Throughout human evolution, body and facial hair have notably diminished, yet eyelashes have remained a distinguishing feature. The physiological or functional purpose of eyelashes-traditionally thought to be for catching dust or filtering air, etc.-has long been debated.
However, a team of Chinese researchers has recently elucidated the characteristics of human eyelashes. Their study reveals that eyelashes consist of a hydrophobic, curved, flexible fiber array, featuring surface micro-ratchets and a macro-curvature approximating the Brachistochrone curve. This structure enables eyelashes to rapidly and directionally expel incoming liquid, thereby preserving clear vision.
The hydrodynamic advantages of eyelashes, particularly their ability to expel unwanted liquids from the eye to maintain visual clarity, have received little attention. For instance, during facial washing or intense physical activity, the eyes are exposed to significant amounts of water or sweat without compromising clear vision.
This study, published in Science Advances on Dec. 20, was conducted by Prof. JIANG Lei and his group from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences.
The research team aimed to investigate the interaction between water and the flexible fiber array of eyelashes. They began by characterizing the structure, wettability, and water drainage process of human eyelashes. Next, they explored how the flexibility, wettability, and curvature of the fiber array influence water drainage.
Based on their findings, the researchers revealed the control mechanism governing transfer direction and contact time, which arise from the multi-scale asymmetric structures and heterogeneous elastic deformations of the fiber array. They also developed a quantitative computational model to calculate the elastic forces acting on the fiber array.
This research has also led to the design of eyelash-mimetic rapid liquid transfer edges, including aesthetically pleasing and protective false eyelashes, waterproof imaging devices, and ventilated structures.
This work was supported by the National Science Fund of China for Excellent Young Scholars and the Young Scientists Fund of the National Natural Science Foundation of China.
Human eyelashes comprised of a hydrophobic fiber array expel water droplets in the shortest time. (Image by Dai Haoyu and Dong Zhichao's group)
