Comparison of static contact angle change and relaxation in EWOD devices

In conventional electrowetting on dielectric (EWOD) devices a static change of the contact angle is observed when a voltage is applied to the electrodes, usually correlated to the Lippmann equation. In this paper we analyze the contact angle changes by using two different dielectrics of different materials and thicknesses and also comparing two different ways of creating the field singularity at the contact line. Although it is known that the contact design may induce a tangential electric field and a spontaneous front wetting, we have observed in our devices that the change in the contact line is still due to the interfacial changing at the air-liquid interface. Several devices were fabricated using Teflon and PDMS coating on top of pyrex wafer and silicon wafer previously oxidized. The results obtained using these devices were compared to a conventional EWOD sample having PDMS deposited on top of metal electrodes previously laid out on top of pyrex wafer. We have compared the static contact angle change, produced using conventional voltage drive source, with a similar measurement after charges were generated in the gas phase. We have observed quite different behaviour of the two films used as coatings, for similar charge densities created, the contact angle on Teflon samples changed much less than in the PDMS samples. Moreover, we have observed that the relaxation time in the Teflon sample was short, in range of seconds, whereas in the PDMS samples the contact angle change remained for minutes and even, opposite sign charge was to be added to the gas phase in order to totally recover the initial value. The paper also describes how the charge created in the gas phase was estimated. An experimental set-up consisting in a micro-amperimeter short circuiting the device, was used and the current transient registered. This arrangement allowed us to quantify the time constant underlying the discharge transient and also to quantify the total amount of charge density created. This is compared in this work with expected Lippmann-Young correlation. We found an experimental result which follows the Lippmann equation but also exhibits a charge shift, not totally understood as for now.