Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 48

be a good alternative to water. These authors concluded the differences in aggrega-
tion between these different surfactants and solvent types mirror the changes in the
solvent dielectric constant.
G. Instabilities in the Course of Spreading
Instability at the contact line of drops of surfactant solutions spreading on prewet-
ted hydrophilic surfaces leads to the formation of fingering patterns at the edge
of drops. Fingering at the contact line of drops of surfactant solutions spreading on
solid substrates was first observed by Marmur et al. [51]. This fascinating behaviour
has, over the years, been studied theoretically, experimentally and using numerical
simulations [45, 52-74]. It was assumed that a pre-existing water film on the sub-
strate was essential for the growth of the instability. In [57, 70, 71], spreading on
oxidised silicon wafers of solutions of non-ionic surfactants in ethylene glycol or
diethylene glycol are investigated. Using polar solvents other than water allowed
the authors to discriminate between the role of a pre-existing adsorbed film of wa-
ter, which is known always to be present on a hydrophilic substrate in contact with
the atmosphere, and the role of a possibly thicker film consisting of the same liquid
as the drop.
In [53] an analytical expression was deduced for the growth rate of a disturbance
on a parallel film trapped between two drops based on linear stability analysis. It
was shown that the identified Marangoni effect resulting from the addition of insol-
uble surfactants dampens the arising instabilities. Under overwhelming presence
of surfactants, the instability becomes insignificant compared to the surfactant-
induced flow along the thin films.
The dynamics of a surfactant influenced thin film on an inclined substrate (grav-
ity driven) were considered [55]. For a constant flux, the formation of a capillary
ridge, with a Marangoni driven fluid 'step' downstream, and a fluid 'hump' up-
stream was observed, while the surfactant concentration reached its maximum at
the capillary ridge. The authors found that the prominent features of the flow, the
ridge, hump, step, and concentration peak become more pronounced via an increase
in inclination angle, Marangoni stresses and precursor layer thickness.
Troian et al. [63] proposed that the Marangoni effect is responsible for the insta-
bility at the edge of spreading surfactant drop. The Marangoni flow in the interface
and the bulk liquid is induced by surfactant concentration gradients along the air-
water interface. In their experiment, drops of aqueous surfactant AOT (sodium
bis-(2-ethyl-hexyl) sulfosuccinate) solution were placed on a hydrophilic surface
covered with a thin water film and immediate spreading, forming fingers advancing
from the contact line, was observed. It was found that the velocity and shape of the
fingers depended on the thickness of the underlying ambient water layer and the
surfactant concentration. On thin water films (0.1 µm), fingers are narrow, sharply
tipped and more branched, than those on a thick (1 µm) film (Fig. 9). The length of