A group of scientists led by Dr David Hu from the Georgia Institute of Technology has discovered that humans and 21 species of mammals – from hedgehogs, to pandas, to camels and giraffes – have eyelash length 1/3 the width of their eye.
Camel. Image credit: Emmanuel Keller, www.tambako.ch / CC BY-ND 2.0.
“Eyelashes form a barrier to control airflow and the rate of evaporation on the surface of the cornea. When eyelashes are shorter than the 1/3 ratio, they have only a slight effect on the flow. Their effect is more pronounced as they lengthen up until 1/3. After that, they start funneling air and dust particles into the eye,” said group member Guillermo Amador, a PhD candidate at the Georgia Institute of Technology.
First, the scientists measured eyes and eyelashes of various mammals. Aside from an elephant, which has extremely long eyelashes, every mammal species studied had evolved to the same ratio of lash length to eye width.
They then built the wind tunnel to re-create air flows on a mimic of an adult, human eye.
A 4-mm deep, 20-millimeter diameter aluminum dish served as the cornea. It sat on top of an acrylic plate, which imitated the rest of the face. Mesh surrounded the dish to replicate the eyelashes.
The team discovered the ideal ratio while varying the mesh 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. 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,” explained Dr Hu, who is the senior author of a paper published in the Journal of the Royal Society Interface.
The opposite process occurred with longer eyelashes. The lashes extended further into the airflow and created a cylinder. The air and its molecules channeled toward the eye and led to faster evaporation.
“This is why long, elegant, fake eyelashes aren’t ideal. They may look good, but they’re not the best thing for the health of your eyes,” Guillermo Amador said.
The scientists said that the findings could be used to create eyelash-inspired filaments to protect solar panels, photographic sensors or autonomous robots in dusty environments.
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Guillermo J. Amador et al. Eyelashes divert airflow to protect the eye. Interface, published online February 25, 2015; doi: 10.1098/rsif.2014.1294
