Moving liquids with light: Photoelectrowetting on semiconductors

TL;DR

This paper introduces photoelectrowetting, a novel method to manipulate liquids on semiconductor surfaces using light, by altering the surface wetting properties through optically generated carriers affecting the insulator-semiconductor interface.

Contribution

It demonstrates the first experimental realization of photoelectrowetting on silicon-based insulator-semiconductor stacks, linking semiconductor physics with wetting phenomena for light-controlled liquid actuation.

Findings

Photoelectrowetting modifies contact angle upon illumination.

Effect confirmed on commercial silicon wafers with various doping levels.

Impedance measurements show the effect is related to semiconductor space-charge regions.

Abstract

Liquid transport in microchip-based systems is important in many areas such as Laboratory-on-a-chip, Microfluidics and Optofluidics. Actuation of liquids in such systems is usually achieved using either mechanical displacement11 or via energy conversion e.g. electrowetting which modifies wetting. However, at the moment there is no clear way of actuating a liquid using light. Here, by linking semiconductor physics and wetting phenomenon a brand new effect "photoelectrowetting" is demonstrated for a droplet of conducting liquid resting on an insulator-semiconductor stack. Optical generation of carriers in the space-charge region of the underlying semiconductor alters the capacitance of the insulator-semiconductor stack; the result of this is a modification of the wetting contact angle of the droplet upon illumination. The effect is demonstrated using commercial silicon wafers, both n- and p-type having a doping range spanning four orders of magnitude (6\times1014-8\times1018 cm-3), coated with a commercial fluoropolymer insulating film (Teflon\textregistered). 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 the above mentioned areas.