Biomedical Engineering Reference
In-Depth Information
to understand and explain experimental results for wetting of 5 different SHS by
various surfactant solutions in Section E, examining how topography, surface chem-
istry, and liquid type/purity affect wetting. But first, some of the literature available
on wetting by surfactant solutions is briefly summarized in Section B.
B. Literature on Surfactant Solution Wetting
The wetting of simple surfaces by surfactant solutions is relatively well understood.
On smooth hydrophilic surfaces (those with water contact angles less than 90
◦
)
,
surfactant solutions tend to spread, and can also demonstrate either a fingering
instability leading to non-symmetric drop shapes [47, 53-65], or the autophobic
effect where adsorption of surfactants create a more hydrophobic surface, leading
to the spontaneous recede of a spreading drop [64-69]. The effect seen (finger-
ing or autophobing) depends on the affinity of the surfactant for the surface. On
smooth hydrophobic surfaces (those with water contact angles greater than 90
◦
),
the addition of surfactants tends to decrease contact angle [48-51, 70, 71]. Some
researchers [48-51] have proposed an autophilic effect to occur on hydrophobic sur-
faces (with surfactants adsorbing ahead of the contact line on areas never touched
by solution). However, solid recent evidence disproves this effect for the surfactants
considered here [70] so the autophilic effect can be neglected. The decreased intrin-
sic contact angle of surfactant solutions on smooth hydrophobic surfaces (which all
researchers agree occurs) could trigger a transition to the Wenzel mode of wetting in
a SHS. Regarding receding contact angle, Varanasi and Garoff suggested [72] that
re-organization of surfactants around the contact line during the recede can lead to
pinning of the contact line and a decrease in the receding contact angle.
Very little consideration has been given to the use of surfactant solutions on
SHS. Shirtcliffe
et al.
[26] studied superhydrophobic porous sol-gels for their use
as switches (switching from superhydrophobic to hydrophilic behavior based on
temperature, liquid type, or impurity concentration). Their study examined the wet-
ting of only one surfactant solution on a single SHS. They reported a decrease in
advancing contact angle from 140
◦
with water to about 120
◦
for concentrations of
Sodium Dodecyl Sulphate (SDS) above Critical Micelle Concentration (CMC), and
a decrease in receding contact angle of 140
◦
for CMC of SDS, but did not present
corresponding results with non-aqueous pure liquids for comparison. They did re-
port that sufficiently low surface tension mixtures of ethanol in water penetrated
the porous SHS. They suggested limited contact of the surfactant solution with the
surface resulted in the high advancing angles, though they did not suggest why the
solution did not penetrate the surface. They also suggest that surfactant films could
be bridging the pores/crevices of the SHS and decreasing the receding contact angle
due to pinning of the contact line.
Chang
et al.
[73] again studied a porous sol-gel, this time with a variety of
surfactant solutions. They found that common single tailed surfactants could not
decrease advancing contact angle below
130-140
◦
∼
(i.e., could not cause a tran-
Search WWH ::
Custom Search