Biomedical Engineering Reference
In-Depth Information
Fig. 9.12
Controllable pinning and transport of a nearly spherical water droplet between two
superhydrophobic MTA membranes. Reprinted with permission from ref. [
50
]. Copyright (2011)
Wiley-VCH
superhydrophobic-like surface has a large adhesive force which can hold water
droplets even when the substrate is turned upside down. This kind of surface
can be used as a “mechanical had” to transfer a water droplet from a normal
superhydrophobic surface to a hydrophilic one. By using the electric field to control
the adhesion on the superhydrophobic surfaces, transporting a water droplet can be
achieved between the same kinds of substrates. Zhao et al. [
50
] have demonstrated
a continuously transportation a small water droplet from one MTA membrane to
another by controlling electrode polarity and magnitude of the applied voltage. As
shown in Fig.
9.12
, a water droplet was first placed on an MTA membrane (M1)
with no voltage applied. Next, another MTA membrane (M2), adopted as a cathode,
was slowly moved toward M1 to make contact and adhere the water droplet by
application of 5 V voltage, as shown in the left side of Fig.
9.7
(b). Then, M2 was
moved away from M1 and the water droplet was completely transferred from M1
to M2. Following the same procedure, this water droplet can be transferred back to
M1 by switching the electrode polarity and applying a 15 V voltage. The final CA of
the water droplet was still higher than 150
ı
. This water transport process has been
repeated more than 30 times in one day using the same MTA membranes, indicating
good durability of the MTA membrane.
9.5.6
Battery and Fuel Cell Application
Superhydrophobic materials have shed light on battery system to solve the problems
in their duration and efficiency. Based on superhydrophobic nanostructured materi-
als, Lifton et al. have developed a novel battery architecture to extend the shelf
life of the battery [
203
]. In principle, they modified both electrodes of a battery
with superhydrophobic materials to separate the liquid electrolyte from the active
electrode materials in order to prevent the reaction happen. To initial the battery
reaction, the electrowetting principle will lead to change the wetting feature of the
liquid electrolyte on the superhydrophobic treated electrodes, and therefore, liquid
electrolyte will penetrate into the electrode. Consequently, the battery is activated
to produce power. The electrowetting phenomenon can be defined as the change
of the solid-electrolyte contact angle due to the applied potential difference between
the solid and the electrolyte. It can be understood in terms of the tendency of
the solid-electrolyte thermodynamic system to minimize energy by changing the
