Biomedical Engineering Reference
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Figure 9. Advancing contact angle on PTFE versus advancing contact angle on Teflon AF coated,
smooth silicon for pure liquids and surfactant solutions of similar intrinsic contact angle. Surfactant
solutions show generally higher contact angles than pure liquids. For the modified Cassie prediction
(Eq. (10e)), f is 0.04. Error bars show one standard deviation.
lineating the apparent contact angle range for pure liquids and the higher intrinsic
contact angle range for surfactant solutions. The difference can also be seen clearly
in Fig. 10b and Fig. 11a and b for OTS aluminum, Teflon AF coated AKD and un-
coated AKD surfaces, respectively. In each of these three graphs, while solutions
and pure liquids show similar apparent contact angles near the data point for ethy-
lene glycol, for lower intrinsic contact angles the pure liquid contact angle drops to
a lower value and with a steeper close than the surfactant solution contact angles. In
Fig. 10a, for Teflon AF coated aluminum, the results are quite close to each other,
and standard deviations of the data point overlap. The Teflon AF coated aluminum
surface gave very high contact angles with pure liquids however, so while the sur-
factant solutions do show slightly higher advancing contact angles, if pure liquids
are already not penetrating the SHS, surfactant solutions could not be expected to
further inhibit penetration.
With the possible exception of Teflon AF coated aluminum, the increases seen
cannot be explained by differences in intrinsic contact angle since the data in Figs
9-11 are normalized for this effect. Therefore, it is proposed that in addition to the
effect of increased intrinsic contact angle, the surfactants are preventing penetration
of the solution into the roughness, either by forming a film over the crevices [26,
77] or by some other means. The specifics of the exact mechanism remain to be
determined, but investigations should be aided though the use of the newly devel-
oped modified Cassie and Wenzel equations, which have been shown capable of
both predicting contact angles on SHS, and interpreting complex situations such as
partial penetration.
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