Metastable water films on hydrophobic silica surfaces

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: -Growing fluctuation waves (spin-odal dewetting) on fluid interfaces under the influence of any kind of attractive force [electrostatic, van der Waals, and maybe a so-called long-range hydrophobic force (LRHF)]. This mechanism was first developed by Scheludko.-Nucleation inside the film first proposed by Derjaguin.Metastable wetting films on glass surfaces either hydrophobized by methylation (negatively charged) or with Al3+ ions positively charged and hydrophilic, are analyzed by film thinning according to the Reynolds law.These experiments demonstrate that -Both mechanisms can be responsible for thin wetting film rupture: in the case of hydrophobic surfaces, the nucleation mechanism; in the case of oppositely charged silica surfaces, the capillary waves mechanism due to the attractive electrostatic double layer force between silica and the air bubble.-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 sites.The results provide deeper insight into the mechanisms of wetting film stability, the adhesion process in flotation and droplet coalescence.