Recent progress of electrowetting for droplet manipulation: from wetting to superwetting systems
Many engineering and industrial technologies as well as biological processes are closely related to surface wettability. The control of surface wettability attracts much attention due to the prospective applications based on liquid and droplet manipulation. However, the dynamic modification of wetting is still a challenging issue both in theoretical models and practical applications. Electrowetting (EW) is one of the most widely used methods for controlling the wettability of liquids due to its fast responding speed, large switching range and excellent durability. Since Gabriel Lippmann first discovered the electrocapillary (ECP) phenomenon in 1875, which is a precursor of electrowetting, there has been considerable progress in this field, especially accompanied with the development of wettability and interface science. In this paper, we review the developments of EW for droplet manipulation in view of wetting to superwetting systems over the past decades, including theoretical basis, electrodes and fluid systems, droplet manipulation and special phenomena induced by EW. Finally, we discuss several difficult challenges in this field and provide future research directions from our perspective.