Figure 1.

Directional and long-range droplet ballistic transport processes. (a) Optical image of drain fly. (d) Microscopic visualization and the corresponding sketches of droplet dynamics on drain fly tentacle. Initially, very small condensate droplets form randomly within individual knot on the tentacle surface and migrate unidirectionally to the apex of tentacle when they grow larger. The chain reaction of droplet transport against gravity is facilitated by frequent coalescence with neighboring droplets and is further synergized by the guided droplet relay between individual knots. (c) Scanning Electron Microscopy (SEM) image of a single tentacle of drain fly. The tentacle consists of several periodical parabola-shaped knots, with a length of ~1.5 mm. (d) The magnified view of the tentacle structure. The knot is parabola-shaped, besieged by seta arrays with a length of ~120 μm. (e) The magnified side view of ratchet arrays on a single seta. The length and the center-to-center spacing of ratchets are ~1.26 μm and ~0.49 μm, respectively. (f) The variation of tilt angle (α) of ratchets (the red triangular dotted line) and the trunk diameter (r_o) of the microseta (the black squared dotted line) as functions of spatial location. Here, 0 μm corresponds to the bottom of seta, while 120 μm corresponds to the apex. The error bars denote the standard deviation of the measurements.