Model and Experimental Study of Surfactant Solutions and Pure Liquids Contact Angles on Complex Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 206

centrations. The fully penetrated Wenzel mode was not seen. Pure liquids, on the
other hand, generally showed lower contact angles than the surfactant solutions,
indicating greater (often full) penetration of the pure liquids into the SHS.
Receding contact angle is high for high surface tension/intrinsic contact an-
gle pure liquids and surfactant solutions. It likewise decreases as surface ten-
sion/intrinsic contact angle decreases. However, the receding contact angles of
surfactant solutions are seen to decrease less than those for pure liquids. In addition,
the PTFE surface shows receding contact angles above 90 ◦ for some high concen-
tration surfactant solutions while the coated aluminum surface did not. The AKD
surface showed zero receding contact angle for all liquids/solutions. This suggests
that topography can play an important role in the receding behavior of surfactant
solutions. In addition to the effects of intrinsic contact angle, for surfactant solu-
tions, the low receding contact angle can be understood by considering stretching
of a surfactant film across the pores [26], re-organization at the contact line [72],
and/or pinning of the solution at the tops of the roughness. Regardless of cause the
results suggests that the effect of surfactants in increasing advancing contact an-
gles (compared to pure liquids) is not necessarily carried over to receding contact
angles.
Taking advancing and receding contact angle data together it is clear that sur-
factant solutions show equal or higher apparent contact angles than pure liquids of
similar surface tension/intrinsic contact angle on all five SHS. Since the analysis
corrects for the slightly higher intrinsic contact angle of surfactant solutions, it is
proposed that surfactant are inhibiting penetration of solutions into SHS, either by
surfactant film formation or by pinning of the contact line at the top of the rough-
ness.
Surface chemistry is seen to affect wetting mainly by means of the intrinsic
contact angle. It is seen that for a given topography saturated hydrocarbon chem-
istry results in similar behavior to fluorinated chemistry in terms of general wetting
trends and mechanisms describing them. However, the contact angles are generally
lower for saturated hydrocarbon chemistry and drop to lower values more abruptly,
and at higher surface tensions. In this way, an unsuitable chemistry can overrule the
beneficial effects of topography.
G. Acknowledgements
The authors acknowledge Nicole Petong, Stefan Michel, Frank Simon and others of
the Leibniz Institute of Polymer Research, Dresden, Germany, for sample prepara-
tion and characterization. Further acknowledgements to C. Blank at the Institut für
Werkstoffwissenschaft at the Technical University of Dresden, for the high qual-
ity, high magnification SEM images and to Pedro J. Ramón-Torregrosa for sample
characterization. Surface characterization was also performed at the Alberta Centre
for Surface Engineering and Science. Funding was provided by the National Sci-
ence and Engineering Research Council of Canada, the Leibniz Institute of Polymer