Electrowetting on polyimide and silicon substrates with high hysteresis

Electrowetting-on-dielectric (EWOD) with atypical liquid-substrate combinations has growing utility for a range of microfluidic devices. However, there are challenges to understanding and utilizing the EWOD behavior on rougher substrates with potentially heterogeneous wetting conditions that cause high contact angle (CA) hysteresis. Accurate and consistent EWOD responses can be obtained for these substrates by measuring advancing and receding CAs at constant applied potential. This method was used to characterize the EWOD (DC) responses of two ionic liquids and an aqueous solution on several conductive polyimide and silicon substrates coated with Parylene C/Teflon AF. The results showed Lippmann–Young behavior with liquid-dependent zero-voltage CAs and 13°–40° control authority, 11° average hysteresis, and CA saturation at approximately 80° for all liquids and substrates. Surface roughness and substrate material had little effect on EWOD response and CA hysteresis. The hysteresis was surprisingly high for a Teflon AF-coated substrate. O₂ plasma etching of the Parylene prior to Teflon coating may have contributed to the high hysteresis conditions by forming Teflon AF islands or embedding charges into the substrate that affected wetting through the thin Teflon AF layer. CA “ratcheting” was observed during advance corresponding to stick–slip of the droplet triple-line. The stick–slip behavior occurred with all liquid-substrate combinations at sufficiently high voltages and on previously polarized surfaces at lower voltages. Experiments with smooth (“ideal”) Si substrates showed ratcheting only at saturation-level voltages suggesting a link between ratcheting and saturation.