Polarity-dependent electro-wetting/-dewetting for efficient droplet manipulation

Droplet manipulation on a substrate by electrical signals is instrumental to the automation and miniaturization of labor-intensive assays in life science and chemistry. Current techniques are primarily based on either electrowetting or a more recent ionic-surfactant-mediated electro-dewetting effect. Here, we report that the two effects can occur simultaneously on the same substrate. Using a dope silicon substrate and an aqueous droplet with a cationic surfactant, the surface exhibits dewetting at positive biases and wetting at negative. Such a polarity-dependent wetting–dewetting transition enables a more significant wettability change (>60° contact angle change between ±3 V), which preserves after multiple wetting–dewetting cycles. We also find that the transition does not experience contact angle hysteresis that sole electrowetting commonly suffers from. Benefitting from these features, we experimentally show that droplet manipulation on a digital microfluidic device is more efficient and robust using this joint mechanism.