Collision and Attachment Interactions of Single Air Bubbles with Flat Surfaces in Aqueous Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 229

case the film thickness can be obtained by numerical solutions or experimental
methods.
When the film becomes thinner, the repulsive forces decelerate the drainage rate
and the attractive forces can cause the film to rupture. There are two traditional
approaches used to describe the film rupture: (1) the thermal fluctuation of the
gas-liquid interface in the presence of the attractive forces, which amplifies the
fluctuation, and (2) the interfacial capillary waves (also called spinoidal dewetting)
which increase the interfacial area and the interfacial energy of the film, causing the
film rupture to minimise the system free energy [53]. The film rupture process can
be established using the linear or nonlinear hydrodynamic stability theories. The
theories can predict the critical film thickness and wavelength, and the time of the
film rupture.
The third theory proposed for film rupture is based on the density fluctuations
inside the film in the vicinity of a hydrophobic solid-liquid interface [53]. Specif-
ically, the film rupture can occurs by a gas nucleation process. The theory was
further developed by considering the stability of 'holes' in the liquid layer on the
solid surface. The film rupture is first triggered by a single hole and then, depending
on the capillary and gravitational forces, the liquid may fill the hole or the hole may
continue expanding. Holes of dimensions smaller than critical ones close and big-
ger than that expand. Sharma and Ruckenstein [64] developed a model for critical
hole diameter which is a function of film thickness and contact angle.
Recently, a number of authors have related the liquid film rupture on hydrophobic
surfaces to the coalescence of the interfacial nanobubbles [21]. After rupturing the
local foam films between the nanobubbles and the big bubbles, a hole with three
phase contact (TPC) in place of nanobubbles is formed which can be followed by
the TPC expansion if the hole is bigger than the critical dimension [69, 70].
Many authors confirmed the existence of the surface nanobubbles using IR spec-
troscopy, force measurement and AFM [20, 41, 75]. Also in a recent study by
Karakashev and Nguyen [32], it has been shown that migration of dissolved gasses
has a strong effect on the film rupture. Therefore, it is likely that the film rupture
is significantly influenced by the surface nanobubbles and/or submicroscopic and
nanobubbles of dissolved gases in the bulk solution.
2. Effect of Surfactants on Wetting Film Drainage and Rupture
Surfactants significantly influence the dynamic properties of thin liquid films and
the film life time. Figure 11 shows the effect of sodium dodecyl sulphate (SDS) on
the lifetime and rupture of wetting films on hydrophobised silica.
In the presence of surfactants the tangential liquid velocity at the film surfaces
may be substantially reduced by an opposing gradient of the surface tension, the
so-called dynamic elasticity or Marangoni effect [40]. Many works have been fo-
cused on the influence of the mechanical properties of the film surfaces (e.g., the
Marangoni stress), the type and concentration of surfactants on the film drainage
and stability [30, 60, 63, 76]. It is shown that surfactant adsorption at the film sur-