Biomedical Engineering Reference
In-Depth Information
value—which results in pressurizing the drop—to a hydrophilic (< 90°) or neutral
(90-95°) value in order to reduce the internal pressure. This decrease in pressure
facilitates the pinching effect on the cutting electrode.
Figure 4.34 shows the time evolution of the drop internal pressure during the
different phases of the extraction process. If the decrease in pressure due to back
pumping is sufficient, the pinching effect on the cutting electrode becomes efficient
and separation occurs.
4.2.3.3 Electrode Shape and System Design
Crenellated Electrodes
In reality, the motion of a droplet from an electrode to the next is not straightfor-
ward. Microfabrication imposes a gap separating the electrodes. This gap is usually
of the order of 10 to 30
m
m, depending on the precision of the lithography process,
compared to an electrode size of the order of 800
m
m. This gap creates a permanent
hydrophobic region between two neighboring electrodes. If the droplet has a volume
such that it is limited by the boundaries of the electrode, it cannot move to the next
electrode when the latter is actuated. This is frequently the case in covered EWOD
microsystems where droplet volumes are carefully controlled, and the size of the
electrodes determines the volume of liquid in each droplet within a margin of a few
percents. In order to remedy to such a caveat, jagged or crenellated electrodes have
been designed as shown in Figure 4.35. The idea behind such a design is that the
droplet contact line with the electrode plane extents over onto the dents of the next
electrode. As soon as the next electrode is actuated, electro-capillary forces act to
produce the motion of the droplet. Such jagged electrodes require more complicated
Figure 4.34
Pressure evolution during drop dispense. Each time an electrode in the electrode row is
actuated, internal pressure decreases and the drop spreads on the new electrode. When the reservoir
electrode is actuated for back pumping with a contact angle of 80°, the pressure decreases to a level
where the pinching effect becomes effective and drop is dispensed (
g
LG
= 40 mN/m).
