Competitive electrowetting of polymer surfaces by water and decane

The present study deals with electrocapillarity effects in the three-phase system water-decane-polymer. This system consists of a decane droplet which adheres under water on a polymer surface. We applied an electric potential between an electrode beneath the polymer film and a platinum electrode imm ersed in the water. The polymer film acts as insulator: no steady electrica l current flows between the electrodes. When the voltage is increased, the contact angle of decane and water changes; in some cases by up to 100 degre es. The shape of the electrowetting curve (contact-angle cosine versus volt age plot) strongly depends on the type of the polymer. A saturation at high er voltages, connected with a hysteresis, is interpreted in terms of charge carrier injection into the polymer surface. Other deviations of the experi mental results from the Lippmann-Young parabola concern a broadening with t he polyolefins, a strong shift, and apparent reduction to one branch with t wo fluoropolymers, and for PET and a heavily oxidized PE sample the deviati ons concern a nearly zero dependence of the voltage.