We report that the fast droplet transport without additional energy expenditure can be achieved onthe spine of cactus (Gymnocalycium baldianum) with the assistance of its special surface structure:the cactus spine exhibits a cone-like structure covered with tilted scales. A single scale and the spinesurface under it cooperatively construct a splayed capillary tube. The arrays of capillary tube formedby the overlapping scales build up the out layer of the spine. The serial drops would be driven bythe asymmetric structure resulted from tilt-up scales-by-scales on the cone-shaped spine, and movedirectionally toward the bottom from top of spine, by means of the Laplace pressure in differences.In addition, after the past of the first droplet, thin liquid film of drop is trapped in the splayedcapillary micro-tube on the surface of spine, which greatly reduces the friction of subsequentialdroplet transport in efficiency. This finding provides a new biological model which could be used totransport droplet spontaneously and directionally. Also this work offers a way to reduce the surfaceadhesion by constructing liquid film on the surface, which has great significance in prompting droplettransport efficiency.
