Biomedical Engineering Reference
In-Depth Information
Figure 4.25
(a) photograph of a “square” droplet in a covered EWOD microchip; (b)the droplet
simulated with Evolver software.
Consequently, the energy of the liquid-air interface is much smaller than that of the
solid-liquid interface and the droplet adopts the shape of the underlying electrode.
This is why a droplet can adopt a nearly square shape.
4.2.3.2 Basic Fluidic Operations
In order to build a biochip, basic fluidic functions must be achievable by EWOD.
In the following, operations such as droplet motion, merging (addition), division,
dispense, and mixing are presented.
Droplet Motion
The principle of droplet motion due to a difference—or a gradient—of wettability
is well known [19, 27, 28]. When a droplet is located on the boundary between a
lyophobic region and a lyophilic region, it moves to the lyophilic region, at least
if hysteresis of contact angle is not too large. The situation is similar in the case of
electrowetting. If a conductive droplet is located on the boundary between an actu-
ated and a nonactuated electrode, an electrowetting force is applied on the contact
line located above the actuated electrode, and a capillary force is exerted on the
contact line located on the lyophobic surface. The resultant of the forces parallel to
the surface is directed towards the electrically actuated region. The droplet is out
of equilibrium, and if the resulting force is sufficient to overcome hysteresis, the
droplet moves (Figure 4.26).
Figure 4.26
(a) View of a droplet of ionic liquid moving to the right; (b) numerical simulation with
Surface Evolver [29]: the droplet moves until it finds an equilibrium state.
