Biomedical Engineering Reference
In-Depth Information
The Cassie-Baxter relation shows that the cosine of the contact angle on a
microscopically inhomogeneous solid surface is the barycenter of the cosine of the
contact angles on the different chemical components of the surface. The Cassie-
Baxter law explains some unexpected experimental results: Sometimes a microfab-
ricated surface may present chemical inhomogeneity and the wetting properties are
not those that were intended. Take the case where a uniform layer of Teflon is de-
posited on a substrate to make it hydrophobic. However, if the layer is too thin, the
Teflon layer may be porous and the coating inhomogeneous; the wetting properties
are then modified according to the Cassie-Baxter law and the gain in hydrophobic-
ity may not be as large as expected.
As for Wenzel's law, an important remark at this stage is that the scale of the
heterogeneities of the different chemical materials of the solid surface is very small
compared to that of the drop [35]. Indeed, if not, it would not be possible to define
a unique contact angle anymore. This latter type heterogeneity is related to drop
pinning as we have seen in Section 3.7.
3.8.2.3 Contact on Microfabricated Surfaces—Superhydrophobic and
Superhydrophilic Substrates
According to the Wenzel and Cassie laws, the contact angle of a liquid on a solid
surface depends on the roughness and the chemical homogeneity of the surface.
By combining the effect of these two laws, special superhydrophobic and super-
hydrophilic substrates have been developed. First, the Wenzel law shows that the
hydrophobic or hydrophilic property of a surface can be increased by increasing
the roughness of this surface. Many techniques have been developed to increase
the roughness of a surface. A widely used method consists in growing nanocrystals
on the surface and deposing a coating on top by chemical vapor deposition (CVD).
Teflon, silanization, or fluorated coating (CFx) have hydrophobic properties and
are widely used in biotechnology (often to reduce the unwanted adsorption of bio-
logic objects on the surface); on nanocrystal-treated surfaces these coatings produce
very hydrophobic substrates [36]. It is even possible to switch a hydrophilic sur-
face—such as gold—to a very hydrophobic surface. An example is the deposit of a
coating of CF 4 -H 2 -He by plasma discharge on a gold plate (Figure 3.68) [37]. The
Figure 3.68  A flat gold plate treated with CF 4 -H 2 -He plasma deposition becomes very hydropho-
bic. (a) A water droplet bounces back from the surface treated with CF4-H2-He plasma deposition.
(b) A water droplet spreads on the original gold surface [37]. Reprinted with permission from [37].
Copyright 2005 American Chemical Society.
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