between R-ratio and crack growth resistance. By performing static crack tip scans on increasingly stretched rubber samples, the mechanisms behind crack deflection and branching were studied.     

Entwicklung allgemeiner Entwurfsgrundsätze für Hängernetze von Netzwerkbogenbrücken

Um die statischen Vorteile von Netzwerkbogenbrücken effizient nutzen zu können, ist die Anordnung der Hänger von großer Bedeutung. Der Tragwerksentwurf wird deshalb wesentlich von der Wahl des Hängernetzes bestimmt. Hängerneigung, Hängeranzahl und die Lage der Hängeranschlusspunkte beeinflussen die Spannungszustände sowohl in Bogen und Versteifungsträger als auch in den Hängern in erheblichem Maße. So wird zum Beispiel die Größe der Biegemomente in Bogen und Untergurt durch die Anzahl der Hänger und die damit verbundenen Abstände der Anschlüsse bestimmt. Des Weiteren ist die Hängerneigung ein entscheidender Faktor für die Anzahl der Ausfälle der druckschlaffen Hänger oder die Größe der Spannungsamplituden der Hänger. Nicht minder wichtig ist jedoch auch das ästhetische Erscheinungsbild des Tragwerkes, welches erheblich durch die Art und Weise der Hängeranordnung geprägt wird. Um das effizienteste Hängernetz zu ermitteln, werden fünf mögliche Hängernetze mit variierenden Hängerneigungsparametern, Stützweiten und Bogenhöhen (f/l‐Verhältnisse) hinsichtlich gezielt ausgewählter statischer Kriterien untersucht und bewertet. Daraus resultierend werden Empfehlungen formuliert, die dem Ingenieur die Wahl eines für entsprechende Rahmenbedingungen geeigneten Hängernetzes erleichtern. Abschließend erfolgt ein Vergleich zwischen einer Brücke mit einem nach den erhaltenen Erkenntnissen konstruierten Hängernetz und einem Tragwerk, bei dem die Hänger ähnlich zu bereits ausgeführten Netzwerkbogenbrücken angeordnet sind. Development of general design principles for the hanger arrangements of network arch bridges. To use the static advantages of network arch bridges efficiently, the arrangement of the hangers is very important. The stress conditions in the arch and lower chord are strongly affected by the slope of the hangers, the number of the hangers and the position of the hanger connection points. For example, the size of the bending moments in the arch and lower chord depends on the number of hangers and the associated distances between the connection points. Furthermore, the slope of the hangers is a decisive factor for the number of relaxed hangers and the size of the stress amplitudes. Not less important is the aesthetic view of the structure, which is influenced by the kind of hanger arrangement. To optimize the hanger‐network, different kinds of hanger arrangement methods have been investigated, considering different hanger slopes for each hanger. Five different and possible hanger arrangements with varying parameters (slope of the hangers, number of the hangers, span of the bridge, arch rise) have been analysed in respect to systematic selected static criterions. On the basis of these investigations, recommendations are given for engineering applications how to choose an optimized hanger arrangement according to different geometrical bridge parameters. Finally, a comparison between a network arch, based on the results of the analyses, and a structure with a hanger slope similar to existing network arch bridges is shown to clarify the advantages of the developed hanger arrangement.     

Temperature‐Responsive Substrates: Adhesion and Mechanical Properties of PNIPAM Microgel Films and Their Potential Use as Switchable Cell Culture Substrates (Adv. Funct. Mater. 19/2010)

Thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) microgel films are shown to allow controlled detachment of adsorbed cells via temperature stimuli. Cell response occurs on the timescale of several minutes, is reversible, and allows for harvesting of cells in a mild fashion. The fact that microgels are attached non‐covalently allows using them on a broad variety of (charged) surfaces and is a major advantage as compared to approaches relying on covalent attachment of active films. In the following, the microgels’ physico‐chemical parameters in the adsorbed state and their changes upon temperature variation are studied in order to gain a deeper understanding of the involved phenomena. By means of atomic force microscopy (AFM), the water content, mechanical properties, and adhesion forces of the microgel films are studied as a function of temperature. The analysis shows that these properties change drastically when crossing the critical temperature of the polymer film, which is the basis of the fast cell response upon temperature changes. Furthermore, nanoscale mechanical analysis shows that the films posses a nanoscopic gradient in mechanical properties.