If you ever want to see some of the coolest, most innovative science and engineering, then look no further than the human body itself. The human body is an incredible masterpiece of scientific engineering design, as is apparent in many of the systems at work in the human body. While many of us are probably aware of how complex the human heart, brain, skeletal system, etc. are, there are a great deal of often overlooked systems that are vastly underestimated in their complexity. Take the eyelashes for example: we’ve long known that the primary purpose of eyelashes is to keep debris (i.e. dust, dirt, and other particulate) from making contact with the surface of our eyes (cornea, or conjuctiva), but what we haven’t fully understood is exactly how the eyelashes perform this function. Sure, there is the obvious fact that eyelashes will act as filters and comb out particulate matter in the air that flows past/through them, and the less obvious fact that they will act as extended sensors to let us know when foreign objects or debris (i.e. particulate matter or insects) are near, so we can quickly close our eyes, but new research suggests that there is another, far more intricate function performed by your eyelashes.
Recent research has concluded that at the optimal length of roughly 1/3 the width of the eye itself, eyelashes will alter the airflow in order to create a stagnation region (a region with stagnant, or still/un-moving air), which helps to divert (possibly debris-containing) airflow from the eye. Because there is a large pocket of still air situated over the eye, when additional airflow is encountered, it is firstly slowed/obstructed by the filter-like eyelashes, and then will drag along the top layer of this stagnation pocket with it as it flows. If there was not a stagnation region, the air flow would be constantly dragging away the layer of air right next to the eye, both leaving it more vulnerable to debris, and requiring constant replenishment of the protective liquid tear film on the surface of your eye (which is produced through the lacrimal glands), thus also leading to high evaporation rates of this tear fluid. However, since there is a stagnation region, there is essentially an air cushion (more accurately, a shear layer) between the surface of your eye and the airflow, that acts to damp out the air movement, thus mostly maintaining the still air region around your eye, and keeping debris out, while increasing the average time to evaporation for the tear fluid. It should also be noted, however, that the research also concluded that if the lashes are too long, they may actually channel the flow directly towards the eye, thus leading to faster tear film evaporation and far more issues, while too short of eyelashes will not provide the protection that the optimal eyelash length would.
How can we put this more simply? When eyelashes are of the optimal length (~1/3 the width of the eye), they help to divert air flow (and debris) from your eye, while also preventing your eyes from drying out as quickly!