Long-range critical wetting: observation of a critical end point

Alkanes deposited on aqueous substrates exhibit two different types of wetting behavior: alternatively to the usual first-order wetting transition, a sequential-wetting scenario of a long-range critical wetting transition preceded by a first-order thin-thick transition may be observed. Here, we present the first successful experimental attempt to locate the transition point between the standard first-order wetting and the long-range critical wetting: a critical end point, observed in a mixture of pentane and hexane which is deposited on an aqueous solution of glucose. Furthermore, we present the first direct measurement of the contact angle in the intermediate wetting state (frustrated-complete wetting) in the sequential-wetting scenario of hexane on brine and compare to theoretical predictions.     

Rod-climbing effect in Newtonian fluids

When a rotating rod is brought into a polymer melt or concentrated polymer solution, the meniscus climbs the rod. This spectacular rod climbing is due to the normal stresses present in the polymer fluid and is thus a purely non-Newtonian effect. A similar rod climbing of an interface between two fluids has therefore been taken as a signature that one of the fluids exhibits normal stress effects. We show here, however, that the effect can occur with simple Newtonian fluids: it occurs when a Taylor-Couette instability happens in the less viscous of the two liquids but not in the more viscous one.     

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Coupling between flow and structure for a lamellar surfactant phase

The flow-structure relation of lamellar phases is studied using rheometry and cross-polarized microscopy under flow. The equilibrium phases show different defects. Low salinities lead to very viscous, "onion" phases, whereas at high salinity, a low viscosity plane lamellar phase is found. Under shear, the latter shows a sudden transition to a viscoelastic gel, with a texture and viscosity very similar to that of the onions. Gelation occurs after a certain delay time, increasing rapidly with salinity, by the nucleation of onions. This allows one to relate the delay time to the defect energy.     

Dynamics of wetting layer formation

We study the formation and growth of wetting layers in the binary liquid mixture cyclohexane-methanol. By progressively deuterating the methanol we can tune the equilibrium wetting layer thickness. Hysteresis of the transition is observed for large thicknesses and is absent for thinner ones. This can be understood by calculating the activation energy for wetting layer nucleation as a function of the film thickness. We also show that the late-stage growth of the wetting layer after the nucleation process follows a power law in time, in agreement with a diffusion-limited growth mechanism proposed theoretically.     

Evolution with temperature of a single sol-gel TIO2 layer on sapphire (α-Al2O3) (0 0 0 1)

TiO₂ sol-gel layer has been deposited on single crystal sapphire (0 0 0 1) substrate. Evolution of the layer microstructure with the thermal treatment in the range 100-1100 °C has been studied using X-ray diffraction and X-ray reflectometry. TiO₂ layer density first increases with temperature up to 800 °C and then decreases with the appearance of a high roughness finally leading to anatase islands formation. The single crystal nature of the substrate seems to contribute to hinder the transformation of the anatase phase into the rutile phase and to induce a preferred orientation of the TiO₂ islands.     

Large intrinsic birefringence in zinc-blende based artificial semiconductors

A new attempt to solve the phase matching problem for semiconductor-based frequency conversion devices, based on the implementation of intrinsic birefringence in artificial materials, is discussed. The first results concerning the growth and characterization of ultrashort period superlattices are presented. To cite this article: J.-M. Jancu et al., C. R. Physique 8 (2007).     

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