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The principle of EWOD has made possible the droplet formation, droplet
separation, and droplet coalesce. The integration of these manipulations has
materialized the possibility of digital microfluidics with relevance to biomedical
applications such as DNA microstamping.
On the nanoscale, the electrowetting force is sufficient to drive the fluid into or
through the carbon nanotube. The electrostatic potential across single-walled
carbon nanotubes [11] overcomes the capillary resistance. This breakthrough
translates electrowetting into the nanoscale technology such as nanolithography
and zepto (10 21 ) liters of liquid handling.
16.3. NANOSCALE COMPONENTS
As the surface tension becomes dominant over the force of gravity in the micro-
and nanoscale domain, design issues such as static friction between two separated
plates in vacuum requires the understanding of quantum effects. Chan et al.
experimentally demonstrated the significance of such a phenomenon at the
submicron scales [12]. Quantum electrodynamics predicts that even in a vacuum
at a temperature of absolute zero, particles exist in the context of Heisenberg
uncertainty principle.
When two parallel, uncharged conducting plates are positioned close together,
the plates will start to move towards each other by a force. The magnitude of this
force is a function of the area A of the plates, and inverse to the fourth power of
the distance of plate separation:
F c
A ¼
p 2
c
240
_
1
z 4 ;
ð 16
:
4 Þ
is called the reduced Plank constant which is
equal to Planck constant h divided by 2 p ,andz is the separation between the
plates. This phenomenon is called the Casimir effect, first predicted in 1948 [13].
When the separation between the surfaces decreases,
where c is the speed of light,
_
the Casimir pressure
increases rapidly, reaching about 1 atmosphere at z
10 nm.The quantum-
mechanical effect of a vacuum fluctuation of the electromagnetic field or the
Casimir effect is not limited to parallel plates, but it occurs in any two conducting
materials in close juxtaposition.
Chan et al. designed an elegantly simple experiment [12]. Using MEMS
fabrication technique, the authors etched a 3.5 m m thick, 500 m m 2 -doped poly-
silicon square plate, anchored in the middle of two opposite sides by small rods so
that the plate was free to rotate. There were two electrodes underneath the plate,
which operate as sensors. A separation of 2 m m existed between the plate and the
electrodes. Considering the experimental difficulty of testing the Casimir effect
with two plates parallel in extremely close proximity, the group at Georgia
Institute of Technology measured the Casimir effect between their micromachined
plate and a gold-coated Styrofoam ball with a radius of 100 m m [14]. This ball was
B
 
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