Nematic pancakes revisited.

The spontaneous spreading of the 5CB nematic liquid crystal on solid substrates has been extensively studied in the last years both at the microscopic(1-4) and macroscopic(5-6) scales. The remarkable feature at the microscopic scale is the presence of a discontinuity in the thickness profile of the films. On the other hand, the spreading dynamics of macroscopic drops is quite specific. The drop first spreads like a simple liquid, and then progressively faster, while a remarkable bell-shaped profile develops at the bottom.(5-6) How the behaviors at the various scales are linked is an open question. Any answer requires reconsidering these wetting experiments deeper into the context of nematic films. More specifically, the anchoring of molecules at the interfaces(7-8) and the competition between nematic elasticity(9) and anchoring(10) must be discussed quantitatively. For the thinnest films, the problem proves to be more complex than expected and contradictory data are found in the literature. Therefore, we decided to complete our previous studies with further experiments using another compound of the cyanobiphenyls series, the 6CB in the nematic phase, and also on liquid substrates, water and glycerol. These new data confirm that the description of the thinnest nematic films is not yet fully understood.

Kinetics of ergodic-to-nonergodic transitions in charged colloidal suspensions: aging and gelation.

There are two types of isotropic disordered nonergodic states in colloidal suspensions: colloidal glasses and gels. In a recent paper [H. Tanaka, J. Meunier, and D. Bonn, Phys. Rev. E 69, 031404 (2004)], we discussed the static aspect of the differences and the similarities between the two. In this paper, we focus on the dynamic aspect. The kinetics of the liquid-glass transition is called "aging," while that of the sol-gel transition is called "gelation." The former is primarily governed by repulsive interactions between particles, while the latter is dominated by attractive interactions. Slowing down of the dynamics during aging reflects the increasing cooperativity required for the escape of a particle from the cage formed by the surrounding particles, while that during gelation reflects the increase in the size of particle clusters towards the percolation transition. Despite these clear differences in the origin of the slowing down of the kinetics between the two, it is not straightforward experimentally to distinguish them in a clear manner. For an understanding of the universal nature of ergodic-to-nonergodic transitions, it is of fundamental importance to elucidate the differences and the similarities in the kinetics between aging and gelation. We consider this problem, taking Laponite suspension as an explicit example. In particular, we focus on the two types of nonergodic states: (i) an attractive gel formed by van der Waals attractions for high ionic strengths and (ii) a repulsive Wigner glass stabilized by long-range Coulomb repulsions for low ionic strengths. We demonstrate that the aging of colloidal Wigner glass crucially differs not only from gelation, but also from the aging of structural and spin glasses. The aging of the colloidal Wigner glass is characterized by the unique cage-forming regime that does not exist in the aging of spin and structural glasses.

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.

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.

Nonergodic states of charged colloidal suspensions: repulsive and attractive glasses and gels.

Two types of isotropic disordered nonergodic states exist in colloidal suspensions: glasses and gels. The difference between the two is that the nonergodicity, or elasticity, of gel stems from the existence of a percolated network, while that of glass stems from caging effects. Despite this clear difference in the origin of nonergodicity, it is not straightforward to distinguish the two states in a clear manner. Taking a Laponite suspension as an explicit example, we propose a general phase diagram for charged colloidal systems. It follows that a transition from the glass to the gel state can be induced by changing the interparticle interactions from predominantly repulsive to attractive. This originates from the competition between electrostatic Coulomb repulsion and van der Waals attraction. If the repulsion dominates, the system forms a Wigner glass, while in a predominantly attractive situation it forms a gel. In the intermediate region, where both repulsive and attractive interactions play roles, it may form an attractive glass.

Stability of soap films: hysteresis and nucleation of black films.

We study the stability of soap films of a nonionic surfactant under different applied capillary pressures on the film. Depending on the pressure, either a thick common black film (CBF), or a micro-scopically thin Newton black film (NBF) is formed as a (metastable) equilibrium state, with a first-order (discontinuous) transition between the two. Studying the dynamics of the CBF-NBF transition, it is found that under certain conditions a hysteresis for the transition is observed: for a given range of pressures, either of the two states may be observed. We quantify the nucleation process that is at the basis of these observations both experimentally and theoretically.

What controls the thickness of wetting layers near bulk criticality?

We study the thickness of wetting layers in the binary-liquid mixture cyclohexane methanol. Far from the bulk critical point, the wetting layer thickness is independent of temperature, resulting from the competition between van der Waals and gravitational forces. Upon approaching the bulk critical temperature [t=(T(c)-T)/T(c)-->0], we observe that the wetting layer thickness diverges as t(-beta) with effective critical exponent beta=0.23+/-0.06. This is characteristic of a broad, intermediate scaling regime for the crossover from van der Waals wetting to critical scaling. We predict beta=beta/3 approximately 0.11, with beta the usual bulk-order parameter critical exponent, showing a small but significant difference with experiment.

Laponite: aging and shear rejuvenation of a colloidal glass.

We study the nonlinear rheological behavior and the microscopic particle dynamics for a colloidal glass, to see whether recently developed models for driven glassy systems can be applied to predict the rheology. Qualitatively, all the findings predicted by the models can be retrieved in our system. Notably, the viscosity decreases strongly with the shear rate. Since it is difficult to predict non-Newtonian viscosities of colloidal systems due to long-ranged hydrodynamic interactions, this shows the promise of this approach for predicting flow behavior. In addition, the measurements allow us to relate the microscopic diffusion dynamics to the macroscopic viscosity of the system.