Eyelashes Protect Eyes

Biological Strategy

Mammals

Jeanette Lim

Image: Laurent DOUSSOT / Pixabay / Public Domain - No restrictions

Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Protect From Wind

Wind subjects living systems to various forces, such as compression, twisting, turbulence, and tension. These forces put living systems at risk of losing the ability to perform life-essential functions, such as when a plant becomes uprooted. Wind can result from weather phenomena or rapid movement through the air, as when flying. Wind is typically not a constant or predictable force, so living systems must be able to function both with and without its presence by adjusting to its direction and speed. A good example is how plants’ leaves and stems are flexible so that they can align with the wind, rather than being battered by it.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Eyelashes on mammals protect the eye from airborne particles by redirecting airflow.

Introduction

Eyes have sensitive surfaces that need to be protected from potentially irritating airborne particles like pollen, dust, and pathogens. For mammals, wet tears are one strategy that helps to protect the eye from foreign particles, but they also have another strategy: eyelashes. Despite their filter-like bristled appearance, eyelashes don’t function like typical filters that trap debris. Instead, they project away from the eye’s sensitive surface and alter the flow of incoming air that may be carrying foreign particles.

The Strategy

Simulations of airflows around eyes with no lashes, long lashes, and intermediate length lashes show that eye surfaces with lashes of an intermediate length should experience weaker flows.

Without lashes, there is nothing to shield the eye from incoming airflows that can deposit foreign particles onto the eye.

As lashes are added, they redirect the incoming airflow to create a region of stagnant air just above the eye’s surface. This layer of stagnant air has weaker flows, which leads to fewer airborne particles landing on the eye’s surface.

Very long lashes, however, project so far into incoming airflow that they begin to direct high-speed flows toward the eye’s surface. The result of these opposing effects is that eyelashes with a length that is approximately one-third the eye width appear to produce the weakest airflows. This ratio of lash length to width seems to match with eyelash measurements in many different species of mammals.

Eyelashes redirecting airflows around eyes constitute a passive mechanism to help keep the eye surface clean. This mechanism may also function in insects, where ocular hairs that project from between eye facets appear to reduce airflow at the eye surface. In mammals, one other possible way that eyelashes protect eyes is to function as a trigger, causing the eye to blink when they’re touched.

Image: Hans / Public Domain - No restrictions

Adapted from Amador GJ; Mao W; DeMercurio P; Montero C; Clewis J; Alexeev A; Hu DL. Eyelashes divert airflow to protect the eye. J. R. Soc. Interface. 2015. 12(105): 20141294, by permission of the Royal Society.

Image: Shad0wfall / Public Domain - No restrictions

Image: stefaanroelofs / Public Domain - No restrictions

The Potential

Sensitive and delicate surfaces or objects are all around us in human-constructed environments. Using an array of fibers to create a protective air-flow barrier around them could provide a more efficient or sustainable alternative to current forms of protection. It could also make it possible to protect objects or surfaces that are not generally protected at all, increasing their functional life, and decreasing the amount of resources needed to repair or replace them.

Last Updated January 20, 2018

References

“Through anatomical measurements, we find that 22 species of mammals possess eyelashes of a length one-third the eye width. Wind tunnel experiments confirm that this optimal eyelash length reduces both deposition of airborne particles and evaporation of the tear film by a factor of two. Using scaling theory, we find this optimum arises because of the incoming flow’s interactions with both the eye and eyelashes. Short eyelashes create a stagnation zone above the ocular surface that thickens the boundary layer, causing shear stress to decrease with increasing eyelash length. Long eyelashes channel flow towards the ocular surface, causing shear stress to increase with increasing eyelash length. These competing effects result in a minimum shear stress for intermediate eyelash lengths.” (Amador et al. 2015:1)

Journal article

Eyelashes divert airflow to protect the eye

J. R. Soc. Interface, 12(105): 20141294 | 25/02/2015 | Amador GJ; Mao W; DeMercurio P; Montero C; Clewis J; Alexeev A; Hu DL

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Reference

“For short lashes, the boundary layer at the eye surface is thickened because of the flow resistance imposed by the lashes and so the shear stress approximately scales as τ∼L⁻¹. Long eyelashes protrude further into the surrounding airflow and channel high-velocity airflow towards the eye; therefore, the shear stress scales as τ∼L². The competition between these two effects results in an intermediate optimal eyelash length L/W=0.3±0.1, where W is the eye’s opening width. At this length, eyelashes reduce particle deposition by 50%.” (Amador and Hu 2015:3171)

Journal article

Cleanliness is next to godliness: mechanisms for staying clean

Journal of Experimental Biology, 218: 3164-3174 | 01/10/2015 | Amador GJ; Hu DL

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