The tear film that covers and moistens our eyes has to serve several functions: in particular, it must keep debris and microorganisms out, while holding the water in and preventing it from evaporating. That is why this liquid film has a protective coating just one or two molecules thick, rather like the surface of a soap bubble. Petrov and colleagues have investigated how this coating, made up of a mixture of many different biological molecules, is arranged, so that they can gain some idea of how it fulfils its role.
Some of the molecular components of a tear film's 'skin' are indeed soap-like molecules: called lipids, they 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 hardly at all in water. On their own, such molecules tend to clump together in droplets on the water surface, like droplets of oil or fat.
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