Limiting phenomena for the spreading of water on polymer films by electrowetting

This paper is about fundamental limitations in electrowetting, used as a to ol for spreading water solutions on hydrophobic surfaces, like the surface of a polymer film. Up to which point can an electric voltage decrease the c ontact angle? The first limitation comes when using pure water, above a thr eshold voltage, little droplets are emitted at the perimeter of the mother drop. We present an analysis of the drop contour line stability, involving competition between electrostatic and capillary forces, which is compatible with observations. The use of salted water solutions suppresses this insta bility, then one faces a second limitation: the evolution of the contact an gle saturates before complete wetting. We show that this saturation is caus ed by ionisation of the air in the vicinity of the drop edge. We analyse th e luminescence induced by gas ionization and measure the related electrical discharges. We explain how air ionization suppresses the driving force for electrowetting and how it induces the formation of anhydrophillic ring aro und the drop.