Biomedical Engineering Reference
In-Depth Information
Figure 4.1
Schematic of a digital microfluidic chip (courtesy of CEA-LETI).
a force on the interface, and if the interface is deformable—like that of a conductive
liquid and a nonconductive fluid or gas—this force can distort the interface. This
especially occurs with electric forces exerted on a liquid-gas interface at the vicinity
of the contact line with a solid, resulting in a change of the contact angle (Figure
4.2).
This property was first observed by Gabriel Lippmann in 1857, but the real
start of electrowetting techniques is recent with the developments of microsystems
and Berge's equation frequently referred as the Lippmann-Young law [3]. We shall
denote in this topic the Berge-Lippmann-Young equation the BLY equation. This
equation describes the change of contact angle with the applied voltage
C
2
cos
θ
=
cos
θ
+
V
(4.1)
0
2
LG
γ
where
q
is the real contact angle,
q
0
the Young contact angle (the one observed
without any electric actuation)
g
LG
the surface tension,
C
the specific capacitance,
and
V
the voltage. In fact, the BLY equation recovers only a part of the physics of
electrowetting. However, it is a clever and convenient engineering approach to con-
vert the effect of the electric forces into an observable change of contact angle.
4.2.2.2 The Different Theories for Electrowetting
The underlying physics behind the electrowetting change of contact angle has been
the object of many investigations, and different approaches have been pursued,
