Biomedical Engineering Reference
In-Depth Information
E. Experimental Results and Discussion
Having derived a model describing contact angles on rough surfaces (wet in the
Wenzel or Cassie mode) and smooth surfaces (Section C), these models are now
applied to experimental data taken for Teflon AF and OTS coated silicon, as well
as uncoated etched PTFE, Teflon AF and OTS coated etched aluminum, and Teflon
AF coated and uncoated naturally rough AKD. For rough surfaces, the predictions
should ideally use estimates/measurements of
f
1
and
f
2
, capturing the rough-
ness/corrugations of the solid-liquid and liquid-vapor interfaces under the drop.
Due to the experimental difficulties in determining values of
f
1
and
f
2
, we will
make single estimates of
f
instead. Calculations [31] suggest that the errors inher-
ent in this simplification are likely less than 5
◦
for the low
f
1
or
f
values seen for
∼
SHS (i.e., for
f
0.1). The use of the correct equation would tend to cause the
predictions of contact angle to decrease more slightly more quickly at higher values
of concentration/lower values of surface tension.
1. Smooth Surfaces—Surfactant Solutions
Figure 4a and b show the contact angles of SDS, HTAB, and MEGA10 on Teflon
AF and OTS coated silicon, respectively. Points are the average of advancing and
receding contact angles (used as a proxy for equilibrium contact angle), and the end
points of the bars mark the advancing and receding contact angles. Also shown (as
lines in each graph) are the predicted contact angles using the model (Eq. (10a))
developed in Section C, with no solid-vapor adsorption since the surfaces are hy-
drophobic [70]. The adsorption constants used in the model are those for SDS on
smooth Teflon AF, with the average water contact angle measured on each surface
used as
θ
y
for each plot. As discussed in Section C, surfaces of different intrinsic
contact angle might experience different solid-liquid adsorption behavior, causing
differences in contact angle and surface tension. Further, different surfactants might
result in different adsorption behavior, again leading to different contact angle and
surface tension curves. Figure 4b shows, however, that the model predicts contact
angles on the saturated hydrocarbon chemistry of OTS coated silicon similarly well
to the Teflon AF coated silicon. Further, while individual plots of surface tension
versus
concentration and average contact angle
versus
concentration show differ-
ences for each different surfactant, when plotted as contact angle (Eq. (10a))
versus
surface tension (Eq. (7)). Figure 4a and b both show that the model successfully pre-
dicts contact angle
versus
surface tension for HTAB and MEGA10 as well as SDS.
The dotted regions of the lines are an extrapolation to super-CMC SDS concentra-
tions, they predict the higher concentration (but still sub CMC) contact angles for
HTAB and MEGA 10. In all, the model is seen to predict surfactant solution contact
angles well in all cases studied here, and so it will be used to explore the wetting of
SHS.
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