Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 46

of high surface tension (flow generation) and in doing so, also deform the interface
resulting in height variations (deformation and instability of liquid films). In this
section, we restrict our discussion to influence of surfactants in thin liquid films.
The understanding of Marangoni induced flows is important, as it can be ei-
ther beneficial or detrimental in many applications. Surfactants, which are normally
present in a healthy mammalian lung to reduce surface tension forces, keep the
lung compliant and prevent collapse of the small airways during exhalation. How-
ever, most prematurely born babies do not produce an adequate amount of these
surfactants, which leads to respiratory distress syndrome. This condition is treated
by surfactant replacement therapy where surfactants are introduced into the lungs.
These surfactants spread by other forces in the large to medium pulmonary air-
ways. In small airways, surface tension gradients dominate and Marangoni flow
distributes the surfactant to the distal regions of the lung [40, 41].
A problem in coating processes where paint films are dried by solvent evap-
oration is that the non-uniformity of the evaporation leads to Marangoni stresses
which cause deformation in the film and hence, permanent defects on the paint sur-
face ('orange peel' effect) [42]. Another example would be in washing latex films
displays surfactant non-uniformities (surfactant islands) that leaves permanent in-
dentations in the film [43]. Another application of the Marangoni effect is the use
for drying silicon wafers after a wet processing step during the manufacturing of
integrated circuits. An alcohol vapour is blown through a moving nozzle over the
wet wafer surface and the subsequent Marangoni effect will cause the liquid on the
wafer to pull itself off the surface effectively leaving a dry wafer surface [44].
Spreading of surfactant solutions on thin liquid films has been reviewed by Afsar-
Siqqiqui et al. [45] a few years ago. A moving circular wave front forms after a
small droplet of aqueous surfactant solution is deposited on a thin aqueous layer
(Fig. 8). The time evolution of the radius of the moving front was monitored [46].
An experimental procedure was designed to investigate the influence of Marangoni
force on spreading of surfactant solutions over thin aqueous layers.
Solubility of the surfactants is also believed to have an important influence on
wetting. Lee et al. [46] considered surfactants of different solubilities at concentra-
tions above CMC, and in all cases observed that spreading considerably depended
on the solubility of surfactants: the higher the solubility, the slower the spread-
ing. Von Bahr et al. [47] also noticed that the presence of micelles significantly
slows surfactant transport, and consequently spreading on the whole. Both groups
detected two stages of the front motion: the first fast stage, followed by a slower
second stage. High solubility in aqueous solutions, typical of ionic surfactants: SDS
and DTAB, slowed down both the first and the second stages [46]; if the solubility is
high enough then during the second stage the front reaches some final position and
does not move any further. Low soluble nonionic surfactants (Tween ® 20, Tergitol ®
NP10) showed faster wetting during both stages and observed dynamics was suc-
cessfully modelled by a power law, R(t) t n ,where n is the spreading

exponent. During the first stage and for low soluble surfactants, n is expected to

= const ·