Four-equation turbulence model for prediction of the turbulent boundary layer affected by buoyancy force over a flat plate

Turbulent convective heat transfer with appreciable buoyancy effect over a heated or cooled horizontal flat plate is numerically analyzed by solving four equations for mean square temperature variance , its rate of destruction _θ, turbulent kinetic energy κ and the rate of kinetic energy dissipation . Turbulent time-scale ratio R of temperature fluctuations relative to velocity fluctuations defined by is found to vary widely across the boundary layer. For both highly stable and highly unstable conditions, the ‘four-equation’ model yields better results for mean temperature profile and surface heat flux than the two-equation model. It is also found that the magnitude of thermal von Karman constant κ_θ is not a universal constant but it depends on the thermal stratification of the boundary layer.     

Simultaneous interpenetrating polymer networks of epoxy and N-phenylmaleimide-styrene copolymers

Ductile thermoplastic N-phenylmaleimide-styrene copolymers (PMS) have been introduced into diglycidyl ether of bisphenol A (epoxy) by a simultaneous polymerization technique. The tensile strength and fracture toughness increased with increasing PMS content and reached a maximum value at a PMS/epoxy ratio of about 20/80 (w/w). The mechanical properties decreased significantly beyond that ratio, because of phase inversion.     

Polynorbornene/fluorosilica hybrids for hydrophobic and oleophobic coatings

Polynorbornene dicarboxylic anhydride (PNA)/fluorosilica hybrid coating materials with good hydrophobicity, oleophobicity, transparency, thermal stability, and hardness were synthesised using a sol–gel method. The surface structure, transparency, hydrophobicity, oleophobicity, and surface free energy of the coating could be controlled by adjusting the fluorosilane ratio. The maximum static water and oil contact angles of films were 112° and 87°, respectively. The PNA/fluorosilica hybrid films exhibited good transparency and colourlessness. The marks written using water and oil-based pens on the films could be erased with a tissue even after eight times. In addition, the hardness of the hybrid films was enhanced to 4H with increasing fluorosilane content.     

Mössbauer spectroscopic and chromaticity analysis on colorative mechanism of celadon glaze

The dependence of the color of a celadon glaze on the chemical composition and the electronic state of Fe was investigated by Mössbauer spectroscopic and chromaticity analysis. The amount of Fe₂O₃ was found to be the main factor influencing L* and b* values, whereas the amount of TiO₂ was found to affect all the parameters (L*, a*, b*). The effect of MnO on the color was significant only by interaction terms. The amount of P₂O₅ was found to be the main factor of the b* value. According to the Mössbauer analysis results, as the amount of divalent iron ions increases, the a* and b* values decreased; on the other hand, the L* value increased. As the amount of titanium increased, Fe²⁺ was found to be destabilized relative to Fe³⁺ due to the structural instability of Fe-O-Ti network.     

Atomistic analyses of the effect of temperature and morphology on mechanical strength of Si–C–N and Si–C–O nanocomposites

3-D molecular dynamics (MD) analyses of SiC–Si₃N₄ nanocomposite deformation and SiCO nanocomposite deformation are performed at 300 K, 900 K, and 1500 K. In SiC–Si₃N₄ nanocomposites, distribution of second phase SiC particles, volume fraction of atoms in GBs, and GB thickness play an important role in temperature dependent mechanical behavior. The deformation mechanism is a trade-off between the stress concentration caused by SiC particles and Si₃N₄–Si₃N₄ GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent. In the case of SiCO nanocomposites, findings indicate that temperature change dependent amorphization of nanodomains, the nanodomain wall placement, the nanodomain wall thickness, and nanodomain size are important factors that directly affect the extent of crystallinity and the strength against mechanical deformation.     

Hypoxia Suppresses Spontaneous Mineralization and Osteogenic Differentiation of Mesenchymal Stem Cells via IGFBP3 Up-Regulation

Hypoxia has diverse stimulatory effects on human adipose-derived stem cells (ASCs). In the present study, we investigated whether hypoxic culture conditions (2% O₂) suppress spontaneous mineralization and osteogenic differentiation of ASCs. We also investigated signaling pathways and molecular mechanisms involved in this process. We found that hypoxia suppressed spontaneous mineralization and osteogenic differentiation of ASCs, and up-regulated mRNA and protein expression of Insulin-like growth factor binding proteins (IGFBPs) in ASCs. Although treatment with recombinant IGFBPs did not affect osteogenic differentiation of ASCs, siRNA-mediated inhibition of IGFBP3 attenuated hypoxia-suppressed osteogenic differentiation of ASCs. In contrast, overexpression of IGFBP3 via lentiviral vectors inhibited ASC osteogenic differentiation. These results indicate that hypoxia suppresses spontaneous mineralization and osteogenic differentiation of ASCs via intracellular IGFBP3 up-regulation. We determined that reactive oxygen species (ROS) generation followed by activation of the MAPK and PI3K/Akt pathways play pivotal roles in IGFBP3 expression under hypoxia. For example, ROS scavengers and inhibitors for MAPK and PI3K/Akt pathways attenuated the hypoxia-induced IGFBP3 expression. Inhibition of Elk1 and NF-κB through siRNA transfection also led to down-regulation of IGFBP3 mRNA expression. We next addressed the proliferative potential of ASCs with overexpressed IGFBP3, but IGFBP3 overexpression reduced the proliferation of ASCs. In addition, hypoxia reduced the osteogenic differentiation of bone marrow-derived clonal mesenchymal stem cells. Collectively, our results indicate that hypoxia suppresses the osteogenic differentiation of mesenchymal stem cells via IGFBP3 up-regulation.     

Optimization of inverted bulk heterojunction polymer solar cells

We have successively fabricated inverted bulk heterojunction polymer solar cells employing ZnO and MoO₃ as electron and hole selective layers, respectively. The device structure is ITO/ZnO/P3HT: PCBM/MoO₃/Al. Differently from conventional polymer solar cells, ITO and Al work as electron and hole collecting electrodes in this inverted structure, respectively. We have found the optimal thickness of ZnO and MoO₃ to be 100 nm and 5 nm, respectively. The highest PCE was obtained to be 3.32% under AM 1.5 illumination at 1,000W/m², which is the highest PCE of inverted solar cells reported previously in the literature.     

Healing behavior of a matrix crack on a carbon fiber/mendomer composite

The cure kinetics of a thermally mendable polymer based on Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions, mendomer 401, was investigated using dynamic differential scanning calorimetry (DSC). The resulting behavior was modeled using a conventional cure kinetic model for thermosetting polymers. A composite panel with two layers of carbon fabric and mendomer 401 was fabricated following the cure cycle suggested by the cure kinetics model. Micro-indentation tests were performed to investigate mechanical properties and dynamic mechanical thermal analysis (DMTA) was conducted to study thermal behavior and to find the trigger temperature for healing. Self-healing behavior of the carbon fiber/mendomer composite was demonstrated using electrical resistive heating over the glass transition temperature.