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.
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.Edit Summary
“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)
Eyelashes divert airflow to protect the eyeJ. R. Soc. Interface, 12(105): 20141294February 25, 2015
“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)