Biomedical Engineering Reference
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sion (for pure liquids) is near the Cassie to Wenzel transition point, at least during
the receding stage. However, near this surface tension value, surfactant solutions
still show high receding contact angles, supporting Mohammadi
et al.
's hypothesis
[77].
3. Aluminum Surfaces—Surfactant Solutions and Pure Liquids
Figure 6a and b show advancing and receding contact angles of surfactant solutions
and pure liquids on Teflon AF coated and OTS coated electrochemically etched
aluminum. For model predictions (
via
Eqs (10b) and (10e)),
r
values were measured
by AFM (see Table 1) and
f
values were estimated for the aluminum surfaces in
the same way as described for the PTFE surface. In addition, another
f
value was
found for OTS Aluminum by fitting the highest concentration data point for SDS
to Eq. (10c). As will be discussed later, this was done to allow a full explanation of
the data.
In Fig. 6a, the successful prediction of the surfactant solution data by Eq. (10e)
indicates that the Teflon AF coated aluminum surface is wet in the Cassie mode
for all surfactant solutions. This surface also shows extremely high advancing con-
tact angles with all pure liquids. Despite this, surfactant solutions are still seen to
display slightly higher contact angles in comparison, lending further support to the
hypothesis of surfactant solutions inhibiting penetration [77].
The receding contact angles for Teflon AF coated aluminum are seen to be simi-
lar to PTFE, and can be understood to occur due to the same mechanisms (stretching
of a surfactant film across the pores [26], re-organization at the contact line [72],
and/or pinning of the solution at the tops of the roughness). However, there are no
high concentration solutions showing high receding contact angle on the aluminum
surface, suggesting that that topography can play an important role in the receding
behavior of surfactant solutions. A possible explanation is that the dual-scale topog-
raphy of the aluminum (see Fig. 3c and e) could give a greater number of pinning
points compared to the sharper topography of the PTFE (see Fig. 3b). Interestingly,
Teflon AF coated aluminum and PTFE otherwise show little difference in terms of
surfactant solution contact angles. On the other hand, the aluminum surface shows
noticeably higher advancing and receding contact angles for pure liquids compared
to the PTFE surface. The main difference between PTFE and Teflon AF coated alu-
minum is topography since liquid chemistry is unchanged and solid chemistry is
similar, with PTFE expected to have a slightly lower intrinsic contact angle com-
pared to Teflon AF [85]. Taking the observations for pure liquids and surfactant
solutions together suggests that the inhibition of penetration caused by surfactant
solutions can override the major differences in topography and minor differences in
chemistry between the 'spiky' PTFE and 'bumpy' aluminum.
OTS coated aluminum (Fig. 6b) shows generally lower contact angles than Teflon
AF coated aluminum (Fig. 6a), with a transition to full wetting on the recede at
higher surface tension values. Individual wetting tests also showed that the receding
contact angle was generally less stable (i.e., exhibited more instances of stick-slip)
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