Environmental Engineering Reference
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in water (or other liquid), which produces the advancing contact angle, and sub-
sequently extracted, generating the receding contact angle.
While the characterization of a superhydrophilic surface (h * 0) is rather
simple—being related to the spreading of water on the surface to cover the largest
area possible, superhydrophobic surfaces (h [ 150) are more complex to
describe. In fact, based on surface tension equilibria and Young's equation, no
known chemistry allows water to configure with a contact angle larger than 120
on a smooth surface (Hunter 2010 ): therefore, surface morphology must neces-
sarily be involved to reach a superhydrophobic state.
When surface roughness comes into play, two models are generally adopted to
describe surface wettability, that is, Wenzel state and Cassie-Baxter state (Fig. 9.5 )
(Wenzel 1949 ; Cassie et al. 1944 ). In 1936 Wenzel stated that adding surface
roughness enhances surface wettability, as shown in Fig. 9.5 a, b, where the contact
angle of a water drop on the same hydrophobic surface is apparently higher when
roughness is introduced, and vice versa on hydrophilic surfaces (Fig. 9.5 c), since it
is measured against an average line that represents the apparent solid surface. This
variation can be calculated by applying Eq. 9.2 :
cos h W ¼ r u cos h 0
ð 9 : 2 Þ
Fig. 9.5 From top to bottom:
a water drop on a smooth
hydrophobic surface;
b Wenzel state; c detail of
apparent decrease in contact
angle predicted by Wenzel
model (h W ) compared to the
one calculated by Young's
equation (h 0 ) on hydrophilic
surfaces; d Cassie-Baxter
state
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