Experimental Validation of the Invariance of Electrowetting Contact Angle Saturation

Basic electrowetting theory predicts that a continued increase in applied voltage will allow contact angle modulation to zero degrees. In practice, the effect of contact angle saturation has always been observed to limit the contact angle modulation, often only down to a contact angle of 60 to 70 degrees. The physical origins of contact angle saturation have not yet been explained successfully and unequivocally. At best, scientists have produced multiple disconnected hypotheses (droplet ejection, charge injection, a thermodynamic limit, etc.) that do not satisfactorily hold for the large body of electrowetting experimental results. Herein we experimentally demonstrate that when using DC voltage, electrowetting contact angle saturation is invariant with electric field, contact line profile, interfacial tension, choice of non-polar insulating fluid, and type of polar conductive fluid or ionic content. The selected experiments were performed and designed using conventional electrowetting materials, without bias toward supporting a particular theory. Because the experimental results show such a strong invariance of saturation angle to multiple parameters, electrowetting saturation parallels many of the trends for Taylor cone formation. However, the contact line geometry is distinct from a Taylor cone, suggesting that some other (though related) form of electrohydrodynamic instability might cause saturation. Although this work does not unequivocally prove what causes contact angle saturation, it reveals what factors play a very limited or no role, and how dominant factors causing saturation may change with time of voltage application. This study thereby provides additional direction to the continued pursuit of a universal theory for electrowetting saturation. (C) Koninklijke Brill NV, Leiden, 2011