Two-dimensional interaction of a wetting front with an impervious layer: Analytical and numerical solutions

Wetting-front movement can be impaired whenever the flow region includes boundaries such as the soil surface, seepage faces, planes of symmetry, or actual layers that are effectively impermeable, such as heavy clays or coarse materials below the water-entry pressure. An approximate analytical solution for interaction of flow from a line source with a parallel plane, impervious layer is derived. The solution ignores gravity and assumes a particular diffusivity that is related to the constant flow rate. It is exact until interaction begins, and provides an accurate approximation for short times thereafter. It can therefore be used to test the accuracy of numerical solutions of the flow equation, which can then be used with confidence for later times when the analytical approximation breaks down, for instance because gravity is ignored. A finite difference solution was tested in this way for both gradual and steep wetting fronts. Agreement between the two solutions was excellent for the gradual front, with the analytical approximation only slightly in error at later times. Numerical errors at the steep front were much greater; an accurate solution needed a finer spatial grid and a restart from the exact analytical values at the beginning of the interaction. The analytical approximation, though not as accurate as for the gradual front, was still good.     

Domain-induced budding in buckling membranes

We present a phase field model on buckling membranes to analyze phase separation and budding on soft membranes. By numerically integrating dynamic equations, it turns out that the formation of caps is greatly influenced by the presence of a little excess area due to the surface area constraint. When cap-shaped domains are created, domain coalescence is mainly observed not between domains with same budding directions, but between domains with opposite budding directions, because the bending energy between two domains is larger in the former case. Although we do not introduce spontaneous curvature like Helfrich model, we obtain some suggestions related to the slow dynamics of the phase separation on vesicles.     

Stimulated emission at the second order stop-zone edge of the two-dimensional opal–zinc oxide photonic crystal

The fabrication of the 2D periodic structures in ZnO thin films by magnetron sputtering on the opal matrices was developed. The microstructures were characterized by AFM and SEM. The spontaneous and stimulated emissions of the ZnO layers on opal were studied at N₂ laser excitation (λ = 337 nm). The stimulated emission near 397 nm was observed at room temperature from ZnO–opal structure. The threshold of the electron–hole plasma recombination laser process was 300 kW/cm² for this structure. This threshold is two orders of magnitude smaller of that one for the flat ZnO–SiO₂ films owing to DFB resonator effect in 2D structure.     

Practical Kedem-Katchalsky equations and their modification

The research problem presented in this work concerns modification of the Kedem-Katchalsky (K-K) equation for volume flow (J _v ) through system (h|M|l), consisting of a membrane M and boundary layers h and l. Such boundary layers appear in the vicinity of the membrane on both sides due to the lack of mixing of solutions. This paper also includes the derivation of the equation for volume flow (J _vr ) dissipated on concentration boundary layers h and l. The derivation of these equations concerns the case in which the substance transport through the membrane is generated by the osmotic pressure gradient . On the basis of the equations for the volume flows (J _v ) and (J _vr ), some calculations for a nephrophane membrane, used in medicine, and for aqueous glucose solutions have been carried out. In order to test the equations for (J _v ) and (J _vr ), we have also carried out calculations for the volume flow (J′ _v ) that is transferred through the membrane in the case of mixed solutions on both sides of the membrane. This volume flux has been calculated on the basis of the original (K-K) equation. The results are presented in Fig. 2.      

Uniform convergence of a weighted average scheme for a nonlinear reaction–diffusion problem

This paper deals with a weighted average scheme (or θ$\theta$-scheme) for solving a nonlinear singularly perturbed parabolic reaction–diffusion problem. The uniform convergence of the weighted average scheme on piecewise uniform and log-meshes is established. Numerical experiments are presented.     

Unusual kinematics-driven chemistry: cleaving C-H but not COO-H bonds with hyperthermal protons to synthesize tailor-made molecular films

Unconventional reaction-design strategies have been developed to exploit the intriguing kinematics that occur when adsorbed organic molecules are bombarded by a beam of hyperthermal protons: kinematic energy transfer is only effective in H-->H collisions and thus only C-H bonds are cleaved. This process yields a cross-linked molecular film with its chemistry governed by the selection of appropriate precursor molecules. Unlike the conventional wet-chemistry synthesis of cross-linked polymeric films, this new route uses no chemical initiators, additives, nor catalysts, and only requires a proton beam with a kinetic energy of a few electron volts in a dry-process mode compatible with molecular-device fabrication. The reaction designs are expressed unconventionally: reaction energy is tuned by the kinetic energy of the proton beam and reactant supply is controlled precisely by the proton fluence. However, conventional considerations such as bond-strength effects on kinematic outcomes and branching-ratio statistics are also important and they can extend the reaction applicability of the kinematics concept. For example, taking advantage of the fact that COO-H bonds are stronger than C-H bonds, we show, with practical reaction conditions, synthesis results, and surface analysis using X-ray photoelectron spectroscopy and atomic force microscopy, that we can break C-H bonds without breaking COO-H and other bonds, in the production of cross-linked molecular layers with any desirable COOH concentration and with no ester nor other chemical contaminations. The new reaction-design strategies are also applicable to the synthesis of molecular layers with other functionalities such as OH, and to the synthesis of a mixture of functionalities, such as OH/COOH, with a controllable concentration ratio.     

具有可调电学和光学性质的双层和三层Janus Ga2SSe

将二维(2D)层状材料的单层堆叠成双层或者少数层,可以很好的调节其光电性质,为该领域发展提供了新的机遇.本文采用第一性原理方法系统地研究了堆叠层数和堆叠次序对双层和三层Janus Ga₂SSe的电学和光学性质的影响.我们发现这些结构的层间距差别很大,而结合能差异却很小.尽管所有的双层和三层Janus Ga₂SSe具有间接带隙,然而其带隙值和载流子有效质量与堆叠层数和堆叠次序密切相关.此外,在Janus Ga₂SSe中,通过增加层数,可以增强其在可见光和紫外区域的吸收系数.同时,通过控制层间堆叠模式,进一步调制其吸收系数,导致在可见光和近紫外区域产生多个吸收峰.我们的结果为双层和三层Janus III族单硫化合物的可调节电学和光学性质提供了有价值的见解,这表明其可能在纳米电子和光电子器件中有着广阔的应用前景.     

Organogelators from self-assembling peptide based dendrimers: structural and morphological features

Two peptide based dendrimers containing l-aspartic acid as the branching unit and succinic acid/terephthalic acid as the core unit were synthesized, characterized, and studied. These dendritic peptides form gels in various organic solvents including n-hexanol, benzene, toluene, chlorobenzene, 1,2-dichlorobenzene, o-xylene, tetralin, and nitrobenzene. Gels were characterized by freeze fracture transmission electron microscopic (FF-TEM), field emission scanning electron microscopic (SEM), transmission electron microscopic (TEM), wide angle X-ray powder diffraction (WAXPD), and variable temperature FTIR (VT-FTIR) studies. The VT-FTIR study indicates that amides and ester groups are involved in intermolecular hydrogen bonding in the gel state. Two transitions have been observed for both the dendrimer gels upon heating: the first one corresponds to the gel to sol transition and corresponds to the breaking of hydrogen bonds between esters and amides; the second one corresponds to the breaking of hydrogen bonds between amides. In the case of dendrimer 1 structural reorganization occurs in the sol state after the first transition, which is absent in the dendrimer 2 in the sol state. FF-TEM observations showed that both dendritic peptides form a platelet structure in gel state. SEM images of these dried gels indicate different geometry in different solvents in their self-assembled gel state.     

Supramolecular Assembly of Double‐Stranded Chains,Helical Chains and Catenane‐like Layers with Flexible Ligands

Four new transition metal coordination polymers, [Co(bpndc)(phen)(H₂O)]_n ( 1 ), [Co₃(bpndc)₃(2,2′‐bpy)₂]_n·0.5n(i‐C₃H₇OH) ( 2 ), and [M(bpndc)(2,2′‐bpy)₂]_n (M = Zn, 3 ; Cu, 4 ; H₂bpndc = benzophenone ‐4,4′‐dicarboxylic acid; phen = 1,10‐phenanthroline; 2,2′‐bpy = 2,2′‐bipyridine) have been synthesized by the hydrothermal reactions and characterized by single crystal X‐ray diffraction, elemental analysis, and IR spectrum. Because of the introduction of different terminal auxiliary ligands, bpndc ligands in complexes 1 and 2 adopt different coordination modes. In complex 1 , bpndc ligands act as tridentate ligand and bridge Co^II ions into 1D double‐stranded chains; while complex 2 possesses 2D (4,4) grids, where bpndc ligands adopt tetradente and pentadentate modes. Two such grids interpenetrate to form a novel catenane‐like layer. Complexes 3 and 4 are isostructural. Bpndc ligands adopt tetradentate mode and bridge metal ions forming 1D helical chains.     

Alkaline fuel cells: Calculating and comparing overall currents of hydrophobized cathodes with thin regular-structure and thick stochastic-structure active layers

A concept of active hydrophobized active layers with regular structure is introduced. In these layers, a hydrophobizer takes part in the development of gas pores representing a set of straight identical rods (cylinders) uniformly distributed over the active layer and extended in a direction perpendicular to the cathode surface. An advantage of cathodes with a thin regular-structure active layer is the reproducibility of their characteristics and a low content of platinum catalyst (up to tenth and even hundredth fractions of mg/cm²). A comparison of current characteristics of thin (with the thickness of several tens of μm) active layers with a regular structure and thick (with the thickness of several hundreds of μm) with the stochastic distribution of the hydrophobizer (with randomly distributed polytetrafluoroethylene) is made. For a fuel cell with an alkaline electrolyte (7 M KOH at 60°C), calculations show that at potentials below 0.5 V (RHE), the cathodes with thin regularstructure active layers demonstrate higher overall currents as compared with cathodes covered with thick active layers with a stochastic structure. However, the opposite trend is observed at potentials above 0.5 V. To increase the current in cathodes with thin regular-structure active layers, it is possible to, first, increase the active layer thickness and, second, decrease the size of hydrophobizer grains in them.