Self-Propelling and Positioning of Droplets Using Continuous Topography Gradient Surface

A radial pattern with continuous topography gradient is presented, which induces a continuous inward wettability gradient and enables self-propelling and accurate positioning of droplets to the pattern center. The effect of droplet size and wettability gradient of the pattern on the self-mobility of droplets is investigated. The wettability gradient is found to increase towards the pattern center, enhancing the self-motion of droplets at the inner area of the pattern. Moreover, larger droplets give rise to a larger solid-liquid contact diameter, which helps to satisfy the self-motion criteria that the advancing contact angle at front edge is smaller than the receding contact angle at rear edge. Consequently, a larger droplet size favors self-motion initiated from the outer area of the pattern. The continuous topography gradient employed here allows the flexible dispensing of droplets at any place within a certain range, and avoids potential pinning defects to droplets at geometrical discontinuities. An average self-motion velocity up to 4.0 cm/s for microliter-sized droplets is achieved on the resultant patterned surface.