A. M. Cazabat

Light—scattering study of water-oil microemulsions

Microemulsions have attracted much interest since their introduction because of their applications in numerous fields of science and technology. They are apparently homogeneous and transparent systems of low viscosity, containing emulsifier molecules, oil, and water. They usually behave as dispersions of very small spherical droplets of water (or oil) surrounded by emulsifier molecules, in a continuous medium containing the oil (or water). The droplet sizes range from 100 to 1000 A in diameter.

Critical behavior in microemulsions

Abstract Interactions in dispersions have been studied using light scattering techniques applied to microemulsions. In these systems, hard sphere interactions are dominant. The remaining interactions (van der Waals, etc.) are usually attractive and short-ranged and can be treated as perturbations. However, close to phase transitions where the microemulsion separates into two other microemulsions, the attractive part of the potential becomes large and behaves as if long range interactions were present; the characteristics of the scattered light can also be interpreted by assuming that the system is close to a critical consolute point. The low interfacial tensions (measured between the two microemulsions in equilibrium using surface light scattering techniques) and the large interfacial thicknesses (deduced from optical reflectivity) are consistent with the picture in terms of critical phenomena.

Diffusion-driven evaporation of sessile drops

The evaporation of wetting drops deposited on a substrate at thermal equilibrium under normal atmosphere is discussed. The evaporation rate appears to be controlled by the stationary diffusion of vapour molecules in the gas phase. Experiments with alkanes and water drops are fairly well accounted for by an isothermal model, taking into account the specific properties of thin films.

Low interfacial tensions in three-phase systems obtained with oil-water surfactant mixtures

Abstract We have studied the quaternary mixture brine-toluene-SDS-butanol, in the domain where it gives three phases, a microemulsion in contact with an organic and an aqueous phase, and measured three low interfacial tensions γmo, γXXX and γow. We find γow stays equal to the largest of the two others and conclude that the origin of low interracial tensions between the microemulsion and organic or aqueous phases are due to different phenomena.

Dynamics of Wetting: From Theory to Experiment

The main available theories for the dynamics of wetting are brieflysummarized and discussed in reference to experiments. In partial wetting,hydrodynamic and molecular theories are equivalently efficient, even if thephysical meaning of parameters is not so clear in the former ones. Incomplete wetting, hydrodynamic theories are the only ones valid at lowangles, but some care has to be taken in the interpretation of the“slip length” introduced to remove the divergence of thedissipation at the contact line. The situation is less favourable at themolecular scale, where the theoretical description is still at itsbeginning, due to the multiplicity of behaviours.

Films driven by surface tension gradients

Fingering instabilities are often observed at the contact line of drops of surfactant solutions spreading spontaneously on solid surfaces. It has been recognised recently that a usual linear stability analysis predicts stable behaviour in contrast to the observed instability. It now seems the instability arises from short-time transients, where the thickness of the film ahead of the main drop is a crucial parameter for amplification. We reconsidered previous experiments and performed new ones along these lines. The strengths and weaknesses of the available models were analysed.

Dynamics of spreading on a liquid substrate

Abstract Experiments on the spreading of liquids on liquid substrates and the concomitant formation of precursor film and extended profile of the central drop are described. Our main findings are (l) compared to spreading on solid substrates the spreading rate on liquid substrates is considerably larger, (2) the spreading rate is in good approximation independent of the viscosity of the spreading liquid, and (3) the dynamic contact angle cannot be unambiguously defined. The results are interpreted in the spirit of a recent theory from Joanny. The general agreement between experiment and theory is satisfactory.

Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films.

We study experimentally the molecular weight

Instabilities during wetting processes: Wetting by tensioactive liquids

M

Instability patterns in ultrathin nematic films: Comparison between theory and experiment

dependence of spreading rates of molecularly thin precursor films, growing at the bottom of droplets of polymer liquids. In accord with previous observations, we find that the radial extension R(t) of the film grows with time as R(t) = (D_{exp} t)^{1/2}. Our data substantiate the M-dependence of D_{exp}; we show that it follows D_{exp} \sim M^{-\gamma}, where the exponent \gamma is dependent on the chemical composition of the solid surface, determining its frictional properties with respect to the molecular transport. In the specific case of hydrophilic substrates, the frictional properties can be modified by the change of the relative humidity (RH). We find that \gamma \approx 1 at low RH and tends to zero when RH gets progressively increased. We propose simple theoretical arguments which explain the observed behavior in the limits of low and high RH.