Biomedical Engineering Reference
In-Depth Information
Using the model with these coefficients, it is seen that in the Wenzel mode, in-
creasing
r
(increasing s-l area under the drop and around the contact line) leads
to an increasing effect of surfactant adsorption on contact angle, as there is more
area to adsorb to. The Wenzel roughness parameter,
r
, also has its usual effect of
increasing contact angle for intrinsic contact angles greater than 90
◦
, and decreas-
ing contact angles otherwise. This means that on rough, hydrophilic surfaces, there
is an amplification both of the intrinsic contact angle and the effect of surfactants,
leading to enhanced wetting.
In Cassie mode wetting (on rough but homogeneous surfaces), it is instead seen
that decreasing
f
1
(decreasing s-l area) leads to a decreasing effect of surfactant
adsorption as there is less area to adsorb to. For the flat-topped pillared surfaces
considered in exploring the model, decreasing Cassie solid fraction,
f
1
, also leads
to an increasing contact angle due to the increasing
f
2
(or
(
1
f)
for flat-topped
surface texture) term in Eq. (10d), which quickly overwhelms the other decreasing
terms in the model. Because of this, wetting in the Cassie mode is less affected
by the intrinsic contact angle of the surface. The dominance of the
f
2
(or
(
1
−
−
f)
for flat-topped surface texture) term explains the high contact angles seen in
literature for SDS solutions on SHS. This key finding is the first of its kind based
on theoretical reasoning.
For Cassie mode wetting of smooth but heterogeneous surfaces, the
f
m
parame-
ters are seen as weighting factors; determining how the behaviors of each material
in a heterogeneous surface are combined. It is seen that small inclusions with high
contact angles at a given concentration have a large effect on surfaces predomi-
nantly composed of a low contact angle material at the same given concentration.
Conversely, low contact angle inclusion have a smaller effect on surfaces predomi-
nantly composed materials with a high contact angle at a given concentration. This
can be understood by considering that the
f
m
parameters act on the cosine of the
concentration dependent contact angles. As a result, a small change in the cosine
of the heterogeneous surface contact angle at low contact angles results in a large
change in contact angle. Conversely, an identical small change in the cosine at con-
tact angles closer to 90
◦
results in a relatively small change in contact angle. Even
more complex behaviors are seen as the
f
m
parameters for each material approach
each other.
The model was also used to investigate advancing and receding contact angles
of nine surfactant solutions of various polarities. Four pure liquids with a sim-
ilar range of surface tensions to the surfactant solutions (
27-73 mN/m) were
also tested. Five superhydrophobic surfaces (SHS) with different topology ('spiky'
etched PTFE, 'bumpy' etched aluminum and 'plate like' AKD) and chemistry (fluo-
rinated or saturated hydrocarbon) combinations were probed along with two smooth
control surfaces.
Based upon advancing contact angle measurements on the SHS, it was de-
termined that the unpenetrated Cassie mode was the most commonly seen, with
occasionally a mode that appears to be partial penetration at higher surfactant con-
∼
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