Frequency and Polarity Effects of Droplet-based LOC Driven by Electrowetting
from: Lab-on-a-Chip Technology (Vol. 1): Fabrication and Microfluidics (Edited by: Keith E. Herold and Avraham Rasooly). Caister Academic Press, U.K. (2009)
Electrowetting has been extensively studied and applied as a pumping mechanism in droplet-based Lab-on-a-Chip (LOC). The electrically controllable liquid-solid contact angle is proportional to the square of the applied voltage and described by the Lippmann-Young equation. Thus, the frequency and polarity effects are easily ignored. In this chapter, the basic electrowetting phenomenon is introduced. Two basic types of electrowetting-based LOC, including sandwiched and open devices, are presented. Moreover, the frequency effect is applied to realize a particle concentrator in a sandwiched device, while the polarity effect is employed to manipulate droplets by a square wave signal on an open device. In the frequency effect study, the applied low frequency signals are consumed mostly in the dielectric layer and cause electrowetting to pump liquid droplets on the millimeter scale. High frequency signals establish electric fields in the liquid and generate dielectrophoresis forces to actuate cells or particles on the micrometer scale inside the droplet. In the polarity effect study, electrowetting on an SU-8 and Teflon coated electrode is investigated. It is found that the Θ-V (contact angle-applied voltage) curve is asymmetric along the V = 0 axis by sessile drop and coplanar electrode experiments. A systematic deviation of measured contact angles from the theoretical ones is observed when the electrode beneath the droplet is negatively biased. In the sessile drop experiment, up to a 10 degrees increment of contact angle is measured on a negatively biased electrode. Asymmetric electrowetting is then applied to oscillate and transport droplets on square-wave-powered symmetric (square) and asymmetric (polygon) coplanar electrodes to demonstrate the manipulation capability of an open device read more ...