There is much more to tears than salty water, report UK chemists. The liquid film that moistens our eyes has a coating similar to a cell membrane.

’A tear is an intellectual thing’, said William Blake in Jerusalem. Peter Petrov of the University of Exeter, UK, and colleagues have shown how right he was. They have found that there’s much more to tears than salty water. The liquid film that moistens our eyes has a coating similar to a cell membrane, which keeps debris and bugs out while holding water in, and is cunningly structured so that it doesn’t get broken up each time we blink. 

This tear film is an organic coating made up of a complex mixture of lipid molecules along with hydrophobic esters and glycerides. Petrov and colleagues used x-ray diffraction and fluorescence microscopy to confirm an earlier hypothesis that bovine tear-film forms a two-layer structure with the lipids arrayed over the water surface and the hydrophobic molecules on top.

The researchers found that the lipid layer contains patches in which the molecules are packed in a dense, crystal-like array surrounded by a disorderly, liquid-like ’sea’ of lipids. These ’flat crystals’ grow or shrink in complex shapes, some resembling flower heads, as the films are compressed. 

The Exeter researchers think that this arrangement enables the tear film to maintain its structure even when it is severely squeezed and stretched when we blink. If the film is squeezed, the lipid crystals grow a bit bigger; stretch it out and they become smaller again. Petrov described the findings today at the Condensed Matter and Materials Physics conference in Exeter, organized by the Institute of Physics. 

Steve Roser, a chemist at the University of Bath who studies organic films and membranes, welcomed the findings. ’Many lipids form exciting and unusual structures at the air-water interface, but here the piggybacking of an oily film on top of a lipid layer, to give the tears flexibility and strength is very interesting,’ said Roser. 

’Monolayer studies have gone in and out of fashion for many years, but it’s nice to see a piece of research looking to understand a basic piece of biophysical chemistry,’ he added. ’Plus I didn’t know that cows cry.’ 

Philip Ball