Method of moments for laminar dispersion in an oscillatory flow through curved channels with absorbing walls

The unsteady dispersion of a solute, when the fluid is driven through a curved channel with absorbing walls by an imposed pulsatile pressure gradient, is studied using the method of moments. The study examines the effect of oscillatory Reynolds number, amplitude/frequency of the pressure pulsation and boundary absorption on the longitudinal dispersion. The methodology involves a set of unsteady integral moment equations obtained by applying the Aris-Barton method of moments on the convective-diffusion equation for a curved channel. Central moments are obtained from the moment equations which are solved by a finite-difference implicit scheme. The effect of curvature and boundary absorption on the effective dispersion coefficient from the initial to the stationary stage of the oscillatory flow is studied. Amplitude of the effective dispersion coefficient is found to increase with curvature and decrease with frequency of the pressure pulsation. For large Peclet number and Schmidt number, the amplitude of the dispersion coefficient can be 1.6 times that in a straight channel at large times. Also, for large times, the amplitude of the dispersion coefficient is twice the amplitude of the dispersion coefficient as α, the frequency parameter changes from 0.5 to 1.0. The axial distributions of mean concentration are determined from the first four central moments by using the Hermite polynomial representation. The effect of curvature is to delay the stationary state and also the approach to normality of the concentration distribution. The study has importance in understanding the spreading of pollutants in tidal basins and natural current fields.     

From Containers to Catalysts: Supramolecular Catalysis within Cucurbiturils

Cucurbiturils are a family of molecular container compounds with superior molecular recognition properties. The use of cucurbiturils for supramolecular catalysis is highlighted in this concept. Both photochemical reactions as well as thermal transformations are reviewed with an eye towards tailoring substrates for supramolecular catalysis mediated by cucurbiturils.     

Application of principal component analysis to gas sensing characteristics of Cr0.8Fe0.2NbO4 thick film array

The transient changes in resistances of Cr_0.8Fe_0.2NbO₄ thick film sensors towards specified concentrations of H₂, NH₃, acetonitrile, acetone, alcohol, cyclohexane and petroleum gas at different operating temperatures were recorded. The analyte-specific characteristics such as slopes of the response and retrace curves, area under the curve and sensitivity deduced from the transient curve of the respective analyte gas have been used to construct a data matrix. Principal component analysis (PCA) was applied to this data and the score plot was obtained. Distinguishing one reducing gas from the other is demonstrated based on this approach, which otherwise is not possible by measuring relative changes in conductivity. This methodology is extended for three Cr_0.8Fe_0.2NbO₄ thick film sensor array operated at different temperatures.     

Photoreactions with a Twist: Atropisomerism‐Driven Divergent Reactivity of Enones with UV and Visible Light

Light‐induced transformation of atropisomeric and achiral enones displays divergent reactivity. Photocyclization leading to 3,4‐dihydroquinolin‐2‐one was observed with achiral enone carboxamide, whereas the atropisomeric enone carboxamides underwent hydrogen abstraction leading to spiro‐β‐lactams. Detailed photochemical, photophysical, and theoretical investigations have provided insight into this divergent reactivity and selectivity.     

Polymer grafted nanoparticles: Effect of chemical and physical heterogeneity in polymer grafts on particle assembly and dispersion

Macroscopic properties of polymer nanocomposites depend on the microscopic composite morphology of the constituent nanoparticles and polymer matrix. One way to control the spatial arrangement of the nanoparticles in the polymer matrix is by grafting the nanoparticle surfaces with polymers that can tune the effective interparticle interactions in the polymer matrix. A fundamental understanding of how graft and matrix polymer chemistries and molecular weight, grafting density, and nanoparticle size, and chemistry affect interparticle interactions is needed to design the appropriate polymer ligands to achieve the target morphology. Theory and simulations have proven to be useful tools in this regard due to their ability to link molecular level interactions to the morphology. In this feature article, we present our recent theory and simulation studies of polymer grafted nanoparticles with chemical and physical heterogeneity in grafts to calculate the effective interactions and morphology as a function of chemistry, molecular weights, grafting densities, and so forth. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Quench collection of nano-aluminium agglomerates from combustion of sandwiches and propellants

An experimental investigation has been carried out to measure the size of nano-aluminium agglomerates emerging from the combustion of nano-aluminized sandwiches and composite solid propellants. Nano-aluminium of median size of 50 nm produced in-house by the electrical wire explosion method is used in these samples. Propellants with different sizes of coarse and fine ammonium perchlorate are considered. Surface features of sandwiches and a propellant whose burning was interrupted by rapid depressurization are examined in a scanning electron microscope. The combustion products of the sandwiches and propellants are quenched close to the burning surface and collected in a quench collection set-up. The surface features of rapid-depressurization quenched sandwiches exhibit relatively large nano-aluminium clusters—of the order a few micrometres—particularly in the binder lamina. Quench-collected nano-aluminium exhibits significant agglomeration, but only a small fraction of the agglomerates are in the 1–3 μm range, except for both the coarse and fine AP particles used in the formulation being large, but even there they do not exceed ∼5 μm in size. This is expected to be benign for reduced smoke propellant applications from exhaust signature point of view, and to decrease the specific impulse losses without sacrificing the energetics of the propellant.     

Synthesis,Structure, Luminescent and Intramolecular Proton Transfer in Some Imidazole Derivatives

A group of novel 2-aryl imidazole derivatives were synthesized and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy imidazoles (dmip and dmtip) have been studied using emission spectroscopy and it was detected that the two distinct ground state rotamers of I and II are responsible for the normal and the tautomer emission respectively. In hydrocarbon solvent, the tautomer emission predominates over the normal emission for both dmip and dmtip. This reveal that rotamer II is responsible for the tautomer emission and it is stabler than rotamer I which causes the normal emission. In alcoholic solvent like ethanol, a dramatic enhancement of normal emission is observed which was due to increased solvation, the more polar rotamer I become stabler than rotamer II. In dioxane—water mixtures it is observed that the addition of water inhibits the ESIPT process due to the formation of the intermolecular hydrogen bonding involving water. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the imidazole derivatives were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state.     

Detection of lead(II) using an glassy carbon electrode modified with Nafion, carbon nanotubes and benzo-18-crown-6

A glassy carbon electrode was modified with Nafion, carbon nanotubes and benzo-18-crown-6 to give an electrode for the selective determination of lead(II) via square wave anodic stripping voltammetry. The use of carbon nanotubes with their extraordinary electrical conductivity and strong adsorption ability warrants high sensitivity. Benzo-18-crown-6 is employed as a “molecular scavenger” because of its excellent selectivity for lead(II). The modified electrode shows enhanced sensitivity, reproducibility and selectivity for lead(II) even without applying an electrical potential during the accumulation time. It responds linearly to lead(II) in the 1 to 30 nM concentration range (with a correlation coefficient of 0.9992) after a 10-min accumulation time. The detection limit is 1 nM. The sensor exhibits excellent selectivity over other heavy metal ions such as Cd(II), Cu(II), Zn(II), and Hg(II).