Biomedical Engineering Reference
In-Depth Information
a way that there is no electric current (and hydrolysis) in the liquid. Usual materi-
als for electrowetting dielectrics are parylene, SiO
2
, Teflon, and Si
3
N
4
. In order to
have a large apparent change of contact angle between actuated and nonactuated
modes, it is best to use a hydrophobic coating. This hydrophobic layer contributes
to increase the amplitude of the change of contact angle between the nonactuated
and actuated state. Hydrophibic layers are often made of Teflon and spread by spin
coating; sometimes they are made of SiOC spread by plasma deposition.
We have seen in the thermodynamical approach that the conductance of a series
of layers is
ε
0
C
=
d
å
i
i
(4.42)
ε
i
=
1,
n
Using the values of usual electrowetting substrates (Table 4.1), it is seen that
the contribution of the electric double layer to the total capacitance is usually neg-
ligible (less than 3/1000). Hence the double layer can be taken out of (4.42) and
we obtain
1
1
1
=
+
(4.43)
C C
C
dielectric
hydrophobic layer
-
An order of magnitude of the capacitance of current EWOD microsystems is
C
~ 2.2 10
-5
F/m
2
. According to (4.42) the capacitance is increased by reducing the
thicknesses of the layers of the substrate. However, there is a limit to this reduction
given by the dielectric breakdown limit, which will be presented in the following
section.
4.2.2.8 Dielectric Breakdown
Breakdown of the dielectric occurs when the electric field in the dielectric exceeds a
limit value called the critical electric field, denoted here
E
BD
. Above this value, the
material is disrupted. This threshold is also called the theoretical dielectric strength
of a material. It is an intrinsic property of the bulk material. At breakdown, the elec-
tric field frees bound electrons. If the applied electric field is sufficiently high, free
electrons may become accelerated to velocities that can liberate additional electrons
during collisions with neutral atoms or molecules in a process called avalanche
breakdown. Breakdown occurs quite abruptly (typically in nanoseconds), result-
ing in the formation of an electrically conductive path and a disruptive discharge
through the material. For solid materials, a breakdown event severely degrades, or
Table 4.1
Values of Thickness and Relative Permittivity for the Different Usual Materials
Material
Thickness
Relative Permittivity
SiOC
1.2
m
m
3.36
Teflon
400 nm
1.9
Si
3
N
4
400 nm
7.8
Teflon
1000 nm
2.2
Water (electric double layer)
< 30 nm
80
