Experiments with human eyelashes and eyelash-mimicking fibers pinpoint several features that help fling water away from the eyes, researchers from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences report December 20 in Science Advances. 

The exterior of an eyelash, or cuticle, acts like a “micro-ratchet,” the researchers report. Water can flow easily from root to tip but not in the opposite direction, thanks to scales that overlap like shingles on a roof. When the scientists dipped loose eyelashes in water and pulled them out again, more force was required to move the eyelash when the water was working against the ratchet than when going with it. And by dripping water on loose eyelashes, the researchers showed that the hairs are hydrophobic, meaning that water beads up on them and tends to roll off. 

Eyelashes approximate a shape called a brachistochrone, a curve that minimizes the time it takes to get from point A to B under the force of gravity. Using arrays of nylon fibers with similar dimensions and elasticity as eyelashes, the researchers compared fibers in the shape of a brachistochrone with fibers that were straight or curved in another shape. The droplets slid fastest off the brachistochrone. 

Known to guard against dust, eyelashes haven’t been recognized for their water-wicking superpower (SN: 2/24/15). In addition to helping out in a rainstorm, the effect could come into play when bathing, sweating or crying. Beauty treatments could mess with this ability, however. Mascara can make eyelashes attract water instead of repelling it, the scientists note, and curling the lashes alters their shape.