Biomedical Engineering Reference
In-Depth Information
butterflies are endowed with superhydrophobic feature to prevent capillary forces
sticking them together [
11
]. The fine structure of butterfly (
Papilio ulysses
) wings is
shown in Fig.
9.1
(c). Inspired by these observations in nature, increasing endeavors
from chemistry, material science, physics, and biology have been made to mimic
those structures for varied functional concern.
The process for fabrication of a superhydrophobic surface via altering the surface
energy and roughening of the surface has been well accepted. Since Kao et al.
[
12
] have demonstrated the artificial superhydrophobic fractal surface made of
alkylketene dimer and reemphasized the importance of geometrical structure in
surface wettability, an increasing number of ways to fabricate superhydrophobic sur-
faces have been reported, either by introducing roughness into a low-surface-energy
material and modifying a rough surface with low-surface-energy materials. Among
those, multifunctional superhydrophobic surfaces have attracted a greater attention.
The conventional super water repellent surfaces are integrated with various features
of transparency, structure color, reversibility, anti-reflectivity, breathability, and
other properties [
13
-
18
].
For structured surfaces, seeking to understand the relationship between surface
roughness and its wetting property, Wenzel's [
19
] and Cassie's [
20
] theories are the
most applied. Essentially, these two theories describe different superhydrophobic
states: the former is complete wetting and the latter is when water droplets sit
on a composite surface with air pockets trapped underneath. Besides, some other
researchers [
21
-
24
] have argued that it is the three-interface contact line (TCL) that
determines the contact angle behavior including advancing, receding, and hysteresis
angles rather than the interfacial area within the perimeter. Even though conceptual
problems may exist within Wenzel's and Cassie's theories, they still have merits
to be applied at certain situations as advocated by their critics [
22
-
24
]. It is fairly
straightforward to use them to characterize two distinguishable superhydrophobic
states: the “slippy” Cassie state and the “sticky” Wenzel state. In general, water
droplets adhere more strongly to the textured surface in the Wenzel state than in
the Cassie state, causing stronger contact angle hysteresis (the difference between
advancing and receding angle). In many cases, water droplets on structured surfaces
are in the metastable Cassie state and the transition from Cassie to Wenzel state can
be induced by external stimuli, such as pressure, electric voltage, or vibration.
In this review, we are not intended to provide an in-depth overview of the
vast body of literatures; rather we want to summarize some selected aspects of
superhydrophobic surfaces with their wetting properties and look ahead to future
developments. The major objective of this chapter is (a) to introduce theoretical
background charactering the lotus effect and the mechanism of wettability, (b) to
review the most recent progress in the fabrication methods of superhydrophobic
