Figure 2. In situ optical microscopic observation of the directional water collection on the cactus spine placed at various angles.

(a–e) A spine placed in fog with the following tilt angles: 90°, 45°, −45°, −90° and 0°, respectively. The deposited water drops were driven from the tip to the base of the spine, even with the spine vertically fixed with the tip pointing down (d). Scale bar, 500 μm.