Rupture of Wetting Films Caused by Nanobubbles

Abstract

It is now widely accepted that nanometer sized bubbles, attached at a hydrophobic silica surface, can cause rupture of aqueous wetting films due to the so-called nucleation mechanism. But the knowledge of the existence of such nanobubbles does not give an answer to how the subprocesses of this rupture mechanism operate. The aim of this paper is to describe the steps of the rupture process in detail: (1) During drainage of the wetting film, the apex of the largest nanobubble comes to a distance from the wetting film surface, where surface forces are acting. (2) An aqueous "foam film" in nanoscale size is formed between the bubble and the wetting film surface; in this foam film different Derjaguin-Landau-Verwey-Overbeek (DLVO) forces are acting than in the surrounding wetting film. In the investigated system, hydrophobized silica/water/air, all DLVO forces in the wetting film are repulsive, whereas in the foam film the van der Waals force becomes attractive. (3) The surface forces over and around the apex of the nanobubble lead to a deformation of the film surfaces, which causes an additional capillary pressure in the foam film. An analysis of the pressure balance in the system shows that this additional capillary pressure can destabilize the foam film and leads to rupture of the foam film. (4) If the newly formed hole in the wetting film has a sufficient diameter, the whole wetting film is destabilized and the solid becomes dewetted. Experimental data of rupture thickness and lifetime of wetting films of pure electrolyte and surfactant solutions show that the stabilization of the foam film by surfactants has a crucial effect on the stability of the wetting film.

Wetting Film Dynamics and Stability

“…10). The detailed analysis [52,53,66,67] shows that gas bubbles formation actually creates non-homogeneities in the wetting film. In the region above the bubble the surface forces are much closer to surface forces in foam (air/water/air) films and differ from those in wetting films (Fig.…”

Dry-spot nucleation in thin liquid films on chemically patterned surfaces

“…Karthaus (1999) observed the formation of ordered arrays of micrometer-sized polymer arrays upon dewetting of a polystyrene solution on mica, Si, and glass substrates. Sharma (1993) and Sharma and Jameel (1993) considered in addition to the well-studied apolar, Lifshitz-Van der Waals interactions also polar, acid-base interactions, and specifically the breakup of aqueous films (Sharma 1998), which have been studied experimentally by Stöc-kelhuber (2003;Stöckelhuber et al 2004). Thiele et al (2001a, b) extended the study of Sharma and Ruckenstein (1986) and elucidated the role of non-uniform initial conditions on the rupture dynamics of unstable thin liquid films.…”

An analytical film drainage model and breakup criterion for Taylor bubbles in slug flow in inclined round pipes

“…Nucleation refers to the presence of nanobubbles on a hydrophobic solid surface [16,44,17,45,24,40,41]. The mechanism here is similar to the film 90 breakup in foam films, which has been extensively studied [7,5].…”