Modeling transitional plane Couette flow

The Galerkin method is used to derive a realistic model of plane Couette flow in terms of partial differential equations governing the space-time dependence of the amplitude of a few cross-stream modes. Numerical simulations show that it reproduces the globally sub-critical behavior typical of this flow. In particular, the statistics of turbulent transients at decay from turbulent to laminar flow displays striking similarities with experimental findings.     

Sphere-to-rod transition of triblock copolymer micelles at room temperature

A room temperature sphere-to-rod transition of the polyethylene oxide-polypropylene oxide-polyethylene oxide-based triblock copolymer, (PEO)₂₀(PPO)₇₀ (PEO)₂₀ micelles have been observed in aqueous medium under the influence of ethanol and sodium chloride. Addition of 5–10% ethanol induces a high temperature sphere-to-rod transition of the micelles, which is brought to room temperature upon addition of NaCl. The inference about the change in the shape of the micelles has been drawn from small-angle neutron scattering (SANS) and viscosity studies.     

Intracluster electron transfer from a metal atom/cluster followed by anionic oligomerization of vinyl molecules

Intracluster electron transfer and oligomerization reaction were investigated by mass spectrometry of clusters of alkali metal atom (M) with acrylonitrile (AN; CH₂=CHCN). In the photoionization mass spectra of M(AN)_n, magic numbers were clearly observed at n = 3k (k = 1-4 for M = Na and K, k = 1 for M = Li). The results of photodissociation of neutral K(AN)_n indicate that the n = 3 cluster has an anomalous stability relative to other sizes of clusters. The C=C bond in vinyl molecules is also found to be necessary to form the magic numbers by measuring the photoionization mass spectrum of K atom with propionitrile. These results strongly support the intracluster anionic oligomerization reaction initiated by electron transfer from the alkali atom. The quantum chemical calculations have revealed that the evaporation induced by excess energy generated by intracluster oligomerization is important to form the magic numbers in the present clusters. Received 29 November 2000     

Catalytic behaviors of ruthenium dioxide films deposited on ferroelectrics substrates, by spin coating process

Catalytic ruthenium dioxide films were deposited by spin-coating process on ferroelectric films mainly constituted of SrBi₂Ta₂O₉ (SBT) and Ba₂NaNb₅O₁₅ (BNN) phases. After thermal treatment under air, these ferroelectric-catalytic systems were characterized by X-ray diffraction and scanning electron microscopy (SEM). SEM images showed that RuO₂ film morphology depended on substrate nature. A study of CH₄ conversion into CO₂ and H₂O was carried out using these catalytic-ferroelectric multilayers: the conversion was analyzed from Fourier transform infrared (FTIR) spectroscopy, at various temperatures. Improved catalytic properties were observed for RuO₂ films deposited on BNN oxide layer.     

Formation of Cs molecules via Feshbach resonances stabilized by spontaneous emission: theoretical treatment with the Fourier grid method

This paper proposes a general method to investigate Feshbach resonances in atomic collisions similar to Cs(6 s ) + Cs(6 p ) in the thermal or cold regime. In order to compute the predissociation widths of the C ¹ Π _u (6 s + 5 d ) bound vibrational levels of Cs₂, coupled both with the (2) ³ Σ ⁺ _u (6 s + 6 p ) continuum and with the (2) ³ Π _u (6 s + 5 d ) vibrational series, a Fourier grid method is implemented, with an optical potential. A convenient way of optimizing the latter is proposed. A large number of resonances are found and calculations of their cross-sections for stabilization into ground state molecules show that the rate may be important. This confirms the interpretation of Lintz and Bouchiat [Phys. Rev. Lett. 80, 2570 (1998)] who observed dimer formation in cell experiments. Possible generalization to the cold regime relies on the possibility to tune the position of a resonance to coincide with the maximum of the collisional energy distribution. Received 14 February 2002 Published online 28 June 2002     

Catalysis of dispersed silver particles on directional etching of silicon

Silver particles are dispersed on silicon by magnetron sputtering and post-annealing to investigate the catalytic effects of individual silver particles on wet etching of silicon surface. According to scanning electron microscopy, dispersed deep holes are present and the major etching direction is vertical to the surface of a Si(1 0 0) wafer or inclined to that on a Si(1 1 1) wafer. Our experiments indicate that the effect of the anisotropy of Si on directional etching is fundamental and the wafer resistivity and experimental process have important influence on the etching results. In addition, aggregation of silver particles and random horizontal etching on the surface of the wafer are caused by the local imbalance between the oxidant and HF. Our results enable better understanding of the catalytic effects of metal particles on silicon and are helpful to the preparation new silicon nanostructures.     

Sensing single electrons with single molecules

We propose a new methodology for probing transport of just one electron, a process of great importance both in nature and in artificial devices. Our idea for locating a single electron is analogues to the conventional GPS where signals from several satellites are used to locate a macro object. Using fluorescent molecules as tiny sensors, it is possible to determine 3D displacement vector of an electron.     

Dissociation of water molecule at three-fold oxygen coordinated V site on the InVO4 (0 0 1) surface

Water molecule adsorption properties at the surface of InVO₄ have been investigated using an ab initio molecular dynamics approach. It was found that the water molecules were adsorbed dissociatively to the three-fold oxygen coordinated V sites on the (0 0 1) surface. The dissociative adsorption energy was estimated to be 0.8-0.9 eV per molecule. The equilibrium distance between V and O of the hydroxyl -OH was almost the same as the V-O distance of tetrahedra VO₄ in the InVO₄ bulk crystal (1.7-1.8 Å).     

Towards the origin of the memory effect in oxide glasses

Glass samples with same composition and properties but different thermal histories can exhibit different behavior upon a subsequent heat-treatment, a phenomenon known as the memory effect. Generally, it is considered that the memory effect is due to nanoscale density fluctuation, which exists in all glasses and causes non-exponential relaxation with more than one relaxation time. Earlier, we studied the memory effect of various silica glasses and found that some pure silica glasses did not exhibit the memory effect while some silica glasses with higher impurity contents exhibited the memory effect. Based upon this finding, we suggested that the phenomenon originated from concentration fluctuation rather than density fluctuation. In this study, the memory effect in 6 mol% K₂O-94 mol% SiO₂ glass was investigated. The K₂O-SiO₂ glass system has a metastable immiscibility below the glass transition temperature and the chosen glass composition is the critical composition corresponding to the estimated critical temperature of the immiscibility boundary. Thus, it is expected that this glass composition would exhibit large super-critical concentration fluctuation, which increases with decreasing heat-treatment temperature. Density fluctuation, on the other hand, increases with increasing heat-treatment temperature. A much larger memory effect was observed at the lower heat-treatment temperature for the present glass. This result supports our earlier contention that the origin of the memory effect is composition fluctuation rather than density fluctuation.