Wetting Film Dynamics and Stability

Abstract

Although the wetting films are similar in many aspects to other thin liquid films, there are some differences in their behavior, too. In contrast to soap and emulsion films, whose surfaces are homogeneous, solid substrates of wetting films are heterogeneous as a rule, unless special measures for their homogenization are taken. Here we mean primarily heterogeneous distribution of surface energy leading to existence of hydrophobic domains on hydrophilic surfaces and vice versa. As is known, such hydrophobic domains could play the role of gasphase nucleation centers and it is widely accepted nowadays that nano-bubbles can be formed there. The present paper reviews the effect of nano-bubbles adhered at solid surface on stability of wetting films. It is shown that the existence of nano-bubbles is crucial for the lifetime of wetting films. Another peculiarity typical for hydrophobic solid surface, the so-called slippage effect, is also investigated and its contribution to the dispersion equation of capillary waves on wetting films is accounted for.Thin films, the building blocks of dispersed systems, are present mainly in three variants: soap, emulsion and wetting films. There are many equilibrium and dynamic characteristics typical for all these kinds of films, e.g. the surface DLVO forces acting therein, the type of hydrodynamics in the films (the so-called lubrication flow), the equilibrium and stability conditions, film rupture, etc. However, besides their common nature each kind of films mentioned above possesses specific features, which should be taken into account when interpreting the corresponding experimental data and their influence on the behavior of dispersed systems. The subject of this paper is to review some of the peculiarities, related to the rheology, stability and rupture of wetting films. As is shown, all phenomena observed are due to effects characteristic of solid/liquid surfaces. A section is devoted to the hydrodynamics of wetting films with special attention paid to a new experimental method for exciting and measuring surface waves on wetting films. Among the most interesting results obtained by this method is the anomalous high slippage observed on solid surfaces. Presence of nano-bubbles adhered at solid/liquid surface is suggested as a possible origin of this effect. The idea of the existence of nano-bubbles in wetting films appears again in the last section as a cause for the film rupture. The detailed analysis shows that the region above a nano-bubble resembles more a foam film, which is unstable as a rule, thus explaining why wetting films with repulsive interactions could also rupture.

Quantitative Analysis of Aqueous Nanofilm Rupture by Molecular Dynamic Simulation

“…13,14 Recent experiments have suggested that nanobubbles can stably form at a hydrophobic surface and in an aqueous solution 15,16 and can be used to explain the liquid film rupture through a number of stages. 17,18 For example, during film drainage between a large air bubble and a hydrophobic surface, the macroscopic bubble first approaches the apex of the largest surface nanobubbles. A nanometer-sized aqueous foam film is effectively formed between the macroscopic bubble and the nanobubble, where the van der Waals attraction is strong.…”

Nonuniform Slip Effect in Wetting Films

“…The interest in thin liquid films has gradually increased from the middle of the previous century due to the importance of disperse systems-such as foams, emulsions and suspensions-to technology. The film studies split naturally into thermodynamic forces and hydrodynamic stability or rupture [1][2][3]. By the onset of nanotechnology in the present century, the flow in wetting films has attracted enormous attention as an essential process in the modern micro-and nano-fluidics [4][5][6].…”

Tribology of thin wetting films between bubble and moving solid surface

“…The last equation corresponds to the stress-free air/water interface, while the first two Navier equations 35 presume a possible slip on the glass/water surface with a slip friction coefficient  .…”