Two-layer flows of generalized immiscible second grade fluids in a rectangular channel

Unsteady one-dimensional flows of two incompressible and immiscible generalized second grade fluids in a rectangular channel are studied. A constant pressure gradient acts in the flow direction, while the channel walls have oscillating translational motions in their planes. The generalization considered in this paper consists into a mathematical model based on constitutive equations of second grade fluid with Caputo time-fractional derivative in which the history of the shear stress influences the velocity gradient. The velocity and shear stress fields in the Laplace transform domain are obtained. Numerical solutions for the real velocity and shear stress have been found by employing the Stehfest numerical algorithm for the inverse Laplace transform. The influence of the fractional parameters on the velocity and shear stress has been studied by numerical simulations and graphical illustrations. It is found that the memory effects are significant only for small values of the time t.     

Global weak solutions to a class of compressible non-Newtonian fluids with vacuum

The global weak solution of an initial-boundary value problem for a compressible non-Newtonian fluid is studied in a three-dimensional bounded domain. By the techniques of artificial pressure, a solution to the initial-boundary value problem is constructed through an approximation scheme and a weak convergence method. The existence of a global weak solution to the three-dimensional compressible non-Newtonian fluid with vacuum and large data is established.     

Cahn-Hilliard vs Singular Cahn-Hilliard equations in simulations of immiscible binary fluids

An efficient semi-implicit spectral method is implemented to solve the Cahn-Hilliard equation with a variable mobility in this paper. We compared the kinetics of bulk-diffusion-dominated and interface-diffusion-dominated coarsening in two-phase systems. As expected, the interface-diffusion-controlled coarsening evolves much slower. Also we find that the velocity field will be caused different greatly by using Singular Cahn-Hilliard equation and using Cahn-Hilliard in the simulation of immiscible binary fluids.     

确定酮、醛、醚和含氮化合物第二维里系数的新方法

本文提出了一套新的用来确定酮、醛、醚和含氮化合物第二维里系数的新方法。在新的非极性流体预估式的基础上,应用对比态原理,给出了酮、醛、醚和含氮化合物极性流体的优化参数,并对其进行了经验关联,使本预估式能应用于大部分非氢键,弱缔合效应的极性流体。并且仅仅需要临界温度、临界压力、偏心因子和对比偶极距作为预估参数。预估结果与最新的高精度的实验数据符合得很好。     

超导磁场穿透深度的拟合简化计算

磁场穿透深度λ与温度T的变化关系一直是超导领域广泛关注的焦点之一,根据BCS理论得到的穿透深度公式虽然与实验符合得相当好,但数学形式比较复杂.本文以BCS理论得出的λ与T的关系曲线为基准,通过计算机拟合的方法,将二流体模型和London极限得出的两条曲线拟合为一条新的曲线,使其数学形式相对简单,同时物理内涵又比较清晰,而且新的曲线与根据BCS理论得到的曲线基本吻合,达到了合理简化λ(T)计算的目的.     

Rapid and Direct Conversion of Graphite Crystals into High‐Yielding,Good‐Quality Graphene by Supercritical Fluid Exfoliation

Graphene has attracted a great deal of attention in recent years due to its unusual electronic, mechanical, and thermal properties. Exploiting graphene properties in a variety of applications requires a chemical approach for the large‐scale production of high‐quality, processable graphene sheets (GS), which has remained an unanswered challenge. Herein, we report a rapid one‐pot supercritical fluid (SCF) exfoliation process for the production of high‐quality, large‐scale, and processable graphene for technological applications. Direct high‐yield conversion of graphite crystals to GS is possible under SCF conditions because of the high diffusivity and solvating power of SCFs, such as ethanol, N‐methyl‐pyrrolidone (NMP), and DMF. For the first time, we report a one‐pot direct conversion of graphite crystals to a high yield of graphene sheets in which about 90–95 % of the exfoliated sheets are <8 layers with approximately 6–10 % monolayers and the remaining 5–10 % are ≥10 layers.     

Enzymatic degradation of 3D scaffolds of starch-poly-(?-caprolactone) prepared by supercritical fluid technology

Starch-based polymers have been proposed for different tissue engineering applications due to their inherent properties. In this work, a polymeric blend of starch-poly-(?-caprolactone) (SPCL) was processed using supercritical fluid technology, namely, by supercritical assisted phase inversion. As SPCL is a biodegradable polymer, the matrices produced are susceptible of undergoing enzymatic degradation upon implantation in the human body. In vitro assessment of the enzymatic degradation of SPCL was carried out in different buffer solutions containing α-amylase and/or lipase. The effect of the presence of these enzymes was studied by monitoring different parameters in order to characterise both bulk and the surface of the scaffolds. As regards to bulk analysis, weight loss of the samples incubated for 1, 3, 7, 14 and 21 days was determined, further differential scanning calorimetry was carried out. The morphology of the scaffolds after these periods was analysed by micro-computed tomography (μ-CT) and surface chemistry was characterised by infra-red spectroscopy and contact angle measurements. Results suggest that SPLC scaffolds undergo bulk degradation, which is typically characterised by hydrolysis of chemical bonds in the polymer chain at the centre of the matrix, resulting in a highly porous material.     

Extraction of lanthanides from aqueous solution by using room-temperature ionic liquid and supercritical carbon dioxide in conjunction

For the first time, the study of a three-step extraction system of water/ionic liquid/supercritical CO2 has been performed. Extraction of trivalent lanthanum and europium from an aqueous nitric acid solution to a supercritical CO2 phase via an imidazolium-based ionic liquid phase is demonstrated, and extraction efficiencies higher than 87 % were achieved. The quantitative extraction is obtained by using different fluorinated beta-diketones with and without the addition of tri(n-butyl)phosphate. The complexation phenomenon occurring in the room-temperature ionic-liquid (RTIL) phase was evidenced by using luminescence spectroscopy.     

Selective oxidation of alkanes with molecular oxygen and acetaldehyde in compressed (supercritical) carbon dioxide as reaction medium

The oxidation of cycloalkanes or alkylarenes with molecular oxygen and acetaldehyde as sacrificial co-reductant occurs efficiently in compressed (supercritical) carbon dioxide (scCO2) under mild multiphase conditions. No catalyst is required and high-pressure ATR-FTIR online measurements show that a radical reaction pathway is heterogeneously initiated by the stainless steel of the reactor walls. For secondary carbon atoms, high ketone to alcohol ratios are observed (3.5-7.9), most probably due to fast consecutive oxidation of alcoholic intermediates. Since C--C scission reactions are detected only to a very small extent, tertiary carbon atoms are transformed into the corresponding alcohols with high selectivity. Detailed analysis of the product distributions and other mechanistic evidence suggest that acetaldehyde acts not only as the sacrificial oxygen acceptor, but also as an efficient H-atom donor for peroxo and oxo radicals and as a crucial reductant for hydroperoxo intermediates. In comparison to other inert gases such as compressed N2 or Ar, the use of carbon dioxide was shown to increase the yields of alkane oxygenates under identical reaction conditions.     

Vorticity banding during the lamellar-to-onion transition in a lyotropic surfactant solution in shear flow

We report on the rheology of a lyotropic lamellar surfactant solution (SDS/dodecane/pentanol/ water), and identify a discontinuous transition between two shear thinning regimes which correspond to the low-stress lamellar phase and the more viscous shear-induced multilamellar vesicle, or “onion” phase. We study in detail the flow curve, stress as a function of shear rate, during the transition region, and present evidence that the region consists of a shear-banded phase where the material has macroscopically separated into bands of lamellae and onions stacked in the vorticity direction. We infer very slow and irregular transformations from lamellae to onions as the stress is increased through the two-phase region, and identify distinct events consistent with the nucleation of small fractions of onions that coexist with sheared lamellae.