Asymmetrical Electrowetting on Dielectrics Induced by Charge Transfer through an Oil/Water Interface

Electrowetting on dielectrics is a fascinating as well as a precise way in microfluid manipulation. As one of the controversial conclusions, charge trapping on the dielectric surface might be one of the causes which induces water contact angle saturation and forms one of the significant issues that bear on the applications of electrowetting on dielectrics. Recently, it was demonstrated that the contact angle saturation can be significantly reduced by employing an oil lubrication layer on the hydrophobic surface. In this work, we have investigated the influence of effects of an oil layer on the electrowetting behavior by dissolving a nonpolar oil-soluble dye in the oil phase. We monitored the contact angle of water drops with varying pH on an oil-lubricated hydrophobic insulator. Interestingly, we found asymmetry in the electrowetting curve. Several analysis methods were proceeded trying to explain this asymmetric electrowetting phenomenon. First and foremost, the electrochemical properties of dye were investigated by cyclic voltammetry which demonstrates that oxidation-reduction reactions of the dye can indeed happen on the electrode and one irreversible peak was found which indicated that the dye molecule might decompose at a higher voltage. Second, thin-layer cyclic voltammetry confirmed ions can transgress the oil/water interface. Also, the conductivity of the oil phase increases with the dissolved dye concentration, which indicates that charges can be transported in the oil phase. Finally, to further understand the transfer mechanism, the transient current of dye-doped oil was measured, which indicates that the formation of inverse micelles in the oil phase at high voltage could be one of the charge carriers. We demonstrated the oil-property-dependent asymmetry phenomenon of electrowetting and its association with charge transfer through the oil/water interface for the first time.