A scaling model for electrowetting-on-dielectric microfluidic actuators

Abstract

A hydrodynamic scaling model of droplet actuation in an electrowetting-on-dielectric (EWD) actuator is presented that takes into account the effects of contact angle hysteresis, drag from the filler fluid, drag from the solid walls, and change in the actuation force while a droplet traverses a neighboring electrode. Based on this model, the threshold voltage, V T , for droplet actuation is estimated as a function of the filler medium of a scaled device. It is shown that scaling models of droplet splitting and liquid dispensing all show a similar scaling dependence on [t/e r (d/L)] 1/2 , where t is insulator thickness and d/L is the aspect ratio of the device. It is also determined that reliable operation of a EWD actuator is possible as long as the device is operated within the limits of the Lippmann-Young equation. The upper limit on applied voltage, V sat , corresponds to contact-angle saturation. The minimum 3-electrode splitting voltages as a function of aspect ratio d/L \ 1 for an oil medium are less than V sat . However, for an air medium the minimum voltage for 3-electrode droplet splitting exceeds V sat for d/L C 0.4. EWD actuators were fabricated to operate with droplets down to 35pl. Reasonable scaling results were achieved.

Bright e-Paper by transport of ink through a white electrofluidic imaging film

“…1) with equal thickness, h, which are hydrophobic such that R V ¼ h/2 and have a value of h such that R V ooR h , the Laplace pressure for the ink in both the upper and lower channel can be approximated as DP ¼ 2g/h. To achieve movement, electromechanical pressure locally reduces the pressure as predicted 4 by DP ¼ À CV 2 /h, where C is the capacitance of the hydrophobic dielectric and V is the voltage applied to the electrode towards which the ink will move 5,9 . The minimum V for movement can be very low, but practically ranges from 5 to 30 V depending on C and/or the required switching speed.…”

“…Figure 4 shows a 150 PPI device switching a 200-mm electrode line. An added advantage of scaling to smaller dimensions is that it leads to a decrease in switching time 9 . Accordingly, both 25 and 150 PPI device switching profiles are plotted in Fig.…”

Electrical actuation-induced droplet transport on smooth and superhydrophobic surfaces

“…This section uses the electromechanical framework developed by Kumari et al [12] to highlight the physics underlying each of the three actuation regimes; similar considerations have been employed by Jones [44] and Chatterjee et al [45] to study frequency-dependent electrical actuation. The framework by Kumari et al [12] is used to develop a scaling analysis-based model to compare the actuation force in each of the three regimes.…”

“…The presence of a surrounding filler fluid leads to an additional viscous drag on the droplet (typically EW actuation is carried out in air or in an oil environment). This viscous drag has also been modeled semi-empirically [26,44,52]. While the presence of an oil carrier medium increases the viscous drag, the contact line friction and the threshold voltage for EW-actuation is reduced in the presence of an oil environment.…”

“…Owing to the complexities of modeling the different phenomena influencing droplet motion, simplified analyses have been conveniently employed to model droplet motion [23,26,27,44,50,54]. As an illustration, Bahadur and Garimella [26] modeled EW-induced transient droplet motion by representing the opposing forces using simplified algebraic expressions and incorporated them in the overall force balance model for a droplet which led to: (13) where m is the droplet mass, F act is the electrical actuation force, F w is the force resulting from wall stress, F f is the filler fluid drag, and F cl is the contact line friction.…”