Rapid water drainage on human eyelashes of a hydrophobic <i>Brachistochrone</i> fiber array

Numerous organisms exploit asymmetrical capillary forces generated by unique fiber or asymmetrical tapered structures to rapidly eliminate undesired liquid for survival in moist or rainy habitats. Human eyelashes, the primary protector of eyes, use a yet-to-be-fully-understood mechanism to efficiently transfer incoming liquid for vision safeguarding. Here, we elucidate that human eyelashes featuring a hydrophobic curved flexible fiber array with surface micro-ratchet and macro-curvature approximating the <i>Brachistochrone</i> is adept at directionally and rapidly expelling incoming liquid to maintain clear vision. These structural attributes are sequentially used for liquid drainage, starting from anisotropic retention via micro-ratchet, followed by the elastic expulsion among deflected hydrophobic flexible fiber arrays and culminating in the fastest sliding off along a <i>Brachistochrone</i> path, which together reduce the contact time by about 20% of that on rigid linear slopes. Investigating the intricate relationship between multistructure and draining efficiency of human eyelashes may inspire the design of advanced liquid-repelling edges on outdoor devices to maintain dryness.