A least-squares finite element method for time-dependent incompressible flows with thermal convection

The time-dependent Navier–Stokes equations and the energy balance equation for an incompressible, constant property fluid in the Boussinesq approximation are solved by a least-squares finite element method based on a velocity–pressure–vorticity–temperature–heat-flux ( u –P–ω–T– q ) formulation discretized by backward finite differencing in time. The discretization scheme leads to the minimization of the residual in the l²-norm for each time step. Isoparametric bilinear quadrilateral elements and reduced integration are employed. Three examples, thermally driven cavity flow at Rayleigh numbers up to 10⁶, lid-driven cavity flow at Reynolds numbers up to 10⁴ and flow over a square obstacle at Reynolds number 200, are presented to validate the method.     

Effect of alkoxides on chain termination and chain transfer of sodium-catalyzed polybutadiene

The reaction of butadiene with n-butylsodium or metal alkoxide-modified n-butylsodium was studied. A loss of “live ends” was observed when the polymerization of butadiene-1,3 with n-butylsodium was conducted in hexane solvent. This occurs, presumably, by elimination of NaH or Na metal in the allylic system. Similar results were obtained when the polymerization was carried out in the presence of sodium-tert-butoxide-modified n-butylsodium. However, the modification of n-butylsodium by lithium or potassium tert-butoxide in the butadiene-1,3 polymerization showed no loss of live ends for a period of two weeks. The results and an explanation are discussed.     

Chemical and biomimetic total syntheses of natural and engineered MCoTI cyclotides

The naturally-occurring cyclic cystine-knot microprotein trypsin inhibitors MCoTI-I and MCoTI-II have been synthesised using both thia-zip native chemical ligation and a biomimetic strategy featuring chemoenzymatic cyclisation by an immobilised protease. Engineered analogues have been produced containing a range of substitutions at the P1 position that redirect specificity towards alternative protease targets whilst retaining excellent to moderate affinity. Furthermore, we report an MCoTI analogue that is a selective low-microM inhibitor of foot-and-mouth-disease virus (FMDV) 3C protease, the first reported peptide-based inhibitor of this important viral enzyme.     

Synthesis, characterization, and ab initio theoretical study of a molecularly imprinted polymer selective for biosensor materials

Despite the complex phenomena involved in encoding template molecule information within stable synthetic polymers to yield selective and efficient molecular recognition processes, molecularly imprinted polymers (MIP) are increasingly finding broad areas of application. Molecular interactions, both during the polymerization of the functional monomers in the presence of the template and during the processes of specific recognition after template removal, are key determinants of an effective MIP. Covalent and noncovalent template imprinting have been employed to achieve specific recognition sites. In the present study, a molecularly imprinted biocompatible polymer, having a high capacity and affinity for the dye template, nickel(II) phthalocyanine tetrasulfonic acid, has been prepared. UV-visible spectroscopy, FTIR spectroscopy, and ICP analysis were used to investigate the aspects of the synthesis, binding capacity, and adsorption kinetics of the system. Poly(allylamine) cross-linked with epichlorohydrin has been used to represent an amino-functional receptor. Binding isotherms and capacities were correlated with the degree of template removal. Kinetic studies of binding allowed diffusion mechanisms to be evaluated for the fine particulate MIP. Ab initio molecular orbital calculations were performed using Hartree-Fock, MP2, and density functional theory methods to determine the most likely mechanisms of molecular imprinting. Suitable theoretical models have been constructed to mimic the interactions between the template molecule and the polymer. Simulation of the vibrational spectra was also undertaken to make meaningful assignments to experimentally determined spectral bands resulting from these template MIP receptor interactions.     

Controlling interfacial curvature in nanoporous silica films formed by evaporation-induced self-assembly from nonionic surfactants. II. Effect of processing parameters on film structure

The double-gyroid phase of nanoporous silica films has been shown to possess facile mass-transport properties and may be used as a mold to fabricate a variety of highly ordered inverse double-gyroid metal and semiconductor films. This phase exists only over a very small region of the binary phase diagram for most surfactants, and it has been very difficult to synthesize metal oxide films with this structure by evaporation-induced self-assembly (EISA). Here, we show the interplay of the key parameters needed to synthesize these structures reproducibly and show that the interfacial curvature may be systematically controlled. Grazing angle of incidence small-angle X-ray scattering (GISAXS) is used to determine the structure and orientation of nanostructured silica films formed by EISA from dilute silica/(poly(ethylene oxide)-b-poly(propylene oxide)-b-alkyl) surfactant solutions. Four different highly ordered film structures are observed by changing only the concentration of the surfactant, the relative humidity during dip-coating, and the aging time of the solution prior to coating. The highly ordered films progress from rhombohedral (Rm) to 2D rectangular (c2m) to double-gyroid (distorted Iad) to lamellar systematically as interfacial curvature decreases. Under all experimental conditions investigated, increasing the aging time of the coating solution was found to decrease the interfacial curvature. In particular, this parameter was critical to being able to synthesize highly ordered, pure-phase double-gyroid films. The key role of the aging time is shown via processing diagrams that map out the interplay between the aging time, composition, and relative humidity. 29Si nuclear magnetic resonance (NMR) spectroscopy and solution-phase small-angle X-ray scattering (SAXS) of the aged coating solutions presented in part I of this series are then used to interpret the effects of aging prior to dip-coating. Specifically, it was found that a predictive model based on volume fractions and the silica cluster stoichiometry obtained from 29Si NMR qualitatively explains the trends observed with composition and aging. However, apart from the effects of relative humidity, a quantitative comparison of the predicted phase with the experimental processing diagram suggests that less-condensed silica clusters are more effective at swelling the EO blocks at early aging times. This enhanced swelling decreases with aging time and results in lower-curvature nanostructures such as the double-gyroid. The decrease in swelling is attributed to the decreased thermodynamic driving force for the more-condensed silica clusters to mix with the EO block of the surfactant.     

Metal-dependent self-assembly of a microbial surfactant

Small-angle neutron scattering (SANS), cryogenic transmission electron microscopy (cryo-TEM), and dynamic light scattering (DLS) were used to study the metal-dependent phase behavior of microbially produced surfactants-marinobactins B, D, and E (MB, MD, and ME). Marinobactins A-E are siderophores that facilitate Fe(III) acquisition by the source bacterium through the coordination of Fe(III) by the peptidic headgroup. All of the marinobactins have the same six amino acid headgroup but differ in the length and saturation of the monoalkyl fatty acid tail. Fe(III) coordinated to ME (Fe(III)-ME) was found to form micelles with a diameter of approximately 3.5 nm that underwent a supramolecular transformation to produce a monodisperse population of vesicles with an average diameter ranging from approximately 90 to 190 nm upon addition of Cd(II), Zn(II), or La(III). SANS profiles of the transition-metal-induced phase exhibit a Bragg peak at QB approximately 0.11-0.12 A-1 and were fit to a SANS model for multilamellar vesicles that have an interbilayer repeat distance of 2pi/QB approximately 5.6-5.0 nm. Cryo-TEM images of the Zn(II)-induced phase reveals the presence of approximately 100 nm diameter approximately spherical aggregates of uniform electron density. The temperature dependence of the Zn(II)-induced transformation was also investigated as a function of the length and degree of unsaturation of the Fe(III)-marinobactin fatty acid tail. The Cd(II)-, Zn(II)-, and La(III)-induced phase changes have features that are similar to those of the previously reported Fe(III)-induced micelle-to-vesicle transition, and this observation has opened questions regarding the role that Cd(II) and Zn(II) may play in bacterial iron uptake.     

Oxygen dynamics in epitaxial YBa2Cu3O7-δ thin films

We have investigated epitaxial YBCO films by Perturbed γγ-Angular Correlation Spectroscopy (PAC) using ¹¹¹In probe atoms. The probes substitute on the yttrium lattice site and hyperfine parameters have been determined unambiguously. Excellent agreement is achieved with full potential electric field gradient (efg) calculations. Good reproducibility has been achieved by our preparation method allowing us to study the influence of oxygen motion on the structural orthorhombic to tetragonal phase transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hierarchy in optical near-fields based on compositions of nanomaterials

Optical near-field interactions exhibit hierarchical responses in the nanometer scale allowing unique functions in nanophotonic systems. Such hierarchical properties in optical near-fields originate various physical entities in the nanometer scale. Engineering nanomaterial compositions, while maintaining geometrically equivalent conditions, leads to characteristic hierarchical responses. We experimentally demonstrate such material-dependent optical near-field hierarchy using core–shell-type nanostructures composed of gold and silver.