Moving liquids with light: Photoelectrowetting on semiconductors

By linking semiconductor physics and wetting phenomena a brand new effect termed “photoelectrowetting-on-semiconductors” is demonstrated here for a conducting droplet resting on an insulator-semiconductor stack. Optical generation of carriers in the space-charge region of the underlying semiconductor alters the capacitance of the liquid-insulator-semiconductor stack; the result of this is a modification of the wetting contact angle of the droplet upon illumination using above band gap light. The effect is demonstrated using commercial silicon wafers, both n- and p-type having a doping range spanning four orders of magnitude (6×1014−8×1018 cm−3), coated with a commercial amorphous fluoropolymer insulating film (Teflon®). Impedance measurements confirm that the observations are semiconductor space-charge related effects. The impact of the work could lead to new silicon-based technologies in areas such as Laboratory-on-a-Chip, Microfluidics and Optofluidics.