Analysis and Measurement of Forces in an Electrowetting-Driven Oscillator
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Analysis and Measurement of Forces in an Electrowetting-Driven Oscillator Nathan Brad Crane1, Alex A Volinsky1, Vivek Ramadoss1, Michael Nellis1, Pradeep Mishra1, and Xiaolu Pang1,2 1 Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Ave ENB 118, Tampa, FL, 33620 2 Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing, 100083, China, People's Republic of
ABSTRACT Electrowetting is a promising method for manipulating small volumes of liquid on a solid surface. This complex phenomenon couples electrical and fluid properties and offers many potential surprises. The complex electrical and capillary interactions in electrowetting are illustrated by an analysis of an electrowetting configuration that produces an oscillating droplet motion from a steady DC voltage input. The paper presents an analysis of the electrowetting forces to explain the oscillation and presents a new method for measuring electrowetting forces using a Hysitron Triboindenter. Initial results are compared with predictions from numerical models and simplified analytical solutions. INTRODUCTION Electrowetting is the change of apparent surface energy in an applied electric field [1]. By varying the electric field applied to liquid drops, they can be moved, split, merged, and mixed to enable lab on a chip microfluidics [2]. Many applications of electrowetting have been proposed in recent years, and many different configurations have been studied. Cooney et.al. have considered the relative advantages of three different electrowetting configurations for control of droplet motion: two plate designs and variations of single plate designs in which all electrical connections are made from the bottom [3]. The electrowetting response is determined by interactions between the electrical and surface energies of the systems that can combine to produce a large variety of behaviors. A system in which oscillatory droplet motion is produced from a steady DC input along with an explanation for this behavior are presented. This phenomenon could be useful in improving mixing in digital microfluidic systems. Oscillation observations This work considers the case in which both electrodes are located in the substrate and are isolated from the droplet by a dielectric layer. Test structures were fabricated by patterning aluminum electrodes on an oxidized silicon substrate and then spin coating an 800 nm layer of diluted Cytop 809M to act as both dielectric and hydrophobic layer. Uncovered 5 µl droplets of 1 M NaCl water solution were positioned asymmetrically over the aluminum pads. When a DC potential is applied, liquid drops frequently exhibit an oscillatory response at around 35 V. The
motion frequency varied from 2 to 7 Hz with an oscillation amplitude of approximately 100-150 µm. The oscillation was seen less often with thicker Cytop coatings. The center of the oscillation is offset from the electrode boundary. This oscillation is caused by variations in the electrowetting force
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