Simulation of the Influence of Surface Wettability on Viscous Fingering Phenomenon in Porous Media

Fingering phenomena are common occurrence in the natural world. It generally takes place when a less viscous fluid displacesa more viscous fluid typically in porous media. Nowadays, such phenomena have extensively been studied due to itsimportance in many industrial fields. In this paper, the effects of surface wettability on finger pattern are studied and simulatednumerically by the Lattice Boltzmann method (LBM). The displacement efficiency is investigated by using two parameters,namely, the breakthrough time and the areal sweep efficiency. The simulation has demonstrated that surface wettability willinfluence the finger pattern no matter the gravity is considered or not, but in the presence of gravity, the finger pattern is muchmore complicated and irregular due to the coexistence and competition of capillary force, viscous force and gravity.     

Isolation and characterization of genes from the marine microalga Pavlova salina encoding three front-end desaturases involved in docosahexaenoic acid biosynthesis

The marine microalga Pavlova salina produces lipids containing approximately 50% omega-3 long chain polyunsaturated fatty acids (LC-PUFA) such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Three cDNA sequences, designated PsD4Des, PsD5Des, PsD8Des, were isolated from P. salina and shown to encode three front-end desaturases with Delta4, Delta5 and Delta8 specificity, respectively. Southern analysis indicated that the P. salina genome contained single copies of all three front-end fatty acid desaturase genes. When grown at three different temperatures, analysis of fatty acid profiles indicated P. salina desaturation conversions occurred with greater than 95% efficiency. Real-Time PCR revealed that expression of PsD8Des was higher than for the other two genes under normal growth conditions, while PsD5Des had the lowest expression level. The deduced amino acid sequences from all three genes contained three conserved histidine boxes and a cytochrome b(5) domain. Sequence alignment showed that the three genes were homologous to corresponding desaturases from other microalgae and fungi. The predicted activities of these three front-end desaturases leading to the synthesis of LC-PUFA were also confirmed in yeast and in higher plants.     

Ordered Hierarchical Mesoporous/Microporous Carbon Derived from Mesoporous Titanium‐Carbide/Carbon Composites and its Electrochemical Performance in Supercapacitor

Novel ordered hierarchical mesoporous/microporous carbon (OHMMC) derived from mesoporous titanium‐carbide/carbon composites was prepared for the first time by synthesizing ordered mesoporous nanocrystalline titanium‐carbide/carbon composites, followed by chlorination of titanium carbides. The mesostructure and microstructure can be conveniently tuned by controlling the TiC contents of mesoporous TiC/C composite precursor, and chlorination temperature. By optimal condition, the OHMMC has a high surface area (1917 m²g^?1), large pore volumes (1.24 cm³g^?1), narrow mesopore‐size distributions (centered at about 3 nm), and micropore size of 0.69 and 1.25 nm, and shows a great potential as electrode for supercapacitor applications: it exhibits a high capacitance of 146 Fg^?1 in noaqueous electrolyte and excellent rate capability. The ordered mesoporous channel pores are favorable for retention and immersion of the electrolyte, providing a more favorable path for electrolyte penetration and transportation to achieve promising rate capability performance. Meanwhile, the micropores drilled on the mesopore‐walls can increase the specific surface area to provide more sites for charge storage.     

Immobilized cells cultivated in semi-continuous mode in a fluidized bed reactor for xylitol production from sugarcane bagasse

Summary Xylitol production from sugarcane bagasse hemicellulosic hydrolyzate was evaluated in a fluidized bed reactor operated in semi-continuous mode, using cells immobilized on porous glass. The fermentative process was performed during five successive cycles of 72 h each one. The lowest xylitol production occurred in the first cycle, where a high cell concentration (12 g l⁻¹) was observed. In the subsequent cycles the xylitol concentration was ever increasing due to the cells adaptation to the medium. In the last one, 18 g xylitol l⁻¹ was obtained with a yield factor of 0.44 g g⁻¹ and volumetric productivity of 0.32 g l⁻¹ h⁻¹.     

Macro-scale topology optimization for controlling internal shear stress in a porous scaffold bioreactor

Shear stress is an important physical factor that regulates proliferation, migration, and morphogenesis. In particular, the homeostasis of blood vessels is dependent on shear stress. To mimic this process ex vivo, efforts have been made to seed scaffolds with vascular and other cell types in the presence of growth factors and under pulsatile flow conditions. However, the resulting bioreactors lack information on shear stress and flow distributions within the scaffold. Consequently, it is difficult to interpret the effects of shear stress on cell function. Such knowledge would enable researchers to improve upon cell culture protocols. Recent work has focused on optimizing the microstructural parameters of the scaffold to fine tune the shear stress. In this study, we have adopted a different approach whereby flows are redirected throughout the bioreactor along channels patterned in the porous scaffold to yield shear stress distributions that are optimized for uniformity centered on a target value. A topology optimization algorithm coupled to computational fluid dynamics simulations was devised to this end. The channel topology in the porous scaffold was varied using a combination of genetic algorithm and fuzzy logic. The method is validated by experiments using magnetic resonance imaging readouts of the flow field.     

Modeling and design of optimal flow perfusion bioreactors for tissue engineering applications

Perfusion bioreactors have been used in different tissue engineering applications because of their consistent distribution of nutrients and flow-induced shear stress within the tissue-engineering scaffold. A widely used configuration uses a scaffold with a circular cross-section enclosed within a cylindrical chamber and inlet and outlet pipes which are connected to the chamber on either side through which media is continuously circulated. However, fluid-flow experiments and simulations have shown that the majority of the flow perfuses through the center. This pattern creates stagnant zones in the peripheral regions as well as in those of high flow rate near the inlet and outlet. This non-uniformity of flow and shear stress, owing to a circular design, results in limited cell proliferation and differentiation in these areas. The focus of this communication is to design an optimized perfusion system using computational fluid dynamics as a mathematical tool to overcome the time-consuming trial and error experimental method. We compared the flow within a circular and a rectangular bioreactor system. Flow simulations within the rectangular bioreactor are shown to overcome the limitations in the circular design. This communication challenges the circular cross-section bioreactor configuration paradigm and provides proof of the advantages of the new design over the existing one.     

Surface‐Modified Porous Carbon Nitride Composites as Highly Efficient Electrocatalyst for Zn‐Air Batteries

Porous carbon nitride (PCN) composites are fabricated using a top‐down strategy, followed by additions of graphene and CoS_x nanoparticles. This subsequently enhances conductivity and catalytic activity of PCN (abbreviated as CoS_x@PCN/rGO) and is achieved by one‐step sulfuration of PCN/graphene oxides (GO) composite materials. As a result, the as‐prepared CoS_x@PCN/rGO catalysts display excellent activity and stability toward both oxygen evolution and reduction reactions, surpassing electrocatalytic performance shown by state‐of‐the‐art Pt, RuO₂ and other carbon nitrides. Remarkably, the CoS_x@PCN/rGO bifunctional activity allows for applications in zinc‐air batteries, which show better rechargeability than Pt/C. The enhanced catalytic performance of CoS_x@PCN/rGO can primarily be attributed to the highly porous morphology and sufficiently exposed active sites that are favorable for electrocatalytic reactions.