Biomedical Engineering Reference
In-Depth Information
Using the capillary equivalence, the electrowetting force exerted on the droplet
by an electrode line of width
e
is
C
2
f
=
e V
=
e
γ θ
(cos
-
cos
θ
)
EWOD
0
2
Division of Droplets
A very important operation requested for the digital manipulation of droplets in
EWOD microsystems is the splitting or cutting of droplets (i.e., the division of a
droplet into two “daughter droplets”). This operation is needed because the merg-
ing of droplets increases at two times the volume of liquid that the system must
treat. Division in two half-volumes restores the possibilities of the system.
The principle of droplet cutting/splitting/division is shown in Figure 4.27. The
liquid is at the same time stretched by two actuated electrodes at both ends of
the droplet, creating two lyophilic (hydrophilic) areas with electro-capillary forces
pulling in opposite directions. The nonactuated lyophobic (hydrophobic) electrode
exerts a pinching force on the triple contact line. Depending on the force balance
and the elasticity of the interface, the droplet can be cut in two.
Splitting a drop into two same daughter drops require an energy input, which
is a function of the increase in free surface. It has been checked by experiments
that the division of a sessile water droplet is impossible by electrowetting actuation,
whereas this operation is feasible in covered EWOD microsystems. Numerical re-
sults confirm the experimental impossibility of splitting sessile water droplets with
EWOD. The electrowetting forces cannot elongate the droplet sufficiently. Figure
4.28 shows that a droplet in the position of Figure 4.27 cannot be sufficiently elon-
gated and escapes randomly to the left or right on a hydrophilic patch.
Furthermore, numerical simulations show that, in an open EWOD configura-
tion, a drop may be split if it has been previously elongated by any other means
(Figure 4.29).
From all the liquid that we have tested, it appears that only ionic liquids may
be split in an open system EWOD, because they have very small contact angles with
the actuated electrodes and their elasticity is larger than aqueous liquids.
Drop division potentially depends on two dimensionless parameters: the initial
elongation ratio
A
between drop length and drop width, and the “cutting” ratio
CR
between pinching length and initial drop length. The domain of possible splitting
Figure 4.27
Drop division. Scheme of the forces on the contact line; the principle is to apply a
stretching force in one direction combined with a pinching force in the other direction.
