Biomedical Engineering Reference
In-Depth Information
4.2.2.5 Hysteresis
As we have seen in the preceding section, at large values of the potential there is
the saturation limit. At small values of the potential there is the hysteresis limit.
Hysteresis is defined as the deviation of the contact angle from its mean value due
to physical phenomena like microscopic surface defects and roughness. During
dynamic motion of an interface, dynamic hysteresis refers to the difference be-
tween advancing and receding contact angles. Hysteresis is currently observed in
electrowetting.
When we have established the BLY law, the value of the macroscopic contact
angle is in reality the average between an advancing and a receding value. If we start
with a nonactuated droplet and we increase the value of the voltage, the droplet
spreads. The contact angle is then an advancing contact angle. When the voltage
decreases, the droplet regains its initial shape and the observed contact angles are
the receding contact angles. The advancing and receding contact angles usually dif-
fer (Figure 4.11).
Another manifestation of electrowetting hysteresis occurs in EWOD Microsys-
tems during the motion of a droplet on a substrate paved with electrodes. Below a
minimum actuation voltage
V
min
, the droplet does not move. In the following sec-
tion we produce the relation between hysteresis and minimum actuation voltage.
Hysteresis and Minimum Actuation Potential
Let us consider the example of electrowetting on dielectric microsystems (EWOD)
schematized in Figure 4.12. It is observed that a droplet of conductive liquid does
not move from one electrode to the next as soon as an electric actuation is applied.
A minimum voltage threshold is required in order to obtain the motion of the drop
[20]. This minimum electric potential (
V
min
) depends on the nature of the conduc-
tive liquid/surrounding fluid/solid substrate triplet. In this section, we relate the
value of the minimum potential to the hysteresis contact angle. We show that the
force balance on the droplet produces an implicit relation linking the minimum
potential
V
min
to the value of the hysteresis contact angle
a
.
Figure 4.11
Experimental evidence of electrowetting hysteresis. Case of a sessile droplet of deion-
ized water immersed in silicon oil (Brookfield) and contacting a SIOC substrate. The arrows show the
advancing and receding phases.
