Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 43

All three adsorption processes result in a decrease of the excess free energy of the
system. However, adsorption processes (i) and (ii) result in a decrease of corre-
sponding interfacial tensions, γ sl and γ lg , but the transfer of surfactant molecules
onto the solid-gas interface in front of the drop results in an increase of a local free
energy, although the total free energy of the system decreases. That is, surfactant
molecule transfer (iii) goes via a relatively high potential barrier and, hence, goes
considerably slower than adsorption processes (i) and (ii). Therefore, they are 'fast'
processes as compared with the third process (iii).
Theexcessfreeenergy, , of the droplet on a solid substrate is given by (see
Fig. 1):
= γ lg A + P e V + π R 2 (γ sl − γ sg ), (4)
where A is the area of the liquid-gas interface; P e = P a − P l is the excess pressure
inside the liquid, P a is the pressure in the ambient air, P l is the pressure inside the
liquid and R is the radius of the droplet base. The last term on the right-hand side
of Eq. (4) gives the difference between the energy of the part of the bare surface
covered by the liquid drop and the energy of the same solid surface without the
droplet. Equation (4) shows that the excess free energy decreases if (i) the liquid-
gas interfacial tension, γ lg , decreases, (ii) the solid-liquid interfacial tension, γ sl ,
decreases, and (iii) the solid-gas interfacial tension, γ sg , increases. This last is a
very important conclusion, which is usually overlooked [27].
E. Spreading over Hydrophobic Substrates
Due to their immense importance in numerous practical and industrial applications,
enhanced wetting of aqueous surfactant solutions has been the subject of consider-
able research interest over the past two decades. Scales et al. [28] were one of the
first groups to point out the importance of the nature of the solid substrate in the
wetting process. They introduced characteristic normalisation of the wetting be-
haviour as a powerful tool to give both a qualitative and quantitative understanding
of adsorption of a series of alkyl aryl polyoxyethylenes. Significance of the sub-
strate hydrophobicity was also shown by Keurentjes et al. [4]. They determined
that a surfactant, when adsorbed onto a hydrophobic solid, exposes its polar head
groups to the solution, whereas in the case of a hydrophilic solid, a by-layer of sur-
factants may form. Eriksson et al. [29] gave further understanding why surfactant
transfer to the three-phase contact line is the dominant process in wetting. Transfer
of surfactant molecules to the interface is widely believed to be the essential ele-
ment in the overall dynamics of wetting and numerous attempts have been made to
quantify this fundamental phenomenon.
Over the years, various wetting and spreading regimes have been revealed,
mostly depending on the type of liquid-solid system investigated. Von Bahr et
al. [30] reported two wetting stages at low surfactant concentrations: a short time
regime where spreading occurs rapidly and a long time regime where spreading is