Wetting Dynamics of Aqueous Surfactant Solutions on Polymer Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 83

current value of the adsorption quantity on the solid surface and e is the corre-
sponding equilibrium value.
While many monographs and reviews are devoted to capillarity and wetting by
pure liquids, the wetting by surfactant solution is not described in monographs,
with the exception for [45, 47, 48]. Interest in wetting dynamics processes has
immensely increased during the past 15 years. In many industrial and medical ap-
plications, some strategies to control drop spreading on solid surfaces are being
developed. One possibility is that a surfactant, a surface active polymer or poly-
electrolyte is added to a liquid. A reduction of water surface tension by adsorption
of surfactant molecules on a water vapour interface and adsorption of surfactant
molecules on solid-liquid and solid vapour interfaces alter non-wetting behaviour
of aqueous solutions on hydrophobic substrates into partial or complete wetting
behaviour.
The same conclusion can be drawn in the case of water penetration into hy-
drophobic porous media. Aqueous surfactant solutions can spontaneously penetrate
into hydrophobic porous substrates and the penetration rate depends on both the sur-
factant type and its concentration. Both the liquid-vapour interfacial tension γ LV
and the contact angle of moving meniscus θ a (advancing contact angle) are con-
centration dependent. Despite the enormous technical importance of spreading of
aqueous surfactant solutions over solid surfaces, information on possible spreading
mechanisms is limited in literature. Disjoining pressure isotherms in the presence
of surfactants are well investigated in the case of free liquid films [49], much less
is known in the case of liquid films on solid substrates [44]. At the present, we are
not able to give an answer how surfactant molecules are transferred in the TPC line
vicinity. In the case of aqueous surfactant solutions our knowledge of the transition
zone behaviour from meniscus to thin films in front is very limited. A systematic
representation of the main areas of this broad topic can be found in a review [45]
recently published.
4. Spreading in Technological Applications
For many applications, surfactants are introduced into the aqueous phase to increase
the rate and uniformity of wetting. Despite their enormous technical importance,
there is a lack of data in the literature about the spreading dynamics of aqueous sur-
factant solutions on surfaces of practical interest, e.g., rough, inhomogeneous. The
knowledge of how surfactant adsorption at the surfaces involved affects the spread-
ing mechanism and dynamics is also limited. Dynamic wetting measurements allow
studying surfactants, polyelectrolytes or other surface-active substances as well as
their mixtures and engineered surfaces. A methodology for screening of technical
surfactant solutions applicable for the wetting dynamic measurement on a wide
range of solid surfaces was developed and is outlined below.
On the one hand, the screening method has to make possible a selection of a
surfactant suitable for certain purpose. On the other hand, new application fields
for known surfactants could be opened up. By a screening method, a methodol-