Fractal substructure of a nanopowder generated by repeated fragmentation and sedimentation: the rôle of the dust

Packings of cohesive nanoparticles, that is nanopowders, may be obtained as the result of repeated fragmentation–reagglomeration cycles (Schwager et al. in Phys Rev Lett 100:218002, 2008) such that the resulting sediment reveals a fractal structure. The size distribution of the fragments after a fragmentation step is a superposition of a narrow distribution of large particles (chunks) whose size is determined by the cutting length and a power-law distribution for small particles, representing scale invariant dust. It was shown that the exponent of the power-law, \(\tau \), is in non-trivial relation to the fractal dimension, \(d_f\), via \(d_f(2-\tau )=1\). This poses the question for the structure of the sediment created by repeated fragmentation–reagglomeration cycles when the dust particles are excluded from the reagglomeration step. We found that even in this case, repeated fragmentation–reagglomeration cycles yield a sediment of fractal structure with slightly reduced fractal dimension while the dust exponent, \(\tau \), remains unchanged.     

Control reconfiguration after actuator failures by Markov parameter matching

Two novel linear control reconfiguration methods for plants subject to actuator failures are described. The common idea is to place a reconfiguration block between the faulty plant and the nominal controller in order to re-route the signals around the broken actuator. The first method uses a computationally simple static reconfiguration block. It recovers the nominal plant input/output-behaviour by assigning the faulty plant the same Markov parameters as the faultless plant. The second method concerns the design of the feedforward part in the virtual actuator using the idea and results of the first approach. The virtual actuator is a dynamical reconfiguration block. Existence conditions and solution algorithms are provided, and it is shown that both approaches guarantee the closed-loop stability if the existence conditions are met. An experimental study demonstrates the practical usability of the proposed approaches.     

Experimentelle und theoretische Untersuchungen zur Filtration mit überlagerter Sedimentation in Drucknutschen

Sedimentation superimposed on industrial cake filtration leads to longer filtration times and often has a detrimental effect on subsequent process steps such as washing and demoisturing. The influence of sedimentation is seldom recognised in laboratory filtration experiments. Methods are presented for evaluation of pressure filter experiments with superimposed sedimentation which avoid the error made in the usual evaluation methods. For the case of zone sedimentation the article presents a graphical evaluation and a numerical method of modelling permitting scale up to any desired cake height. In superimposed classifying sedimentation simultaneous measurement of filtrate volume and cake height provide information about local variation in filter cake resistance. It is shown for a model system that, owing to sedimentation, the cake resistance shows a minimum at mean cake height and increases rapidly towards greater heights. The local cake resistance corelates with particle size distributions measured for layers of a horizontally cut filter cake. The method of evaluation presented permints determination of the flow resistance of the uppermost layers of a cake and hence estimation of the gas pressure necessary for demoisturing. The profiles of local filter cake resistance show that the relative cake layering is largely independent of the level of suspension filling. A scale-up model is presented for use in those cases where classifying sedimentation cannot be suppressed.     

Laser-induced desorption of small molecules from oxide surfaces: A first-principles study

Recent efforts in the theoretical simulation of laser-induced desorption of small molecules from surfaces are summarized. As a representative example, photodesorption of CO molecules from a Cr₂O₃(0001) surface is investigated since detailed quantum state resolved experimental results are available for this system. In particular, vectorial properties such as the alignment of the desorbing species are considered. Furthermore, the influence of surface temperature as a control parameter is investigated, and lateral velocity distributions are calculated and compared with experimental results. All simulations presented in the present study are based on ab initio potential energy surfaces (PESs) for the electronic ground state as well as electronically excited states involved in the desorption process. These PESs provide the prerequisite for extensive high-dimensional quantum mechanical simulations of the dynamics of nuclear motion based on a stochastic wave packet scheme. These wave packet calculations allow for a detailed microscopic understanding of experimental results and provide a perspective for future experiments.     

Macrokinetic analysis of polarisation characteristics of gas-diffusion electrodes in contact with liquid electrolytes, Part II: Oxygen reduction as example for a higher order reaction

Classical partial oxidation processes often suffer from low selectivities. Promising alternatives are electrochemical processes where the oxidation takes place at a packed-bed anode while an oxygen-consuming gas–liquid membrane is used as cathode. As a basis for the reliable design of such a process, the performance of oxygen-consuming gas-diffusion electrodes (GDE) is investigated experimentally and is analysed based on a rigorous model accounting for the reaction microkinetics and all relevant mass and charge transport phenomena. The results indicate that oxygen is transported in the gas-filled pores by Knudsen diffusion and that the cathodic oxygen reduction follows a parallel reaction scheme forming hydrogen peroxide at the carbon black support and water at the applied platinum catalyst particles.     

Experimental frames in a modern modeling and simulation system

We investigate the experimental frame concept for discrete event simulation. We first give a brief history of experimental frames and then present and discuss the desired properties of a modern approach to experimental frames. Lastly, we describe the experimental frames implemented in the Hierarchical Modeling and Simulation System-Java (HiMASS-j). HiMASS-j is a prototype modeling and simulation system that uses the Hierarchical Control Flow Graph Model paradigm, visual interactive modeling, and the modern approach to experimental frames.     

Increased Emulsion Stability for Reverse Y‐Shaped Sugar‐Based Surfactants

A series of reverse Y‐shaped surfactants containing aromatic and aliphatic linkers to combine two short hydrocarbon chains and one carbohydrate head group was prepared. Liquid crystalline behavior, air–water interfacial properties, and efficiency as an emulsifier was investigated for each reverse Y‐shaped surfactant. All reverse Y‐shaped surfactants mediated higher emulsion stabilities for water‐in‐oil compared to common typical reference surfactants, reflecting an improved ability to cope with a curvature towards water. The introduction of a benzene ring into the linker substantially increased the affinity of the surfactant for hydrophobic media, resulting in improved emulsion stability for both water‐in‐oil and oil‐in‐water.