Review on Kinetics of Spreading and Wetting by Aqueous Surfactant Solutions - Drops and Bubbles in Contact with Solid Surfaces - page 52

instabilities. However, at the current time it is difficult to extract information from
these investigations related to the properties of surfactants.
H. Spreading of Surfactant Solutions over Porous Substrates
Spreading and penetration of liquids on porous media is a fundamental property
that affects applications including printing, painting, adhesives, oil recovery, imbi-
bition into soils, health care, and home care products [78-82]. In inkjet printing, the
resolution is directly associated to the degree of liquid extension and spreading on
the printing media after deposition [82]. The spreading of a drop on a thin perme-
able medium proceeds in two parts: (a) spreading on the surface of the medium and
(b) penetration into the underlying medium. Knowledge of the spreading rate and
area covered is critical as drop-to-drop contact would result in unwanted and detri-
mental effects. Fast penetration of the liquid would limit the time the drop spent
on the surface, thereby decreasing coalescence of drops. However, penetration of
liquid and medium are usually slow due to the poor wettability by the liquid of the
porous medium. Surfactants from this point of view may play a crucial role.
Furthermore, surfactants' role in oil recovery processes is especially important.
It is highly desirable to extract oil trapped in the pores of rocks. The injection of sur-
factants reduces the interfacial tension between the oil and water phases, allowing
the extracting of trapped oil in small pores [83]. The importance of such knowledge
leads to on-going research on the wetting kinetics of porous media influenced by
surfactants.
Recent publications reveal a growing interest in exploring the simultaneous
spreading and imbibition processes of aqueous surfactant solutions. However, these
studies have so far mostly been restricted to pure liquids simultaneously spreading
and imbibing into the porous substrate [15, 84-94].
Clarke et al. [95] developed a theoretical model for simultaneous spreading and
imbibition by incorporating the molecular kinetic theory of spreading [15] with the
modified Lucas-Washburn equation.
Starov and co-workers developed a theoretical model [86, 87] for the case of
complete wetting of a drop spreading over a pre-wetted or a dry porous layer. The
lubrication theory approximation was used, neglecting gravitational influence, so
that only capillary forces are taken into account in the model. They established
a system of two differential equations to describe the evolution of the radius of
both the drop base ( L(t) ) and wetted region ( l(t) ) inside the porous layer, and
an equation describing the dynamic contact angle. Experiments were performed
in order to test the theoretical model. Silicon oils were used as the liquid and as
porous layers, nitrocellulose membranes with different pore size. By comparing
the theoretical model and experimental data on appropriate dimensionless scales, a
universal behaviour was observed where experimental data was in good agreement
with theoretical prediction [86, 87].