Digital, Two-Phase, and Droplet Microfluidics - Microfluidics for Biotechnology - page 148

Figure 4.18 Interval V min - V max as a function of the capacitance C .

advantage in trying to increase the specific conductance of the dielectric layer: the
actuation requires lower potentials as well for V min as for V max .
Equation (4.40) can be equivalently written

(4.41) f = 2 γα θ sin < f < γ γ θ - cos = f

min 0 EWOD C 0 max

showing the range of the electrowetting force. The optimal solution would be the
lowest possible f min and a largest possible f max . First, let us analyze the conditions
for obtaining the smallest possible f min . For aqueous liquids, a very hydrophobic
contact angle q 0 is optimal for having a very low minimum force for droplet mo-
tion, because sin q 0 ~ ( p - q 0 ) ~ 0. On the other hand, the maximum electrowet-
ting force is f max = g SL 0 . The hydrophobic substrate that maximizes g SL is the most
suitable.

4.2.2.7 Substrate Capacitance
Usually the substrate comprises of an electrode, a dielectric layer, and a hydropho-
bic layer (Figure 4.19). The dielectric layer guaranties the electric insulation in such

Figure 4.19 Total capacitance includes the contribution of the dielectric layer (Parylene, SiO 2 ,
or Si 3 N 4 ) and the hydrophobic layer (Teflon, SIOC, and so fourth); the contribution of the electric
double layer can be neglected.