RESUMEN
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.
RESUMEN
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.
RESUMEN
We study the intrinsic friction of monolayers adsorbed on solid surfaces from a gas phase or vapor. Within the framework of the Langmuir model of delocalized adsorption, we calculate the resistance offered by the mobile adsorbate's particles to some impure tracer molecule, whose diffusive random motion is biased by a constant external force. We find that for sufficiently small driving forces the force exerted on the tracer shows viscouslike behavior. We derive then the analog of the Stokes formula for two-dimensional adsorbates, calculate the corresponding friction coefficient, and determine the stationary particle distribution in the monolayer as seen from the driven impurity.
RESUMEN
The same approach used by Boender, Chesters, and van der Zanden in the context of an advancing liquid-gas meniscus in a capillary tube is extended to the case of spontaneous spreading of a droplet on an ideal solid surface. The result is an ordinary differential equation for the droplet profile which can be solved if the meniscus inclination phi0 is specified at some distance lambda from the solid. As in the capillary-tube case, good agreement is obtained with experimental data obtained by the authors and by others if phi0 is set equal to the static contact angle (zero in cases investigated experimentally), taking lambda of the order of a molecular dimension (1 nm). A comparison of predicted dynamic contact angles in the spreading-drop and capillary-tube cases for given values of the capillary number indicates only a weak dependence of the behavior on the system geometry. De Gennes and co-workers have predicted that during the final stages of spreading the inner length scale lambda should be determined by the effects of disjoining pressure in the thin film adjacent to the contact line rather than by molecular dimensions. The lambda value implied by their model is derived, thereby establishing the regime of spreading in which such effects should be dominant. The observed behavior in this regime is found to correspond somewhat better with a lambda value of the order of a molecular dimension, although the differences are small. Although the explanation probably lies in the nonideality of even the smoothest surfaces, this result suggests that the simplest model, based on a single lambda value of the order of 1 nm, should provide an excellent predictive tool. Copyright 1998 Academic Press.