Effect of Contact Angle Hysteresis on Electrowetting Threshold for Droplet Transport

Abstract

This paper describes the effect of the contact angle hysteresis caused by the adhesion between the liquid droplet and the solid surface on the threshold voltage for droplet transport in three different ground-type electrowetting configurations. It was found that the threshold voltage equals the hysteresis voltages in both single-plate and twin-plate configurations and the square root of 2 times the hysteresis voltage in the two-plate configuration by analyzing the contact angles at electrowetting threshold theoretically. The fabricated devices using 2.5 mu m thick polyimide (epsilon(r) = 3.3) as the dielectric layer and thin Teflon AF as the hydrophobic layer (theta(0) = 116 degrees and alpha = 4-6 degrees) showed the threshold voltages for droplet transport initiation of 28, 40 and 28 V in the single-plate, two-plate and twin-plate devices, respectively, and the threshold voltages for stable droplet transport of 35, 50 and 35 V, respectively. All the results are in very good agreement with theoretical values. (C) Koninklijke Brill NV, Leiden, 2011