Liquid Dielectrophoresis on Electret: A Novel Approach Towards CMOS-Driven Digital Microfludics

Abstract

Electrically-driven droplet-based digital microfluidics analog to integrated chips has undergone tremendous growth; however, its applications are restricted due to high-voltage requirement and lack of versatile droplet manipulation methods. We propose liquid dielectrophoresis on electret (L-DEPOE) as a CMOS-driven general droplet manipulation method for various conductive or dielectric droplets. By mimicking electric relays to operate the electret as a virtual voltage source with a low-voltage signal, L-DEPOE dramatically reduces the driving voltage to only 5 VDC for transporting oil droplets. The driving force in liquid dielectrophoresis (L-DEP) is compared with dielectrophoresis (DEP) and electrowetting on dielectric (EWOD) in a unified approach by introducing the concept of electropolarization in non-uniform electric fields. In this scenario, molecular electropolarization forces act in dielectric liquid bulk for L-DEP, in the electrical double layer (EDL) near the three-phase contact line for EWOD. The vanishing of electropolarization forces may contribute to the contact angle (CA) saturation in EWOD. The apparent CA reduction in EWOD is formulated within the classical electromechanical framework as a consequence of electropolarization forces in the EDL. To electrically manipulate various liquids, we transported both conductive water and dielectric oil droplets in the same L-DEPOE configuration using external voltages, and we propose a modified configuration for tuning liquid shapes. Improved results on electret charge stability are presented, and approaches to enhance electret performances and droplet motion velocity are analyzed based on qualitative analysis and quantitative simulation.