Fig. 2. Droplet transport on the different samples.

(A) On PNAS, water harvested from a fog self-propels after successive coalescences. Transport takes place along positive x, as highlighted by dashes (see also movie S1). (B) On equally high Janus pillars, transport starts the same way: Droplets sit on the right side of the Janus pillars and get ejected in the +x direction after coalescence. However, interpillar coalescence later launches droplets in the opposite direction (movie S2). (C and D) On conical needles, coalescence can eject droplets in both +x and −x directions, due to the absence of orientation of droplets at the needle tips (movies S3 and S4). Photo credit: Shile Feng, City University of Hong Kong.