Surfactant‐mediated electro‐dewetting of droplets in oil for liquid‐shape manipulation

Abstract The capability to manipulate liquid shape at the microscale has enabled numerous microfluidic devices. Due to its simple electric actuation, electrowetting‐on‐dielectric has been widely used in a variety of microfluidic applications that require reversible liquid‐shape modulation. However, its use of dielectric and hydrophobic layers raised operation voltage, caused reliability issues, and increased fabrication cost. As an alternative mechanism, ionic‐surfactant‐mediated electro‐dewetting has recently been demonstrated to enable digital microfluidics in air with a much lower voltage, higher reliability, and simpler chip fabrication. However, electro‐dewetting for liquid‐shape manipulation has remained poorly explored due to its limited contact‐angle changes. Here, we investigated electro‐dewetting in oil by testing various droplet liquids and hydrophilic substrate materials. To guide device development, cationic surfactants with varying hydrocarbon chain lengths and concentrations are tested. A contact‐angle change of 100° is obtained for electro‐dewetting of a dimethyl sulfoxide droplet in hexadecane with mere 4 V. To evaluate the utility of electro‐dewetting in oil, proof‐of‐concept devices are assembled to explore the potential in optical applications such as reflective displays and liquid lenses. Compatible with various liquids and substrates, electro‐dewetting with the liquid‐in‐oil configuration opens a door for simpler and more reliable microfluidic devices.