Stability analysis of gravity driven shear flows with free surface for power-law fluids

Summary We study the stability of thin films of fluids subject to gravity along inclined planes, obeying a power-law constitutive relation of the Ostwald-de Waele type. A first analysis, in which the inertia terms are ignored, shows such flow to be stable against small, linear perturbations; a second analysis, in which the inertia terms are included, proves that there are stable and unstable regimes that are separated by a critical Ostwald-de Waele number O. Numerical computations for selected values of O demonstrate the decay and growth rate behavior of some finite amplitude disturbances. Received 12 May 1997; accepted for publication 23 July 1997     

Nickelmodifiedlarge poremesoporoussilicas ascatalysts for methanol decomposition

Summary Ni-modifiedlarge poremesoporous silicasarecharacterized by XRD, N₂physisorption and TPR with H₂. Theeffectof the supportpore structure on their catalytic behaviorin methanol decomposition to H₂, CO and CH₄is studied.     

Maximum entropy deconvolution of AFM and STM images

The Maximum Entropy Method (MEM) has been applied successfully to the deconvolution of images obtained by Atomic Force (AFM) and Scanning Tunneling Microscopy (STM) using a NanoScope III system. The images have been taken on graphite (STM) and NaCl (AFM) substrates. Image processing has been performed running the Cambridge MaxEnt Fortran 77 library MEMSYS-5 on an IBM RISC 6000/360. Among the possible hypotheses the optimal solution was selected using the standard entropy method. ICF and response function have been generated artificially to fit the correlation of physical structures for atomically resolved images. Comparison of MEM and FFT revealed, that the main advantage of MEM is its ability to reproduce atomic defects on regular structures, whereas FFT deconvolution tends to eliminate these perturbations.     

Ionic liquids from Car-Parrinello simulations, part I: liquid AlCl3

The properties of isolated AlCl3 clusters and the bulk system are investigated by means of static and dynamic electronic structure methods. We find important structural motifs with the edge connectivity dominant in a dimer and the corner connectivity dominant in a trimer. Furthermore, the trimer cluster exhibits an interesting ring structure with large cooperative effects relative to the dimer. Comparing the found structural motifs in isolated molecule calculations with the structure of the liquid allows us to determine the dominance of edge connectivity in the liquid. The size of the clusters present in the liquid indicates indeed that the dimer is the most abundant species, but there are also trimers, tetramers, and pentamers present. From the local dipole analysis both for the isolated clusters as well as for the liquid, further proof for the edge connectivity is given. However, all results point to the fact that there is also some small percentage of corner connectivity present that might be attributed to the most stable corner-connected cluster, namely the trimer. Importantly, we find that energetic considerations of isolated (static) clusters only do not represent the findings in liquid phase. Instead, a quantum cluster equilibrium approach or simulations are needed.     

Particle image velocimetry in a foam-like porous structure using refractive index matching: a method to characterize the hydrodynamic performance of porous structures

We present a method to measure two-dimensional velocity fields inside an artificial foam-like porous structure using particle image velocimetry and a refractive index matching technique to avoid optical distortion. The porous structure is manufactured by stereolithography with the epoxy resin WaterShed^? XC 11122 as solid material, and anisole is used as refractive index-matched fluid. It was found that the direction of build-up of the stereolithographic structure plays an important role for the quality of the recorded images. The velocity fields measured in this study and the turbulent statistics derived thereof allow to characterize the hydrodynamic performance of the artificial foam-like structure and clarify the mechanisms of mixing. Results from this study compare well to results from a large eddy simulation reported by Hutter et?al. (Chem Eng Sci 66:519–529, 2011b) and hence reinforce these simulations.     

On flows of granular materials

This article reviews the behavior of materials made up of a large assemblage of solid particles under rapid and quasi static deformations. The focus is on flows at relatively high concentrations and for conditions when the interstitial fluid plays an insignificant role. The momentum and energy exchange processes are then primarily governed by interparticle collisions and Coulomb-type frictional contact. We first discuss some physical behavior —dilatancy, internal friction, fluidization and particle segregation — that are typical to the understanding of granular flows. Bagnold's seminal Couette flow experiments and his simple stress analysis are then used to motivate the first constitutive theories that use a microstructural variable — the fluctuation energy or granular temperature — governing the subscale fluctuating motion. The kinetic theories formalize the derivation of the field equations of bulk mass, momentum and energy, and permit derivation of constitutive relations for stress, flux of fluctuation energy and its dissipation rate for simple particle assemblages and when frictional rubbing contact can be ignored. These statistical considerations also show that formulation of boundary conditions needs special attention. The frictional-collisional constitutive behavior in which both Coulomb-type rubbing contact and collisional encounters are significant are discussed. There is as yet no rigorous formulation. We finally present a phenomenological approach that describes rapid flows of granular materials under simultaneous transport of heat and close with a summary of stability analyses of the basic flow down an inclined plane.Dedicated to Professor Dr.-Ing. Franz Gustav Kollmann on the occasion of his sixtieth brithday     

On stability of rapid granular shear flows

We study stability properties of rapid granular flows that are described by the balance laws of mass, momentum and fluctuation energy with phenomenological relationships based upon dimensional arguments [12, 13]. Small disturbances propagating perpendicular to the shear plane are only studied. Calculations show that such flows are stable at small free path lengths of the granules but unstable if free path lengths are large. Received June 17, 1997     

Boron dipyrromethene chromophores: next generation triplet acceptors/annihilators for low power upconversion schemes

In the present study, the red-light absorbing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) was used as a triplet sensitizer in conjunction with two distinct iodophenyl-bearing BODIPY derivatives independently serving as triplet acceptors/annihilators poised for photon upconversion based on triplet-triplet annihilation. In deaerated benzene solutions, extremely stable and high quantum efficiency green (Phi(UC) = 0.0313 +/- 0.0005) and yellow (Phi(UC) = 0.0753 +/- 0.0036) upconverted emissions were observed from selective red excitation of the PtTPBP sensitizer at 635 +/- 5 nm. The current systems represent the first examples of photon upconversion where aromatic hydrocarbons do not serve the role of triplet acceptor/annihilator. Notably, the nature of the current chromophore compositions permitted highly reproducible upconversion quantum efficiency determinations while permitting the evaluation of the triplet-triplet annihilation quantum yields in both instances.