The 'Magic Number' of Eyelash Length for Healthier Eyes

There's a magic number when it comes to eyelashes: one-third. Animals that range from humans to hedgehogs to giraffes all have eyelashes that are one-third the width of their eyes. Now, scientists have taken a closer look to find out why that is.

"Eyelashes form a barrier to control airflow and the rate of evaporation on the surface of the cornea," said Guillermo Amador, one of the researchers, in a news release. "When eyelashes are shorter than the one-third ratio, they have only a slight effect on the flow. Their effect is more pronounced as they lengthen up until one-third. After that, they start funneling air and dust particles into the eye."

In order to examine this ratio, the researchers measured the eye and eyelashes of various animals. Aside from an elephant, which has extremely long eyelashes, every species studied had evolved the same ratio of lash length to eye width.

Intrigued by this, the scientists built a wind tunnel to recreate air flows on a mimic of an adult, human eye. They discovered that the ideal ratio while varying the recreated "eyelash" length during evaporation and particle deposition studies.

"As short lashes grew longer, they reduced air flow, creating a layer of slow-moving air above the cornea," said David Hu, one of the researchers, "This kept the eye moist for a longer time and kept particles away. The majority of air essentially hit the eyelashes and rolled away from the eye."

When eyelashes grew even longer, though, the lashes extended further into the airflow and created a cylinder. The air and its molecules were then channeled toward the eye and led to faster evaporation. This shows that long, elegant fake eyelashes may look nice, but aren't healthy for your eyes.

The findings could be used to create eyelashes-inspired filaments to protect solar panels, photographic sensors or autonomous robots in dusty environments.

The findings are published in the Journal of the Royal Society Interface.

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