The effect of the lipid layer on tear film deposition and thinning

Abstract

This work mathematically models the effects of surfactants during
tear film deposition and subsequent thinning. The surfactants
occur naturally on the surface of the tear film in the form of a lipid
layer. A lubrication model is developed that describes lipid spreading
and the evolution of the tear film height during and after an
opening blink. Numerical methods are applied to the model,
which includes the effects of lipid concentration, surface tension,
viscosity, evaporation and gravity. It is found that the lipid layer
continues to spread upwards on the cornea after the opening
blink, in agreement with experimental work. It is also shown that
lipids play an important role in drawing the tear film up the cornea
during the opening blink. Further non-uniform distributions of
lipids may lead to a rapid thinning of the tear film behind the advancing lipid front, resulting in the formation of surface capillary
waves or shocks.
Preliminary tear-scope and fluorescein experiments were
undertaken to visualise the motion of the lipid layer and any associated
shocks immediately after a blink. Using the experimental
data, lipid particles were tracked to determine the surface velocity
and these results were compared to the model predictions.