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 immersed in the water. The polymer film acts as insulator: no steady electrical 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 degrees. The shape of the electrowetting curve (contact-angle cosine versus voltage plot) strongly depends on the type of the polymer. A saturation at higher voltages, connected with a hysteresis, is interpreted in terms of charge carrier injection into the polymer surface. Other deviations of the experimental results from the Lippmann-Young parabola concern a broadening with the polyolefins, a strong shift, and apparent reduction to one branch with two fluoropolymers, and for PET and a heavily oxidized PE sample the deviations concern a nearly zero dependence of the voltage.