Biomedical Engineering Reference
In-Depth Information
Figure 4.12
Sketch of a droplet starting to move towards the actuated electrode.
Experiments have been conducted using different substrates (Teflon and SiOC)
and different conductive liquids and surrounding gas/fluids (deionized water in oil
or air, biological buffers with surfactants in oil or air, ionic liquids in air). It will
be shown that the results of the models are in agreement with the experimental
results.
Our starting point is the BLY law
C
V
2
cos
θ
-
cos
θ
=
(4.27)
0
2
γ
At the onset of the motion, there is no dynamic effect; hence, we will interpret
the BLY law as a pseudocapillary effect. A droplet starts to move under an “appar-
ent wettability gradient” between an actuated and a nonactuated electrode. We are
remind here that the electrowetting line force density on a triple line is given by
C
2
f
=
V
=
γ θ
(cos
-
cos
θ
)
(4.28)
EWOD
0
2
This line force acts on that part of the triple line located above the actuated
electrode (Figure 4.12). On the part of the triple line located on the initial nonactu-
ated electrode, the forces are just the capillary line forces. Usually the substrate is
hydrophobic so the forces are exerted in the same direction as the electrowetting
forces.
We recall that the capillary line force acts on the triple line in the plane of the
substrate, perpendicularly to the triple line. In the case of a hydrophilic contact,
cos
q
> 0 and the line force points away from the surface, whereas in the case of
hydrophobic contact, the line force is negative and points inside the droplet. The
EWOD line force acts on the triple line and has a component located in the plane of
the substrate perpendicular to the triple line, and also a vertical component.
If the surface is perfect, using the pseudocapillary equivalence for the electrowet-
ting force, the drop will immediately move as soon as the neighboring electrode
is actuated. However, experiments have shown that there is an electric potential
threshold below which the droplet does not move (i.e., there is a pseudogradient of
wettability below which the droplet does not move).
Figure 4.13 shows experimental results for a microdrop of deionized water im-
mersed in silicon oil and placed on a SiOC substrate. The contact angle is not the
same when the droplet is spreading on the substrate (advancing) or receding from
the substrate. After having performed different plots using different substrates and
liquids, it is concluded that the plot of Figure 4.13 is typical. The vertical shift be-
tween the two curves defines the electrowetting hysteresis angle. Hysteresis angles
are usually of the order of a 2 to 15°.
