Instability of a vertical columnar vortex in a stratified fluid

In this article, we study experimentally the evolution of a vertical columnar vortex in a stratified fluid. Three different measurement techniques are used. Particle image velocimetry allows us to monitor the time evolution of the characteristics of the vortex (Froude and Reynolds numbers). Dye visualizations reveal the existence of an instability for Froude numbers smaller than one, which creates an undulation of the vortex centerline. Synthetic schlieren visualization shows that the density structure of the unstable mode is very similar to the structure found recently numerically for the radiative instability of a Lamb–Oseen vortex (Riedinger et al. in J Fluid Mech, 2010). The experimental stability diagram and unstable wavelengths are compared with these numerical results. A secondary instability associated with the presence of critical layers is also observed for Froude numbers larger than one.     

Wetting of Alkanes on Water from a Cahn-Type Theory: Effects of Long-Range Forces

We apply the phenomenological wetting theory of Cahn to fluids with van der Waals forces, and in particular to the wetting of pentane on water. Taking into account explicitly the long-range substrate–adsorbate interaction allows us to reproduce the experimentally observed critical wetting transition, which arises from the vanishing of the Hamaker constant at T53°C. This transition is preceded by a first-order transition between a thin and a thick film at a (much) lower temperature. If long-range forces are neglected, this thin–thick transition is the only wetting transition and critical wetting is missed. Our study focuses on the development of useful theoretical tools, such as phase portraits and interface potentials adapted to systems with van der Waals forces.     

New insights into the reaction mechanism and the rate-determining step of n-butane hydroisomerisation over reduced MoO3 catalysts

The comparison of the proportions of the minor products formed during the hydroisomerisation of n-butane over reduced MoO3 with thermodynamic data demonstrates that a bifunctional mechanism operates and that the rate-determining step is the isomerisation of the linear butene intermediates to iso-butene.     

Titanium isodicyclopentadienide hemimetallocenes for ethylene/styrene copolymerization

The syndiotactic polymerization of styrene with exo-[(η⁵-isodiCp)TiCl₃] 1/methylalumoxane (MAO; isodiCp=isodicyclopentadienyl) was studied as well as the ethylene/styrene copolymerization with exo-[{η⁵:η¹(N)-isodiCp(SiMe₂Nt-Bu)}TiCl₂] 2/MAO. These two catalytic systems are stable during polymerization. The half-sandwich titanocene 1 exhibits good syndiospecificity and average activity. The bridged half-sandwich amino complex 2 was found to incorporate styrene into polymer chains at 70 °C. Activity results decrease with increasing styrene concentrations.     

Continuity of type-I intermittency from a measure-theoretical point of view

We study a simple dynamical system which displays a so-called type-I intermittency bifurcation. We determine the Bowen-Ruelle measure and prove that the expectation (g) of any continuous functiong and the Kolmogoroff-Sinai entropyh() are continuous functions of the bifurcation parameter. Therefore the transition is continuous from a measure-theoretical point of view. Those results could be generalized to any similar dynamical system.     

Synthesis and Characterization of New Chiral Schiff Base Complexes with Diiminobinaphthyl or Diiminocyclohexyl Moieties as Potential Enantioselective Epoxidation Catalysts

New chiral Schiff base complexes have been obtained by condensation of 2,2'-diamino-1,1'-binaphthalene or 1,2-diaminocyclohexane and various salicylaldehydes and by subsequent metalation with manganese, iron, cobalt, nickel, copper, or zinc. The complete (1)H and (13)C NMR characterization of the ligands is reported, as are the X-ray crystal structures of (1R,2R)-(-)-N,N'-bis[3-(N,N-dimethylamino)salicylidene]-trans-1,2-cyclohexanediimine and [(1R,2R)-(-)-N,N'-bis(salicylidene)-trans-1,2-cyclohexanediiminato]copper(II). The new chiral manganese complexes have been evaluated in the oxygenation of prochiral olefins and sulfides using sodium hypochlorite, hydrogen peroxide, or N-methylmorpholine N-oxide/m-chloroperbenzoic acid as oxidant.