Spontaneous high-speed transport of subnanoliter water droplet on gradient nanotextured surfaces

We present water droplets that undergo spontaneous self-directed motion upon contact with a chemically patterned nanotextured surface with wedge-shaped gradient. The surface exhibits two distinct wetting properties and low hysteresis. The droplet velocity depends on the droplet position and gradient angle. A wide range of droplet volume can be transported and a droplet velocity as high as 0.5 m/s has been achieved herein. Ascension of water droplets with all-round acclivity and a subnanoliter droplet movement were also demonstrated. We conclude that it is the combination of surface tension gradient and nanowetting actuation that governs the droplet motion.