Superhydrophobic tracks for low-friction, guided transport of water droplets.

anti-fogging, [ 6 ] anti-icing, [ 7 ] buoyancy [ 8 ] and drag reduction. [ 9 ] By defi nition, a surface is superhydrophobic if the contact angle between a water drop and the surface at the solid/liquid/air interface is larger than 150 ° , and the contact angle hysteresis is small, i.e., drops readily slide or roll off when the surface is tilted slightly. [ 10–12 ] Here we explore the feasibility of using superhydrophobicity for guided transport of water droplets. We demonstrate a simple yet effi cient approach for droplet transport, in which the droplet is moving on a superhydrophobic surface, using gravity or electrostatic forces as the driving force for droplet transportation and using tracks with vertical walls as gravitational potential barriers to design trajectories. Although the slope of the platform is as small as a few degrees, the drops move at a considerable speed up to 14 cm s − 1 , even in highly curved trajectories. We further demonstrate splitting of a droplet using a superhydrophobic knife and drop-size selection using superhydrophobic tracks. These concepts may fi nd applications in droplet microfl uidics and lab-on-a-chip systems where single droplets with potential analytes are manipulated. [ 13–16 ]