Model and Experimental Study of Surfactant Solutions and Pure Liquids Contact Angles on Complex Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 200

predictions (at progressively higher values of f , i.e., increased amounts of penetra-
tion into the pores of the roughness) are thus necessary to model both surfaces. In
contrast, the contact angles of bromonapthalene and hexadecane strongly suggest
Wenzel mode wetting, supporting the hypothesis [77] that surfactant solutions are
inhibiting penetration of surfactant solutions into the roughness of the SHS.
Considering that both coated and uncoated AKD surfaces seem to be wet in the
Cassie mode by surfactant solutions, it is counterintuitive that they would display
zero receding contact angle, both with surfactant solutions and pure liquids. This is
expected to be due to the hydrophilic heterogeneities in the structure of the AKD
pinning the contact line, and/or due to pinning on the microstructure of the AKD.
5. Investigating the Role of Intrinsic Contact Angle
Summarizing Fig. 5 through Fig. 7, one can see that surfactant solutions wet SHS
differently than non-aqueous pure liquids of similar surface tension. Teflon AF
coated aluminum shows higher contact angles than the PTFE surface for pure liq-
uids, but both surfaces show roughly similar behavior with surfactant solutions.
This indicates that on rough surfaces, surfactants can make up for a less repellent
topography by preventing penetration (thereby maintaining a low f value). Con-
sidering the low contact angles on AKD, however, shows that surfactants cannot
always do so. Another, more important, difference is that surfactant solutions show
higher contact angles, indicating a preserved Cassie mode, compared to pure liquids
of similar surface tension. In literature, the surface tension of a liquid is often taken
as the controlling factor determining if said liquid will penetrate a given rough sur-
face. However, intrinsic contact angle is a better measure, since as Eq. (4) suggests,
a critical intrinsic contact angle must be surpassed to transition between the Cassie
and Wenzel modes. As the analysis of experimental results in this chapter has, so
far, relied upon surface tension, the question remains if the preserved Cassie mode
for surfactant solutions on SHS is due to differences in intrinsic contact angle, or
due to an additional effect of the surfactants. To answer this question, contact an-
gles of pure liquids on smooth Teflon AF coated and OTS coated silicon surfaces
were also measured. The results comparing surfactant solutions and pure liquids on
smooth surface are presented in Fig. 8a and b.
For both Teflon AF and OTS coated silicon surfaces, pure liquids are seen to
show generally similar contact angles to surfactant solutions of similar surface ten-
sion. At low surface tensions however, surfactant solutions show a slightly higher
contact angle than the corresponding pure liquids (bromonapthalene and hexade-
cane). To investigate if this increase in intrinsic (smooth surface) contact angle
explains the higher contact angles on SHS for surfactant solutions, one can plot
contact angle on the SHS versus intrinsic contact angle. The range of intrinsic con-
tact angles considered is shown by the dotted boxes in Fig. 8a and b.
Plotting the contact angle on the SHS versus intrinsic contact angle in Figs 9-
11, one can see that advancing contact angle for pure liquids on SHS are generally
significantly lower than the range of advancing contact angle for surfactants solu-