The Aerodynamics of Eyelashes


Although we find that children with uncorrected visual skill deficits have symptoms that overlap with dry eye or allergies, and are ineffectively treated with lubricating drops or lid hygiene which deflects attention from the real source of the problem, there are in fact children who experience ocular surface disease problems.  Hence the dynamic interplay between visual comfort and ocular integrity.

I was reminded of this while reading How to Walk on Water and Climb Up Walls, that rare breed of a breezy science read, written by the bio-locomotion expert David L. Hu.  In Chapter 4, Of Eyelashes and Sharkskin, Hu writes:

“Since 1972, allergy doctors had relied on a list of symptoms to determine if a child had an allergic disease.  These symptoms included dark rings under the eyes due to poor sleep, creases on the nose due to frequent rubbing, and ‘long silky eyelashes’.  This latter feature had no explanation, although along with the other symptoms, it had been used in medical practice for years.  In 2004 Israeli allergy doctors decided to put it to the test.  They measured the eyelash length of patients that were having allergic reactions to house mites, and those that had no allergic symptoms.  They found that children with allergies and eyelashes that were 10 percent longer than if they were allergen-free.  It was an intriguing discovery, and the authors had an idea about how it happened.  They speculated that specialized mast cells at the edges of the eyelid acted as ‘central switchboards’, releasing chemicals that cause hair growth in response.”

Professor Hu felt like there might be a missing link in that explanation.  He asked, “what benefit was conveyed by having longer lashes?”  It was known that eyelashes play a role in maintaining human health.  Specifically, a lack of eyelashes is correlated with higher rates of eye infection.  Professor Hu recruited a a student from his fluid mechanics class and collected data on eyelash length from a wide variety of mammals.  They found that on average, eyelashes had a length of one third the width of the eye.  As the eye gets bigger, so do the lashes in proportion, a relationship known as isometry.  We’re talking specifically about eyelash length, not density.  So the plot in this story line lengthens …

Guillermo Amador, a mechanical engineering student, assisted the Hu laboratory in investigating the aerodynamics of eyelashes.  They published their results in a 2015 issue of the Journal of the Royal Society Interface in an article titled Eyelashes Divert Airflow to Protect the Eye.

Hu Photo

In his book, Professor Hu writes: “For years, I had this amazing demonstration in the back of my mind.  We usually think of evaporation as unimaginably slow, but here was an experiment that could actually measure it.  I realized the analytical balance was the perfect tool for studying evaporation of the eye’s tear film … We were shocked by the results.  The eyelashes had a dramatic effect on water evaporation.  A cup with no lashes took ten minutes to evaporate.  By adding lashes of the optimal length, we could lengthen this duration to twenty minutes.  Strangely enough, these effects disappeared once we used longer lashes.  Long lashes had a deleterious effects, as bad as if there were no eyelashes at all.”

Professor Hu explains that without lashes, dry air immediately impacts the eye, stealing water molecules from the eye’s surface.  Lashes of the appropriate length act as a speed bump, slowing down evaporation.  But longer lashes act as a funnel increasing air flow and hastening evaporation.  The next time you admire beautiful long eyelashes on a patient, bear in mind that while long on appearance those lashes may be short on functionality.  Recall the speculation above by Israeli researchers that unchecked mast cells aggregating around the lids results in longer eyelash growth.

We might therefore conceive of these considerations as a borderland between lid hygiene and visual hygiene.  It also lends further context to how the the lashes and lids frame eyeballs as specialized joints adapted for sight, and how optimizing the ocular surface minimizes drag that can be disruptive to ocular kinematics, eye movement and binocular coordination.

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