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 um thick polyimide (ϵ_r = 3.3) as the dielectric layer and thin Teflon AF as the hydrophobic layer (θ₀ = 116° and α = 4–6°) 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.

Keywords:

• Digital microfluidics
• electrowetting
• droplet transport
• threshold voltage
• contact angle hysteresis
• adhesive friction

Acknowledgements

This work was supported by Seoul R&BD Program (No. 10920) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. K20902001448-10E0100-03010 and NRF-2010-355-D00024).