Natures wonders: ideal eyelash length (Introduction)

by David Turell @, Sunday, March 01, 2015, 18:55 (1662 days ago) @ David Turell

One-third of the width (vertical) of the mammalian eye. Protects against dust and evaporation:

"Our study demonstrates that eyelashes divert airflows, acting as a passive dust controlling system for the eyes. They reduce evaporation and particle deposition up to 50%, indicating the evolution of eyelashes may have played a role in reducing the frequency of endogenous blinks, which replenish and clean the tear film [2]. Our experiments show that eyelashes of an intermediate length accrue the greatest benefits in terms of flow reduction and particle deposition. In our simulations, the optimum eyelash length is assessed through shear rate as a proxy for particle deposition and evaporation. This optimum is unique to porous eyelashes, and does not apply for completely impermeable eyelashes, such as paper cylinders. Thus the porosity of surfaces can induce non-intuitive and advantageous flows. Through scaling, we find that the optimum arises because the aerodynamic drag imposed by short eyelashes thickens the boundary layer above the ocular surface, while long lashes channel flow with high kinetic energy towards the ocular surface. As a result, the shear stress at the eye surface scales as the inverse of eyelash length, or ? ? L?1, for short lashes; shear stress scales with eyelash length squared, or ? ? L2, for long lashes. The combination of these two competing effects gives rise to an optimum length for eyelashes that minimizes shear stress, shear rate, evaporation and contamination at the ocular surface. We find the optimal eyelash length to be L/W = 0.35 ± 0.15, following anatomical measurements, aerodynamic scaling analysis, numerical simulation and wind tunnel experiments."

http://rsif.royalsocietypublishing.org/content/12/105/20141294

http://www.livescience.com/49934-optimal-length-for-eyelashes-discovered.html

"At this point you might be wondering: Are you telling me I should toss my fake eyelashes? Well, maybe not. But perhaps you should take your beauty cues from the camel.

"'The more dense you could make your eyelashes, the better it would be," Hu said.

"Camels have two rows of dense eyelashes — likely an extra defense against the dry, sand-laden desert air. Eyelashes of high density (or low "porosity" as the researchers say) are better at blocking airflow from the surface of the eye, the experiments showed.

"This benefit might even hold true for humans with unnaturally long lashes. In their paper, Hu and colleagues wrote that "wearing curved false eyelashes of low porosity can potentially provide extra protection to the eye and reduce dry eye.'"

Interesting thought: how did hunt and peck Darwinian evolution get to this exact solution? Eyes first, lashes second? Or both appearing at the same time fortuitously and then perfecting the size and parts of each.


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