Ionic-surfactant-mediated electro-dewetting for digital microfluidics

The ability to manipulate droplets on a substrate using electric signals¹-known as digital microfluidics-is used in optical^2,3, biomedical^4,5, thermal⁶ and electronic⁷ applications and has led to commercially available liquid lenses⁸ and diagnostics kits^9,10. Such electrical actuation is mainly achieved by electrowetting, with droplets attracted towards and spreading on a conductive substrate in response to an applied voltage. To ensure strong and practical actuation, the substrate is covered with a dielectric layer and a hydrophobic topcoat for electrowetting-on-dielectric (EWOD)^11-13; this increases the actuation voltage (to about 100 volts) and can compromise reliability owing to dielectric breakdown¹⁴, electric charging¹⁵ and biofouling¹⁶. Here we demonstrate droplet manipulation that uses electrical signals to induce the liquid to dewet, rather than wet, a hydrophilic conductive substrate without the need for added layers. In this electrodewetting mechanism, which is phenomenologically opposite to electrowetting, the liquid-substrate interaction is not controlled directly by electric field but instead by field-induced attachment and detachment of ionic surfactants to the substrate. We show that this actuation mechanism can perform all the basic fluidic operations of digital microfluidics using water on doped silicon wafers in air, with only ±2.5 volts of driving voltage, a few microamperes of current and about 0.015 times the critical micelle concentration of an ionic surfactant. The system can also handle common buffers and organic solvents, promising a simple and reliable microfluidic platform for a broad range of applications.