Electrowetting of Ionic Liquids: Contact Angle Saturation and Irreversibility

In this work, electrowetting of several room temperature ionic liquids having in common the cation or the anion, [EMIM][EtSO4], [EMPy][EtSO4], [EMIM][NTf2], [BMIM][BF4], [BMIM][TFA], and [OMIM][BF4], on poly(tetrafluoroethylene) (PTFE) was investigated. The typical behavior of aqueous salt solutions was found: symmetric parabolic curves of contact angle versus (positive and negative) applied voltage, at low voltages, and contact angle saturation after a threshold voltage. Furthermore, the contact angle did not recover its initial value when the voltage decreased, and this irreversibility was found even at low voltages. The dependence of the contact angle with the applied voltage is generally described by the Young-Lippmann equation before contact angle saturation. In contrast, the Langevin function as well as a modified form of the Young-Lippmann equation were found to fit the experimental electrowetting curves in the whole range of voltages. A correlation between the electrowetting behavior and the surface tension of the liquids was reported. Some physical effects that have been pointed out as possible causes for the contact angle saturation were investigated. The most plausible explanation for contact angle saturation in our systems seems to be the charge trapped across the solid/liquid interface.