Electrowetting Properties of Micro/Nanostructured Black Silicon

This paper reports on the electrowetting on dielectric (EWOD) properties of dual rough black silicon (Si) surfaces, produced by pulsed laser structuring of Si wafers and subsequently coating with a thermally grown oxide and a chloroalkylsilane layer. By varying the laser fluence, it was possible to tune the black Si wettability, from hydrophobicity to water repellence, through a systematic and reproducible variation of the surface roughness at micro- and nanoscales. It is shown that a liquid droplet on these surfaces can be readily switched between superhydrophobicity and hydrophilicity by applying moderate external electric fields. This behavior is reversible or irreversible depending on the geometry of the patterned structures and the water repellence characteristics of the different surfaces. The fundamental role of structural and dynamic wettability characteristics on the switching behavior during the EWOD process is investigated and discussed. The results indicate the potential use of dual rough black Si for EWOD applications.