Biomedical Engineering Reference
In-Depth Information
Fig. 9.10
Schematic
illustration of electrowetting
set-up. Partially wetting
liquid droplet at zero voltage
(dashed) and at high voltage
(solid). See the text for
details. Reprinted with
permission from ref. [
50
].
Copyright (2011) Wiley-VCH
monotonically with increasing negative voltage, reaching a maximum of 130
N
at 22 V, 25 times higher than the original value. It follows that the nearly spherical
water droplet can be controllably pinned on the substrate, even if the substrate
is turned upside down. Moreover, the electrically adjustable adhesion is strongly
polarity-dependent; only a fivefold increase is found when a positive bias of 22 V is
applied. This remarkable electrically controlled adhesive property is ascribed to the
change of contact geometries between the water droplet and MTAs, on which water
droplets exhibit different continuities of TCL.
Alternatively, self-assembled conducting polymers with different wettability
conformations are found to be used to control the wetting behavior on the surfaces
by electrical potential. Choi [
183
] has found that (16-mercapto)hexadecanoic
acid molecules undergo transition of conformation between straight and bent by
applying an electrical potential. In the straight conformation, the molecules exhibit
hydrophilic feature due to the carboxylate anion on the topmost layer, while the
conformation re-orientating to bent state, they display hydrophobicity because of the
exposure of the hydrophobic tails of the molecules outwards. Hence, by controlling
these two inverse conformations, surface wettability changes within the range of
CAfrom20to30
ı
. To amplifying the range of wettability switching, Xu et al.
[
184
] have demonstrated a simple facile electropolymerization of superhydrophobic
polypyrrole (PPy) films and the reversible transition of the PPy films between super-
hydrophobicity and superhydrophilicity. The PPy films are endowed with a unique
porous structure, on which the reversible superhydrophobic-to-superhydrophilic
switching is achieved by simply adjusting the electrical potential.
9.5
Superhydrophobic Surfaces for Various Functional
Applications
The research on superhydrophobic surfaces is driven by various functional appli-
cations. We will discuss below not only how superhydrophobic coatings are used
to improve the performance of conventional materials by surface modification, but
also how superhydrophobic modification brings about new functions which are not
available for the materials themselves.
