Turbulent drag reduction by spanwise wall oscillations

In the present work a technique is numerically investigated, which is aimed at reducing the friction drag in turbulent boundary layers and channel flows. A cyclic spanwise oscillation of the wall with a proper frequency and amplitude is imposed, allowing a reduction of the turbulent drag of up to 40%. The present work is based on the numerical simulation of the Navier-Stokes equations in the simple geometry of a plane channel flow. The frequency of the oscillations is kept fixed at the most efficient value determined in previous studies, while the choice of the best value for the amplitude of the oscillations is evaluated not only in terms of friction reduction, but also by taking into consideration the overall energy balance and the power spent for the motion of the wall. The analysis of turbulence statistics allows to shed some light on the way oscillations interact with wall turbulence, as illustrated by visual inspection of some instantaneous flow fields. Finally, a simple explanation is proposed for this interaction, which leads to a rough estimate of the most efficient value for the frequency of the oscillations.     

Temperature-dependent optical band structure and defect mode in a one-dimensional photonic liquid crystal

We analysed the propagation of an electromagnetic wave in a one-dimensional periodic system composed of a finite set of E7 liquid crystal mixture slabs in a twisted configuration alternated by homogeneous and isotropic dielectric layers. For different incident angles of the circularly polarised wave, we studied the optical band structure for reflectance and transmittance considering that the dielectric matrix of the device depends on temperature and wavelength. We demonstrated that the position of the band can be moved from visible to short-infrared spectrum region by increasing the thickness of the layers. We found that for a fixed incident angle, the band spectrum shifts towards the short-wavelength region as the temperature gets increasing, whereas, for a constant temperature, such a spectrum moves towards larger frequencies as the incident angle increases. We show that when one of the homogeneous and isotropic slabs has a different size compared with the remaining ones, a defect mode is induced in the band structure whose frequency can be thermally tuned.     

Integration of Mesoporous Bioactive Glass Nanoparticles and Curcumin into PHBV Microspheres as Biocompatible Composite for Drug Delivery Applications

Bioactive glasses (BGs) are being increasingly considered for biomedical applications. One convenient approach to utilize BGs in tissue engineering and drug delivery involves their combination with organic biomaterials in order to form composites with enhanced biocompatibility and biodegradability. In this work, mesoporous bioactive glass nanoparticles (MBGN) have been merged with polyhydroxyalkanoate microspheres with the purpose to develop drug carriers. The composite carriers (microspheres) were loaded with curcumin as a model drug. The toxicity and delivery rate of composite microspheres were tested in vitro, reaching a curcumin loading efficiency of over 90% and an improving of biocompatibility of different concentrations of MBGN due to its administrations through the composite. The composite microspheres were tested in terms of controlled release, biocompatibility and bioactivity. Our results demonstrate that the composite microspheres can be potentially used in biomedicine due to their dual effects: bioactivity (due to the presence of MBGN) and curcumin release capability.     

A Combined Computational and Experimental Study of Copolymerization Propagation Kinetics for 1‐Ethylcyclopentyl methacrylate and Methyl methacrylate

The crosspropagation of 1‐ethylcyclopentyl methacrylate (ECPMA) and methyl methacrylate (MMA) has been studied using a combination of quantum chemistry calculations and experiment. Our computational work utilizes a trimer‐to‐tetramer reaction model, coupled with an ONIOM (B3LYP/6‐31G(2df,p): B3LYP/6‐31G(d)) method for geometry optimization and an M06‐2X/6‐311+G(2df,p) method plus SMD solvation model for single point energy calculations. The results show several trends: the identity of the ultimate unit of a trimer radical affects not only the preferred conformation of the region where the reaction takes place, but also the reactivity of the radical; the addition of an ECPMA monomer to the radicals is generally favored compared to an MMA monomer; the pen‐penultimate unit of a trimer radical shows a nonnegligible entropic effect; the penultimate unit effect is implicit for the ECPMA–MMA copolymer system. Finally, terminal model reactivity ratios fitted based on the explicit rate coefficients calculated from the quantum chemical results are compared with those from experimental measurements. The computations not only agree qualitatively with experimentally derived results in terms of the selectivity of ECPMA–MMA crosspropagation, but also give reasonable quantitative predictions of reactivity ratios.

     

High efficiency nanotextured silicon solar cells

We report the computational modeling of forward scattering phenomena arising in Au nanoparticles array near their localized surface plasmon resonance, which by producing a strong field enhancement effect on the substrate leads to higher optical absorption and, therefore, higher efficiencies of operation. Computational calculations indicate that the ultimate efficiency of an optimized silicon nanoholes (SiNH) array texture surface in combination with the surface and bottom-of-a-trench Au nanoparticles array described herein, is 39.67%, which compares favorably with the ultimate efficiency of 31.11% for an optimized silicon nanoholes array texture surface. Furthermore, the utilization of an optimized silicon nitride antireflection coating increases the ultimate efficiency to a promising value of 41.88%, while the utilization of a single-crystal silicon layer of thickness 2.8 μm will be instrumental in drastically reducing solar cell manufacturing cost.     

Calix[4]Arene Cavitands: A Solid State Study on the Interactions of their Aromatic Cavity with Neutral Organic Guests Characterised by Acid CH3 or CH2 Groups

Abstract

To gain information on CH-π aromatic interactions involved in the formation of host-guest adducts, the geometrical parameters which define the solid state structures of the complexes of calix[4]arenes in the cone conformation with guests having acid CH₃ or CH₂ groups have been studied. Most of the data have been obtained from the CH₃CN and CH₂Cl₂ calix[4]arene complexes retrieved from the literature. To understand the effect of the acidity on these parameters, p-cyclohexylcalix[4]arene-biscrown-3 ? CH₃CN, p-tert-butylthiacalix[4]arene ? CH₃CN, p-tert-butylthiacalix[4]arene ? CH₃NO₂, 1,3-dipropoxy-p-tert-butylcalix[4]arene ? ClCH₂CN and 1,3-dipropoxy-p-tert-butylcalix[4]arene ? CH₂(CN)₂ complexes were prepared, crystallised and investigated in the solid state. CH₃X guests are bound preferentially by hosts having a C₄ symmetry. The interaction is directional, but it is independent from the basicity of the host and acidity of the guest, indicating that classic hydrogen bond do not play a major role. On the contrary CH₂XY guests find the best matching with hosts having a C_2v symmetry, interacting specifically with two diametrical aromatic rings. These interactions are directional and show a correlation between the acidity of the guest and the CH-π aromatic distance, thus supporting a stronger contribution of “classic” hydrogen bond in these latter complexes. These results are in agreement with the hypothesis that CH-π aromatic interactions derive from the superimposition of different types of intermolecular forces, whose contribution depends on several factors as the nature of the interacting partners.     

Adiabatic vs. non-adiabatic determination of specific absorption rate of ferrofluids

The measurement of temperature variations in adiabatic conditions allows the determination of the specific absorption rate of magnetic nanoparticles and ferrofluids from the correct incremental expression, SAR=(1/m_MNP)C(ΔT/Δt). However, when measurements take place in non-adiabatic conditions, one must approximate this expression by SAR≈Cβ/m_MNP, where β is the initial slope of the temperature vs. time curve during alternating field application. The errors arising from the use of this approximation were estimated through several experiments with different isolating conditions, temperature sensors and sample-sensor contacts. It is concluded that small to appreciable errors can appear, which are difficult to infer or control.     

Poly (phenylmethylsilane) obtained by electrochemical reduction in a divided cell containing a polymeric membrane

The electrochemical synthesis of poly(phenylmethylsilane) was carried out in a divided cell using a polymeric anion-exchange membrane. The solution contained phenylmethydichlorosilane (PhMeSiCl₂), mixed solvent tetrahydrofuran + hexamethylphosphoramide, with tetrabutylammonium perchlorate as support electrolyte. The electrodes were stainless steel as the cathode, stainless steel as the sacrificial anode or platinum as the resistant anode. Poly(phenylmethylsilane) was obtained in yields from 1.5 to 4.5 % from phenylmethyldichlorosilane concentrations equal or higher than 1 M. The number-average molecular mass M_n of poly(phenylmethylsilane) was in the range from 5600 to 9500. A monomodal molecular weight distribution was obtained with a polydipersity Index of 2. The spectra of ¹H, ¹³C, ²⁹Si MMR and IR were determined. From the ²⁹Si NMR analysis, the poly(phenylmethylsilane) showed a clear difference in the tacticity related to the chemical synthesis of the same polymer. The UV spectra showed a strong absorption in a 331 to 335 nm characteristic for the poly(phenylmethylsilane).     

Autoacceleration and inhibition: Free volume. Epoxy-amine kinetics

The epoxy-diamine cure process was studied. We found that the mechanism can be described in three steps: (i) initiation up to 20–25% of conversion. (ii) autoacceleration and (iii) inhibition. It has been observed that after the initiation there is a clear autoacceleration effect which has been explained in terms of free volume, considerations: the volume occupied by the products of reaction diminishes the available volume increasing the “local concentration” of the reactants and therefore the rate of reaction. The reaction was followed by FTIR (near infrared) as the main technique. We used a purified epoxy resin as well as phenyl glycidyl ether cured with m-xylenediamine. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1001–1016, 1998