M.P. Valignat

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.

Spontaneous spreading of surfactant solutions on hydrophilic surfaces

Abstract The spontaneous spreading of surfactant solutions on hydrophilic surfaces is investigated. The systems are solutions of nonionic C n E m surfactants in ethylene (EG) and diethylene glycol (DiEG) . The parameters of the study are the ratio c * of the concentration of surfactant to the critical micellar concentration, and the relative humidity RH of the atmosphere. Above some RH threshold, the spreading is controlled by Marangoni effect and is accompanied by dendritic contact line instabilities. The mobility of the pure surfactant on the solid surface is the key parameter for the start of that flow, and for the structure of the instability pattern.

Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films.

We study experimentally the molecular weight

Binary separation in very thin nematic films: thickness and phase coexistence.

M

Structure and dynamics of liquid films on solid surfaces

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.

Ultrathin liquid films. Ellipsometric study and AFM preliminary investigations

The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl on silicon substrates is reported. In the vicinity of the nematic-isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h(2) law ( h is the film thickness). Microscope observations and small-angle x-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first-order phase transition where thickness plays the role of an order parameter.

Structure and dynamics of thin liquid films on solid substrates

Liquid films spreading spontaneously on solid surfaces may exhibit very different behaviour, depending on the surface energy, atmospheric relative humidity, presence of temperature or concentration gradients, etc. In homogeneous situations, wetting liquids build molecularly thin films, which can be conveniently studied at the edge of spreading microdroplets. Spatially resolved ellipsometry is the relevant technique for this study. We present new results showing the various roles of atmospheric humidity in the spreading of polydimethylsiloxanes, silane derivatives and trisiloxane surfactants (which do not wet completely, but build a well organized bilayer on hydrophobic surfaces). In the presence of surface tension gradients, induced by temperature or concentration gradients, the films are much thicker and grow more rapidly. Contact line or interfacial Rayleigh-like instabilities frequently occur. Microscopy or interferometry are the relevant techniques for studying these cases. We present examples of such behaviour in the systems ethanol–water and ethylene glycol–non-ionic surfactant C12E10.

Examples of wetting transition: the effect of long and short range interactions

Films of molecular thickness exhibit a large variability, both in their structural properties and in their spreading dynamics. The thickness profiles of nonvolatile liquid microdroplets spreading spontaneously on smooth solid surfaces provide information on short range liquid-liquid and liquid-solid interactions. These profiles can in principle be recorded by various techniques, including spatially resolved ellipsometry and atomic force microscopy (AFM). The main part of the paper deals with results obtained using ellipsometry. In particular, the influence of water adsorbed on the substrate on the spreading dynamics of various liquids is reported. Unexpected absorbed on the substrate on the spreading dynamics of various liquids is reported. Unexpected problems met with the AFM technique are discussed, further work is under way.

Some specific aspects of the behaviour of ultrathin films

In the absence of surface tension gradients, and on smooth, chemically homogeneous substrates, wetting liquids build molecularly thin films, conveniently studied at the edge of spreading microdroplets. Spatially resolved ellipsometry is a relevant technique for this study. At the microscopic scale, microdroplets usually take a stepped pyramidal shape due to the layering induced by the solid surface. The thickness and growth dynamics of the successive layers give information on the behaviour of the liquid close to the solid. In particular, the friction process between the first molecular layer and the substrate is still only partially understood. We report studies of the spreading rate of polydimethylsiloxanes (PDMS) with different chain lengths. For moderate or high friction, the successive layers grow at a comparable rate and the spreading of the droplet is mainly controlled by the mesoscopic scale. Thus, the relevant parameter is the bulk viscosity h. For low friction, only the first layer grows significantly. Then the relevant parameter is the number N of monomers or the molar mass M of the polymer chain.

Surface Induced Effects in Liquid Crystals: A Microscopic Study

The thickness profiles of microdroplets spreading spontaneously on solid surfaces have been studied using spatially resolved ellipsometry. Information on interaction and rheology can be extracted. We present experimental findings illustrating how long or short range interactions may induce a wetting transition in an isotropic liquid. The parameters modified to approach the transition are the silica thickness of a silicon substrate and the atmospheric relative humidity. The first one induces a change in long range interaction and the second one a change of conformation in liquid molecules in contact with the solid.