Model and Experimental Study of Surfactant Solutions and Pure Liquids Contact Angles on Complex Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 180

sition from Cassie to Wenzel mode wetting). Stronger wetting, double/branched
tailed surfactants could decrease advancing contact angle to zero however, indicat-
ing a Cassie to Wenzel transition. They did not report much information on receding
contact angle, and it is unclear whether their technique of decreasing drop volume
by 20% would in fact access the receding mode of the drop. They explain their re-
sults by suggesting that the common surfactants cannot decrease the surface tension
sufficiently to achieve the Wenzel mode (i.e., liquid penetrating the pores/crevices
of the SHS), while the double/branched tailed surfactants can. They did not test
pure liquids to compare their results/examine if any other effects besides surface
tension played a role.
The results of Tang et al . [74] support the findings of Chang et al. , i.e., suf-
ficiently high concentrations of Silwet L-77 (a 'superspreading' surfactant) can
drastically decrease contact angle on a lotus leaf surface. The main thrust of the
work was spreading, rather than measuring contact angles [74]. Receding contact
angles were not reported, nor was a comparison with pure liquids performed.
Ferrari et al. [75, 76], reported on wetting of SHS by mixtures of non-ionic,
semi-polar, and ionic (SDS and hexadecyltrimethylammonium bromide (HTAB))
surfactant solutions (with or without salt). They created their SHS by roughening a
glass slide, covering it with silica nanoparticles, and coating them with hydropho-
bic treatments. They tested a single concentration of each surfactant far below CMC
and at 2 CMC, with no intermediate concentrations, with and without the addition

of 20 mM NaCl. They found superhydrophobic behavior for low concentrations,
and decreased but non-zero contact angles at 2

×

CMC. Salt was seen to have
little effect at low surfactant concentration, and was somewhat detrimental to re-
pellency at 2

×

×

CMC. Their explanation for the observed behavior suggested that
self-limiting surfactant adsorption (decreased surface tension) explained the results.
The mixed surfactant studies they performed showed more complicated behaviors
that are beyond the scope of discussion in this chapter. Mixed surfactants still could
not fully wet SHS, however.
If the hypothesis of Ferrari et al. and Chang et al. were correct (i.e., if the decreas-
ing surface tension with surfactant concentration were the sole controlling factor of
contact angle on SHS), then the same SHS would be expected to behave equally
poorly with pure liquids of similar surface tension to the solutions. This was not
seen by Mohammadi et al. [77], who studied a SHS produced by the natural forma-
tion of a rough microstructure on alkyl ketene dimmer (AKD), a wax that presents a
saturated hydrocarbon surface. They found that pure liquids showed an abrupt drop
in advancing * contact angle around a surface tension of 45 mN/m. This is similar to
the work of Shibuichi et al. [36], who also studied AKD SHS, but with pure liquids

* The receding contact angle data reported in reference [77] has been discovered to be erroneous; using
the traditional definition of receding contact angle (a sustained and constant value while the contact line is
receding) reanalysis of the data showed that the receding contact angle for all liquids is zero. Shibuichi et
al. [36] did not measure receding contact angle, and may be unaware if it was zero for their AKD surfaces.