Biomedical Engineering Reference
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Figure 5. Advancing (closed symbol) and receding (open symbol) contact angles of surfactant solu-
tions on naturally fluorinated plasma etched PTFE. Pure liquid contact angles are given as open circles
(advancing) and '
's (receding). Lines show the model prediction of contact angles (using Eqs (10b)
and (10d) for Wenzel and Cassie mode wetting, respectively) using parameters found for aqueous so-
lutions of SDS on Teflon AF. Standard deviations are generally of the same order as symbol size, and
are not shown except for the large deviation of the receding contact angle of ethylene glycol (discussed
in text).
×
SHS, respectively. The model requires values of r and f * for the surface. The r
value was measured using AFM (see Table 1). The f value was estimated by fitting
the advancing contact angle for water on the PTFE surface to Eq. (3). The advanc-
ing contact angle was used instead of the average contact angle since the AKD
surfaces (studied later) have zero receding contact angle (which makes using an av-
erage contact angle problematic). For the case of the PTFE and aluminum SHS, the
advancing and receding contact angles for water are nearly identical, meaning that
an estimate of f based on the advancing contact angle is essentially no different
from an estimate based on average contact angle.
Using the measured r value, the model predictions of the modified Wenzel equa-
tion (10b) fails to predict the advancing contact angles of surfactant solutions on
the PTFE surface considered in Fig. 5 (dashed-dotted line). Using an r value fit to
the water contact angle via Eq. (2) produces similarly poor results (not shown), and
results in an r value that does not reconcile with the measured value. The failure of
the modified Wenzel model is not surprising since the PTFE surface has shown SHS
behavior (high advancing and receding contact angle, therefore likely wetting in the
Cassie mode). The modified Cassie equation (10e) predicts the advancing contact
* Predictions should use values of f 1 and f 2 in Eq. (10d). Single estimates of f , and Eq. (10e) are used
instead for the sake of simplicity, with errors likely less than 5 and with the correct equation (Eq. (10d))
resulting in contact angle decreasing slightly more quickly [31].
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