The influence of acting forces on the rupture mechanism of wetting films -: nucleation or capillary waves

The mechanism of the rupture process of liquid films is not fully understood yet, particularly in the case of an asymmetric film between a solid surface and a gas bubble. There are two theoretical approaches describing this problem, (a) growing fluctuation waves (spinodal dewetting) on fluid interfaces under the influence of any kind of attractive forces (electrostatic, van-der-Waals, and maybe a so-called long-range hydrophobic force (LRHF)). This mechanism was first developed by Scheludko. (b) Nucleation inside the film, first proposed by Derjaguin. Metastable wetting films on negatively charged, hydrophobic glass surfaces (gaseous phase methylated), or on hydrophilic, positively charged glass surfaces (with Al-3 (+) ions), have been analyzed by means of the kinetics of film thinning according to the Reynolds law. These experiments demonstrate that (a) both mechanisms can be responsible for the rupture of a thin wetting film, in the case of hydrophobic surfaces the nucleation mechanism is preferred; in the case of an opposite charged silica surface the capillary waves mechanism due to an attractive electrostatic double layer force between silica and air bubble takes place, (b) the existence of a LRHF on a hydrophobic surface can be excluded. The apparent interaction can be explained by the presence of gas nuclei formed on heterogeneous surface sites. The results provide deeper insight into the mechanisms of wetting film stability, adhesion process in flotation and droplet coalescence. (C) 2001 Elsevier Science B.V. All rights reserved.