The eyes have it: Long lashes not so lovely

Here’s one aerodynamics research finding no supermodel should miss: Long, luscious eyelashes can, in fact, get too long.

There’s a sweet spot in the range of lash lengths —about one-third the width of the eye —reports David Hu of Georgia Tech in Atlanta. Eyelashes much longer than that funnel airflow and grit into the eye instead of deflecting about half of it. And lashes that are too stubby offer too little protection. Those conclusions emerge from computer simulations, measuring the lashes of animals and experiments in a custom-built slightly windy tunnel, Hu and colleagues report February 25 in the Journal of the Royal Society Interface.

The notion that longer is not always better for protective eyelashes was very surprising, Hu says. It’s the spaces between the long lashes that cause this counterintuitive effect, creating a porous barrier that lets a bit of air drift between the individual lashes.

The project began with Hu staring at his baby daughter’s beautiful long eyelashes. Wondering about the rest of the animal kingdom, he sent a student to measure lash length in a basement full of pelts at the American Museum of Natural History in New York and at a taxidermy center with the motto “Only God can make them better.” Studying pelts proved the most feasible approach. “If you bring sharp instruments like measuring instruments near animals’ eyes, they get annoyed,” Hu notes.

In 22 species of mammals ranging from hedgehogs to giraffes, most lash lengths were about a third of the eye width. That’s within the range that a computer analysis suggested would be the best for giving eyes some measure of protection as air flows over the face of an animal moving forward. “The eye is a wet organ,” Hu says. Too much air dries out tissues and raises the risks of dust and debris blowing in.

To test real-world airflow, “we decided to build a miniature wind tunnel for eyelashes,” Hu says. A computer fan and some cardboard created a gentle breeze, and a small lab dish of water represented the vulnerable moisture of the eye. Without any lashes, the water evaporated about twice as fast as it did when surrounded by a fringe of upright lashes.

Biologists have also mused about how eyelashes might protect against airborne microbits of junk bombarding the eye. So Hu and his colleagues loaded a home humidifier with dyed water and gently blew a green mist of microscopic droplets at their stand-in eye. The experiment showed that right-sized lashes can deflect about half of incoming particles.

It’s not protection like that of a solid barrier or a filter that catches dust from air whooshing through. Eyelashes do snag some bits, Hu says, but much of their effect comes from deflecting airflow and sending its particle burden wafting safely in another direction. Thus deflectors may not require as much cleaning as solid barriers or as frequent replacement as filters do. Designs for protective rims for cameras, sensors and other delicate instruments often rely on solid barriers, Hu says, but he’s hoping for more experiments with eyelash-inspired designs.

Hu also suggests that artificial lashes might someday ease the widespread human misery of dry eye. The idea sparked both interest and caution from Richard Braun of the University of Delaware in Newark, a mathematician who studies the outer film of moisture covering eyes. That film can vary by more than a factor of 20 in how well it protects the eye from evaporation.  The right lashes in the experiment can double protection, but, Braun says, “there are a host of other factors that may be more significant in treatments for dry eye.”

There are also a host of other significant factors influencing cosmetics, so the impact of aerodynamics on fashion may be sadly small.