Electro-capillary Induced Droplet Wetting on Structured Surfaces*

Electrowetting on microstructured surfaces enhances the mobility of the three-phase contact line to a greater extent due to the higher initial contact angle encountered on such surfaces. We have observed that a greater maneuverability is possible only through directional electrowetting and the initial wetting state of the droplet has an important role to play. We have obtained a transition in the initial wetting state by varying the feature size of the surface microstructures. The initial Cassie state of wetting has a limited directionality but a greater mobility of the contact line due to the higher initial contact angle. However, even though the initial Wenzel state of wetting has a lower mobility of the contact line, it has a higher directional wetting property. This is due to the additional electro-capillary effect due to the presence of micro-ridge like structures, which manifests themselves as open micro-capillaries beneath the droplet. This observation can be useful in designing droplet-based microfluidic systems for better control over the directionality of the droplet movement.