Peter Petrov of the University of Exeter and colleagues have found that tears are a much more complex fluid than previously thought. Their surfaces are, they say, highly structured, almost like cell membranes with a protective coating just two molecules thick.
The tear film that covers and moistens our eyes must keep debris and microorganisms out as well as holding water in and keeping the eye lubricated. Petrov’s team has investigated how this two-molecule coating, made up of a mixture of many different biological molecules, is ordered with the aim of getting a clearer view of its role.
Some of the molecular components of the tear film’s “skin” are soap-like lipid molecules. These are similar to the key constituents of cell membranes, and have a water-soluble ‘head’ and an insoluble ‘tail’. At the surface of water, these molecules tend to sit in layers one molecule thick, with their water-loving heads immersed and their insoluble tails poking up out of the water. But some other constituents of tear films are wholly ‘water-fearing’ (hydrophobic) — they will dissolve readily in fats, but very poorly in water. On their own, such molecules tend to clump together in droplets on the water surface, like droplets of oil or fat.
Petrov and colleagues have attempted to explain how this mixture organize itself in a tear film by bouncing X-rays off the surface of both natural tear films (taken from cows) and artificial analogues composed of a comparable mixture of lipids and oily compounds. Their results show that, in both the real and the synthetic films, the molecules seem to line up at the water surface in regular, orderly arrays, rather like two-dimensional crystals.
When the researchers added fluorescent molecules to synthetic tear films containing just the lipid components, they saw that the lipids separated into two different states: a relatively disorderly state, like a two-dimensional liquid, interspersed with blobs of a more closely packed, crystal-like state. These lipid crystals grew into remarkable patterns shaped rather like flower heads. When the fat-like components were added to these artificial films, they seemed to form a separate later on top of the lipids, which enabled them to remain out of the water. Petrov and colleagues think that this arrangement enables the tear film to keep a relatively constant structure even when it is severely squeezed and stretched, as is likely to happen for example when we blink: squeeze the film and the lipid crystals grow a bit bigger; stretch it out and they become smaller again.
Petrov and colleagues describe their findings at the Condensed Matter and Materials Physics conference in Exeter today.