Biomedical Engineering Reference
In-Depth Information
In addition, temperature/PH dual responsive surfaces with tunable wettability
have been investigated by introduction of multi-responsive functional groups. For
instance, integrating NIPAAm films with the pH-dependent component acrylic acid
(AAc) leads to temperature/pH dual-stimulated P(NIPAAm-
co
-AAc) copolymer
films. Jiang et al
.
[
170
] have demonstrated wetting behavior is controlled simul-
taneously by temperature and pH in a wide range. Figure
9.9
shows the wetting
behavior on these films becomes more hydrophobic as temperatures increases when
the pH is fixed, and becomes more hydrophilic as pH value increases at certain
temperature. The extreme hydrophobic state with water CA 148.8
ı
is found when
the temperature is at 45
ı
C and pH value is 2. While the extreme hydrophilic state
with water CA 7.6
ı
is obtained when the temperature is at 21
ı
C and pH value is 11.
9.4.3
Electrical Potential as External Stimulus
Electrical potential as another attractive external stimulus is able to control the sur-
face chemistry and/or morphology in a few seconds or less. Hence, electrowetting
is expected as an effective and versatile method to manipulate surface wettability
[
171
-
173
]. Electrocapillarity, the basis of modern electrowetting, was first described
in detail in 1875 by Gabriel Lippmann [
174
]. Recently, Berge [
175
] has developed
the idea to isolate the conductive liquid from the substrate using a dielectric layer in
order to eliminate the problem of electrolysis. This concept has also become known
as electrowetting on dielectric as shown in Fig.
9.10
. Basically, the application of
an external electrical bias across the solid/liquid interface leads charges to build up
both at the liquid side and at the solid electrode, while decreasing the solid/liquid
interfacial tension. Consequently, wetting behavior can be tuned to become more
hydrophilic without changing the surface morphology and composition.
Even though studies on electrowetting have been conducted for decades, re-
searches are limited on superhydrophobic surfaces. In 2004, Krupenkin et al
.
[
176
]
had demonstrated dynamic electrical control of the wetting behavior of liquids
on designed nanostructured silicon wafer surfaces. For the first time, over the
entire possible range from superhydrophobicity to superhydrophilicity was achieved
based on electro-wetting with a small electrical bias. Since then electrowetting
studies on superhydrophobic surfaces have been increasingly investigated on varied
materials, such as CNTs [
177
-
181
], ZnO [
182
], and MnO
2
[
50
]. Wang et al.
[
181
] have demonstrated that water can be efficiently wet and pumped through
superhydrophobic aligned multiwalled CNTs membranes via electrowetting. The
CNTs membranes were found be strong polarity-dependent and an abrupt transition
from superhydrophobicity to hydrophilicity was observed at a critical bias ( 1.7 V),
with the membrane connecting to anode. Zhao et al. [
50
] have demonstrated
electrowetting on a superhydrophobic membrane of MTAs. In particular, the electro-
driven adhesion was enhanced on the superhydrophobic surfaces, on which a water
droplet can be immobilized by application of a small DC bias, despite of its large
contact angle. For a 3-
L water droplet, the measured adhesive force increases
