Biomedical Engineering Reference
In-Depth Information
it was found that Wenzel drops could sometimes make a transition to the Cassie
state either entirely or over part of their footprint area through coalescence during
the condensation process [
42
]. The authors suggest that the calescence of two drops
eliminates liquid-air interfacial area, and then enough energy is brought into the
system for this energy barrier to be overcome.
9.3
Biomimicking Technologies
It is found that suitable roughness combined with low-surface energy are essential
to obtain superhydrophobic surfaces no matter what material is used (organic
or inorganic) and what kind of structure formed on the surface (particles, rod
arrays, or pores). As a result, roughening the surface followed by hydrophobization
or transforming low-surface-energy materials into rough surfaces are common
procedures to produce superhydrophobic surfaces.
The most used substrates for fundamental research on superhydrophobic surfaces
involve rigid solid substrates such as silicon wafers, glass slides, and metal surfaces,
which might limit the practical applications and the large-scale production of
superhydrophobic surfaces [
63
]. Flexible substrates such as polymer films and
fibrous substrates outperform the rigid substrates for superhydrophobic surfaces
in industrial applications. The rough surfaces could be cast, spray coated, or
post treated on polymer films with intrinsic low-surface energy or imparted by
hydrophobization. Fibrous substrates include woven or nonwoven textiles with
natural or synthetic microfibers.
Practically, the methods for superhydrophobic surface preparation can be gen-
erally ascribed into three categories: top-down, bottom-up, and combination of
bottom-up with top-down approaches. Top-down approaches involve lithographic
and template-based techniques [
64
], as well as surface plasma treatment [
65
-
72
].
Bottom-up approaches encompass mostly self-assembly and self-organization [
65
-
72
], such as chemical deposition [
73
-
77
], layer-by-layer (LBL) deposition [
77
-
81
],
hydrogen bonding [
73
,
77
,
78
], and colloidal assemblies [
82
]. Methods combining
bottom-up with top-down approaches contain casting of polymer solution, phase
separation [
64
,
76
,
83
-
85
], and electrospinning [
86
]. In the subsequent section,
different approaches are illustrated in detail.
9.3.1
Top-Down Approaches
The top-down approach is a general term referring to the manufacture of materials
and devices by carving, molding, or machining bulk materials with tools and
lasers in microelectronics, including technologies such as templation [
79
,
87
,
88
]
and lithographic approaches [
39
,
89
], micromachining [
8
,
33
,
45
,
90
-
92
], and
plasma treatments. The technologies are frequently combined in order to produce
