Physical chemistry of the normal and disordered tear film.

The preocular tear film is viewed as a thin, mostly aqueous film covering a hydrophilic solid and bordered by a meniscus which is also supported by the solid. The aqueous layer is coated by an even thinner lipid layer. Part of the solid boundary is mobile and capable of compressing the lipid layer rapidly thereby simulating blinking. By the use of such a model, the properties of the tear film system can be described and its behavior can be predicted by applying the principles of interface science to the model. Gravity has a negligible effect on the tear film. Hydraulic flow of tears can only occur in the meniscus. The rupture of the tear film is attributed to lipid contamination of the supporting solid making its surface hydrophobic. Blinking compresses the lipid layer and removes the lipid-contaminated, hydrophobic mucus from the epithelium-tear interface. The instability of the preocular tear film, which can be caused by several factors, appears to be characteristic of all dry eye states irrespective of aetiology.