The dynamics of single droplets in a bounded shear flow is experimentally and numerically investigated for blends that contain one viscoelastic component. Results are presented for systems with a viscosity ratio of 1.5 and a Deborah number for the viscoelastic phase of 1. The numerical algorithm is a volume-of-fluid method for tracking the placement of the two liquids. First, we demonstrate the validation of the code with an existing boundary integral method and with experimental data for confined systems containing Newtonian components. This is followed by numerical simulations and experimental data for the combined effect of geometrical confinement and component viscoelasticity on the droplet dynamics after startup of shear flow at a moderate capillary number. The viscoelastic liquids are Boger fluids, which are modeled with the Oldroyd-B constitutive model and the Giesekus model. Confinement substantially increases the viscoelastic stresses and the elongation rates in and around the droplet. We show that the latter can be dramatic for the use of the Oldroyd-B model in confined systems with viscoelastic components. A sensitivity analysis for the choice of the model parameters in the Giesekus constitutive equation is presented. 相似文献
A covering of the Euclidean plane by a polygon P is a system of translated copies of P whose union is the plane, and a packing of P in the plane is a system of translated copies of P whose interiors are disjoint. A lattice covering is a covering in which the translates are defined by the points of a lattice, and a lattice packing is defined similarly. We show that, given a convex polygon P with n vertices, the densest lattice packing of P in the plane can be found in O(n) time. We also show that the sparsest lattice covering of the plane by a centrally symmetric convex polygon can be solved in O(n) time. Our approach utilizes results from classical geometry that reduce these packing and covering problems to the problems of finding certain extremal enclosed figures within the polygon. 相似文献
The entanglement process between porphyrins and some amphiphiliccyclodextrins and the occurrence of different species have been proved by the combination of UV-Visabsorption, fluorescence anisotropy, resonance light scattering and 1H NMR spectroscopy. 相似文献
The chemical compatibility between YBa2Cu3O6.8 (Y123) and Pt, Ag or Au was studied using quasi-infinite diffusion couples which were encapsulated and annealed at 650 to 800°C for 5 to 80 h. The phase formation at the interface was analyzed in cross sections of these couples using optical and scanning electron microscopy together with energy- and wavelength dispersive X-ray microanalysis. In addition, bulk powder mixtures of Y123 with Pt, Ag or Au were annealed at 800°C for 100 h and phase analysis was performed using X-ray diffraction.
At the Pt/Y123 interface a reaction zone grows slowly but decisively at 800°C. Its microstructure is multiphase with Y---Cu-oxides and a fine-grained dispersion of Y2BaCuO5 (Y211) with Ba---Cu-oxides. Additional Ba---Pt oxides and other phases are seen in powder mixtures annealed for longer times. Barium is suspected to diffuse out from the superconductor along grain boundaries from as deep as 1 mm, causing the decomposition of Y123 into Y---Cu-oxides in the depleted regions. Ag and Au form a stable contact at the interface to Y123 with no reaction zone or new phases. Interdiffusion at the Ag/Y123 interface at 800°C was too low to be clearly detected. In contrast, Au diffuses very fast into Y123 and at 800°C the solubility is 4.2 mass% Au. Yttrium and barium diffuse much slower into the (Au) phase, Cu diffusion was not detected. Weak traces of decomposition products, mostly Y211 and Ba---Cu-oxides, were observed in bulk powder mixtures of Y123 with Ag or Au annealed at 800°C for 100 h in closed capsules. These decomposition products are considered to be due to the high oxygen pressure in the closed capsule, exceeding the stability limit of Y123, and not due to the reduction of Y123. Both Ag and Au are virtually non-reactive with Y123. 相似文献
An approximate analytical solution is provided for one-dimensional, counter- current, spontaneous imbibition of a wetting
phase (water) into a semi-infinite porous medium. The solution is based on the assumption that a similarity solution exists
for the displacement process. This assumption, in turn, rests on the assumption that the set of relative permeability and
capillary pressures curves are unique functions of saturation and do not depend on the nature of the displacement. It further
rests on the assumption that the saturation at the imbibition face does not vary with time. It is demonstrated that the solution
is in agreement with results obtained from experiments and also numerical analyses of these experiments. The experiments utilize
cylindrical samples with the radial surface and one end-face sealed, and with counter-current imbibition occurring at the
open end-face. The stage of the experiment that is modeled by the present solution is the period before the imbibition front
contacts the sealed end-face. An important finding of the present analysis is that the pressure upstream of the advancing
invasion front is a constant. A second, improved solution is also presented; this solution is an iterative, series solution
of an integral-differential equation. It converges to a stable solution in very few terms. 相似文献
Palladium(II) acetate is readily converted into [Pd3(μ2‐OH)(OAc)5] ( 1 ) in the presence of water in a range of organic solvents and is also slowly converted in the solid state. Complex 1 can also be formed in nominally anhydrous solvents. Similarly, the analogous alkoxide complexes [Pd3(μ2‐OR)(OAc)5] ( 3 ) are easily formed in solutions of palladium(II) acetate containing a range of alcohols. An examination of a representative Wacker‐type oxidation shows that the Pd‐OH complex 1 and a related Pd‐oxo complex 4 can be excluded as potential catalytic intermediates in the absence of exogenous water. 相似文献
Methods for the highly stereoselective and regiodivergent hydrosilylation of 1,3‐disubstituted allenes have been developed. The synthesis of E allylsilanes is accomplished with palladium NHC catalysts, and trisubstituted Z alkenylsilanes are accessed with nickel NHC catalysts. Unsymmetrically substituted allenes are well tolerated with nickel catalysis and afford Z alkenylsilanes. Evidence for a plausible mechanism was obtained through an isotopic double‐labeling crossover study. 相似文献
Homoatomic polyanions of post‐transition main‐group metals, namely, Zintl anions, are precast in analogous Zintl phases and can react in solution to form new materials. Despite comprehensible reaction approaches, the formed products cannot be planned in advance, as hitherto undetected and therefore disregarded side reactions take place. The outcomes and interpretations of the reactions of Zintl anions are so far based mainly on crystal structures, which only allow characterization of the product that has the lowest solubility. Here we present the results of our investigation of the stability of highly charged tin Zintl anions in liquid ammonia, which is not exclusively based on solution effects but also on the oxidative influence of the solvent. This allows for a deeper understanding of the ongoing processes in solution and opens doors to the directed synthesis of transition metal complexes of Sn44?, here shown by its reactivity towards MesCu. 相似文献