Precursor films in wetting phenomena.

The spontaneous spreading of non-volatile liquid droplets on solid substrates poses a classic problem in the context of wetting phenomena. It is well known that the spreading of a macroscopic droplet is in many cases accompanied by a thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter with a much higher speed. Such films have been usually associated with liquid-on-solid systems, but in the last decade similar films have been reported to occur in solid-on-solid systems. While the situations in which the thickness of such films is of mesoscopic size are fairly well understood, an intriguing and yet to be fully understood aspect is the spreading of microscopic, i.e. molecularly thin, films. Here we review the available experimental observations of such films in various liquid-on-solid and solid-on-solid systems, as well as the corresponding theoretical models and studies aimed at understanding their formation and spreading dynamics. Recent developments and perspectives for future research are discussed.

Experimental study of hybrid nematic wetting films.

Liquid crystal layers, with thickness less than 1 µm, are deposited on isotropic - solid or liquid - substrates and investigated in the bulk nematic range of temperatures. The boundary conditions at interfaces are antagonist ones, therefore the layers are distorted due to nematic elasticity. These films are referred to as "hybrid nematics". The consequences are complex. First, a forbidden range of film thickness is observed, depending only on temperature. Second, the anisotropy of the elastic response gives rise to striking stripe patterns in the thicker films. This behavior is common to several members of the series of n-cyanobiphenyls deposited on oxidized silicon wafers, water and glycerol. The aim of the study is to collect data, and determine which ones find a place within a common theoretical framework.

Coalescence driven by line tension in thin nematic films.

Thin nematic films deposited on liquid substrates provide a unique situation to investigate coalescence: the whole process can be followed under microscope over a wide range of times, and temperature allows us to monitor the surface viscosity of the surrounding fluid. For the first time, the complete scenario of 2D coalescence has been recorded for a given system in both inviscid limit and viscous environment, enabling us to identify the successive routes of dissipation. In particular, 2D "viscous bubbles" of the surrounding viscous fluid with a bulbous shape formed in the gap between coalescing films are observed. Available models are adapted to our specific case and account satisfactorily for the whole process.

Thin nematic films on liquid substrates.

Thin films of cyanobiphenyl liquid crystals (nCB) deposited on water or glycerol have been studied in the nematic temperature range. A common property of the systems is the hybrid anchoring conditions at the film interfaces. The preferred orientation of the nematic director is planar at the liquid interface, and it is homeotropic and somewhat weaker at the air interface. The resulting structure of the film depends on its thickness. Films thicker than 0.5 microm show the usual defects of nematics. Between 0.5 microm and 20-30 nm, complex instability patterns such as stripes, "chevrons", or squares are observed in extended films. Then there is a forbidden range of thickness below in which much thinner structures (usually monolayers and trilayers) are present. The present paper investigates this common behavior in various systems and gives arguments for its analysis.

Some specificities of wetting by cyanobiphenyl liquid crystals.

The present paper provides an up to date restatement of the wetting behaviour of the series of cyanobiphenyl liquid crystals (LCs) on usual substrates, i.e. oxidized silicon wafers, water and glycerol, at both the macroscopic and microscopic scale, in the nematic range of temperature. We show that on water the systems are close to a wetting transition, especially 5CB and 7CB. In that case, the wetting behaviour is controlled by the presence of impurities. On a mesoscopic scale, we observe for all our (thin LC film-substrate) systems an identical, complex, but well defined general scenario, not accounted for by the available models. In the last part, we present a study on line tension which results from the specific organization of LCs at the edge of the nematic film. We report preliminary results on two-dimensional film coalescence where this line tension plays a major role.

Post-Tanner stages of droplet spreading: the energy balance approach revisited.

The spreading of a circular liquid drop on a solid substrate can be described in terms of the time evolution of its base radius R(t). In complete wetting, the quasistationary regime (far away from initial and final transients) typically obeys the so-called Tanner law, with R∼t(α(T)), α(T) = 1/10. Late-time spreading may differ significantly from the Tanner law: in some cases the drop does not thin down to a molecular film and instead reaches an equilibrium pancake-like shape; in other situations, as revealed by recent experiments with spontaneously spreading nematic crystals, the growth of the base radius accelerates after the Tanner stage. Here we demonstrate that these two seemingly conflicting trends can be reconciled within a suitably revisited energy balance approach, by taking into account the line tension contribution to the driving force of spreading: a positive line tension is responsible for the formation of pancake-like structures, whereas a negative line tension tends to lengthen the contact line and induces an accelerated spreading (a transition to a faster power law for R(t) than in the Tanner stage).

Post-Tanner spreading of nematic droplets.

The quasistationary spreading of a circular liquid drop on a solid substrate typically obeys the so-called Tanner law, with the instantaneous base radius R(t) growing with time as R∼t(1/10)-an effect of the dominant role of capillary forces for a small-sized droplet. However, for droplets of nematic liquid crystals, a faster spreading law sets in at long times, so that R∼t(α) with α significantly larger than the Tanner exponent 1/10. In the framework of the thin film model (or lubrication approximation), we describe this 'acceleration' as a transition to a qualitatively different spreading regime driven by a strong substrate-liquid interaction specific to nematics (antagonistic anchoring at the interfaces). The numerical solution of the thin film equation agrees well with the available experimental data for nematics, even though the non-Newtonian rheology has yet to be taken into account. Thus we complement the theory of spreading with a post-Tanner stage, noting that the spreading process can be expected to cross over from the usual capillarity-dominated stage to a regime where the whole reservoir becomes a diffusive film in the sense of Derjaguin.

The leading edge of evaporating droplets.

New experiments on drops evaporating in normal atmosphere from smooth substrates in the situation of complete wetting are reported and compared with the available theoretical model. They are the continuation of previous work with alkane or water sessile drops, which is first briefly summarized. The model accounts very well for the dynamics of the drop radius, but the predictions are only qualitative for the contact angle, especially for small angles. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drops dynamics. Then the main part of the paper concerns new experiments with polydimethylsiloxane oligomers. These silicone oils are similar to alkanes as far as evaporation rate is concerned, but have lower surface tensions, and therefore smaller dynamic contact angles. The purity of the oils appears to be critical for the experiments, and requires a preliminary investigation. Then a systematic study of the drops dynamics is presented, as a basis for forthcoming theoretical work.

Rescaling the dynamics of evaporating drops.

The dynamics of evaporation of wetting droplets is investigated experimentally in an extended range of drop sizes to provide trends relevant for a theoretical analysis. A model is proposed, which generalizes Tanner's law in the presence of evaporation. A qualitative agreement is obtained, which represents a first step toward the solution of a very old, complex problem.

Spontaneous spreading of nematic liquid crystals.

The spontaneous spreading of macroscopic drops of nematic liquid crystals on hydrophilic substrates has been investigated by interferometric techniques. There is a complex interplay between the elastic energy, due to antagonist anchoring at the interfaces, and the radial flow in the spreading drop. A relevant parameter appears to be the relative humidity of the atmosphere, because it controls the amount of water molecules adsorbed on the substrate and, therefore, the strength of anchoring defects. The spreading laws differ from the ones of simple wetting liquids, and contact line instabilities coupled to short- (anchoring) or large-scale (disclinations) defects of the nematic film are observed.

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.

Defect-induced perturbations of atomic monolayers on solid surfaces.

We study long-range morphological changes in atomic monolayers on solid substrates induced by different types of defects; e.g., by monoatomic steps in the surface, or by the tip of an atomic force microscope (AFM), placed at some distance above the substrate. Representing the monolayer in terms of a suitably extended Frenkel-Kontorova-type model, we calculate the defect-induced density profiles for several possible geometries. In case of an AFM tip, we also determine the extra force exerted on the tip due to the tip-induced dehomogenization of the monolayer.

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

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.

Organization of cyanobiphenyl liquid crystal molecules in prewetting films spreading on silicon wafers.

We observe prewetting films of 8CB (4'-n-octyl-4-cyanobiphenyl) spreading at room temperature on silicon wafers by ellipsometry and x-ray reflectivity. Ellipsometry indicates the formation of a nondense monolayer spreading in front of a 45-A-thick film. X-ray reflectivity, performed using a ribbon geometry for the liquid crystal (LC) reservoir, allows us to determine the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar monolayer with tilted molecules. We show that the thickness of the bilayer is equal to the smectic periodicity in the bulk material and that the tilt angle of the molecules in contact with the solid surface is close to 60 degrees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using x-ray reflectivity because it induces a modulation of the electron density along the normal to the surface. Furthermore, a study of the ellispometric profile of a drop heated in the nematic phase, where we observe a complete spreading of the LC, shows the complex structuration of the LC close to the solid interface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific interaction between the trilayer and silicon wafer.

Structure of human cervical mucus from light scattering measurements.

Using laser light scattering, photon correlation and spectral analysis, it was shown that cervical mucus is a non-Newtonian Hydrogel with large meshes (approximately 5 microns). The experimental results are in agreement with a model of hydrogel with weak linkages and are definitely not compatible with a model of entangled macromolecules. Large oscillations, induced by both thermal and mechanical excitation, have been observed in this medium, probably due to its non-Newtonian character.

Proteins in membrane mimetic systems. Insertion of myelin basic protein into microemulsion droplets.

The insertion of myelin basic protein into microemulsion droplets of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) has been studied by quasi-elastic light scattering. Measurements were made at both low and high molar ratios of water to surfactant, as a function of protein occupancy. The hydrodynamic radii of filled and empty droplets were experimentally evaluated. These were compared to values calculated using a water shell model of protein encapsulation, and excellent agreement was obtained. At low molar ratio of water to surfactant (w0 = 5.6), the hydrodynamic radius of filled droplets is significantly larger than the radius of empty ones. Under these conditions, about three empty (water-filled) droplets are required to build up a droplet of sufficient size to accommodate a single protein molecule. At maximum solubilization, which occurs at w0 = 5.6, a small fraction of droplets are found containing protein aggregates. In contrast, results at high values of w0 (22.4) reveal radii for empty and occupied droplets of comparable dimension, and the absence of aggregates. The results are discussed in terms of the model and the mechanism of interaction of this protein with the aqueous interfaces provided by these membrane-mimetic systems.