Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 44

slow. This was confirmed by Dutschk et al. [31] who investigated wetting behaviour
of dilute ionic and nonionic aqueous surfactant solutions over highly hydrophobic
and moderately hydrophobic polymer surfaces. They found that nonionic surfac-
tants enhanced spreading on both type of surfaces, whereas ionic surfactants did
not spread over highly hydrophobic surfaces. However, they argued that the long
time regime goes much slower and concluded that a possible explanation is that
adsorption at the expanding solid-liquid interface is slower than the diffusion.
Starov et al. [32] described the spreading mechanism of aqueous surfactant so-
lutions over hydrophobic surfaces as a transfer of surfactant molecules on the bare
hydrophobic surfaces in front of the moving liquid at the three-phase contact line
(Fig. 5). This mechanism was first mentioned elsewhere [33-35], but Starov and
co-workers broadened the idea assuming surfactant transfer onto the hydrophobic
solid interface to take place only from the liquid-gas interface. As explained be-
fore, this increases the solid-gas interfacial tension and 'hydrophilises' the initially
hydrophobic solid substrate. Adsorption of surfactant molecules at the liquid-solid
and liquid-gas interfaces results in a decrease of the relevant interfacial tensions,
and consequently of the total free energy of the system, and the drop spreads.
Spontaneous adsorption of surfactant molecules in front of the moving three-phase
contact line controls the rate of spreading [36].
The spreading of aqueous solutions of the sodium dodecyl sulphate on two types
of hydrophobic substrates, PTFE film and PE wafer, has also been studied [32]. It
was found that the evolution of drop radius was in agreement with the theoretical
model suggested (Fig. 6). Drop surface coverage was found to be an increasing
function of the bulk surfactant concentration inside the drop, and the maximum
was reached close to the CMC. Hence, according to the above mechanism, at low
surfactant concentrations inside the drop, the characteristic time scale of the sur-
factant molecular transfer, τ , decreases with increasing concentration, while above
the CMC, τ should level off and reach its lowest value. Both of these effects are
observed in experimental results [32] (see Fig. 7). The theoretical prediction of τ
dependency on surfactant concentration corresponded well to experimental findings
and served as a justification for the assumption concerning the transfer mechanism
of surfactant molecules.
F. Spreading of Surfactant Solutions over Thin Aqueous Layers: Influence of
Solubility and Micelle Disintegration
Thin liquid films can be found in many engineering, geology, and biophysics en-
vironments. Their application is significant in many coating processes [37-39] and
physiological applications [40]. Presence of non-uniform temperature or surface ac-
tive compounds across thin liquid films will lead to the formation of shear stresses,
also known as Marangoni gradients, at the air-liquid interface. These gradients
cause mass transfer on, and in, a liquid layer due to surface tension non-uniformity.
Marangoni stresses distribute the liquid from areas of low surface tension to areas