Engineering surface roughness to manipulate droplets in microfluidic systems

Systematic variation of surface roughness has been employed to create microstructured guide rails for droplets propelled by vibration. The wetting mechanism of rough hydrophobic surfaces has been utilized to design surface energy gradients that form the guide-rails. Microfabricated pillars in silicon have been employed to control the roughness which determines the contact angle and thus the surface energy. Droplets were moved down the surface energy gradient overcoming hysteresis by supplying energy through mechanical vibration. This work introduces roughness as a new control variable in any scheme of manipulating droplets, and presents fabricated structures and experimental results that validate the approach.