these surfaces have multiple-scale roughness. The main bumps can act
as Wenzel-type surfaces, while the smaller bumps can act as Cassie–
Baxter surfaces. As is expected, very low hysteresis and very large contact
angles have been reported on fractal solids. Another important benefit of
fractal solids is their ease of fabrication—because these surfaces are
usually created by chaotic movement of material, they require less specialized machinery and can be made with a variety of materials. In fact,
fractal superhydrophobic surfaces have been reported using semiconducting polymers, giving these particular surfaces the ability to switch
properties under the influence of UV light or an applied voltage.
One very common downside of fractal solids, however, is their tendency
to be opaque. Their multiscale roughness is very beneficial to their
superhydrophobic properties, but because these features range from
nanoscale to microscale, they interact with visible light. However, if
transparent fractal solids could be synthesized, they could very well be
used for windshield coatings on cars or as protective layers for photoactive devices such as LEDs or photovoltaic cells.
This problem is not present in another type of superhydrophobic surface
fabrication—uniform nanostructures. These structures can be generated
in many ways, from carbon nanotube growth to lithographic etching.
These structures are the opposite of fractal patterns in that although they
are sometimes random in arrangement, they are generally uniform in
size. These materials can be easily generated using laser lithography as
well as chemical etching to produce surface morphologies of a specific
predetermined shape and pattern. As well, these surfaces can be easily
“decorated” with additional coatings and features to produce increased
hydrophobicity. Structures of this type have been reported to reach
contact angles up to 178° using dodecanoic acid–coated, cobalt hydroxide
nanopins. As well, significant (168°), although not quite as substantial,
Wenzel contact angles were reported using decorated carbon nanotubes.
Perhaps most importantly, these kinds of structures are of one size and
thus do not often absorb in the visible range. This feature makes these
kinds of structures more viable for the kinds of surfaces previously discussed. The drawback of these kinds of materials is that they are much
less effective at repelling water than fractal structures. They lack multiscale roughness, and even methods that generate more random arrays
are less hydrophobic than fractal surfaces.
What has emerged in recent years has not been the development of one
dominant method of constructing superhydrophobic surfaces. Rather,
FUNDAMENTALS OF SURFACE SCIENCE 229
as Wenzel-type surfaces, while the smaller bumps can act as Cassie–
Baxter surfaces. As is expected, very low hysteresis and very large contact
angles have been reported on fractal solids. Another important benefit of
fractal solids is their ease of fabrication—because these surfaces are
usually created by chaotic movement of material, they require less specialized machinery and can be made with a variety of materials. In fact,
fractal superhydrophobic surfaces have been reported using semiconducting polymers, giving these particular surfaces the ability to switch
properties under the influence of UV light or an applied voltage.
One very common downside of fractal solids, however, is their tendency
to be opaque. Their multiscale roughness is very beneficial to their
superhydrophobic properties, but because these features range from
nanoscale to microscale, they interact with visible light. However, if
transparent fractal solids could be synthesized, they could very well be
used for windshield coatings on cars or as protective layers for photoactive devices such as LEDs or photovoltaic cells.
This problem is not present in another type of superhydrophobic surface
fabrication—uniform nanostructures. These structures can be generated
in many ways, from carbon nanotube growth to lithographic etching.
These structures are the opposite of fractal patterns in that although they
are sometimes random in arrangement, they are generally uniform in
size. These materials can be easily generated using laser lithography as
well as chemical etching to produce surface morphologies of a specific
predetermined shape and pattern. As well, these surfaces can be easily
“decorated” with additional coatings and features to produce increased
hydrophobicity. Structures of this type have been reported to reach
contact angles up to 178° using dodecanoic acid–coated, cobalt hydroxide
nanopins. As well, significant (168°), although not quite as substantial,
Wenzel contact angles were reported using decorated carbon nanotubes.
Perhaps most importantly, these kinds of structures are of one size and
thus do not often absorb in the visible range. This feature makes these
kinds of structures more viable for the kinds of surfaces previously discussed. The drawback of these kinds of materials is that they are much
less effective at repelling water than fractal structures. They lack multiscale roughness, and even methods that generate more random arrays
are less hydrophobic than fractal surfaces.
What has emerged in recent years has not been the development of one
dominant method of constructing superhydrophobic surfaces. Rather,
FUNDAMENTALS OF SURFACE SCIENCE 229