Deposition and Thinning of the Human Tear Film

The exposed part of the eyeball is covered by a tear film, which is vital for the proper function of the eye. The film thickness has been measured to be roughly 10 μm; however, how a tear film of this thickness is generated has not been clearly explained. It is proposed that the tear film is deposited analogous to a coating process by the rising meniscus of the upper lid during a blink. A coating model is formulated that not only predicts correctly the film thickness, but also captures the postblink lipid spreading commonly observed in experiments. A deposited tear film thins rapidly near the tear meniscus surrounding the film. Numerical simulation of this thinning reveals that the minimum film height obeys a power law. When the minimum height reaches the effective range of dewetting intermolecular forces, the film ruptures. The thinning time therefore defines a breakup time, and the thinning law shows explicitly how this breakup time is related to tear viscosity, surface tension, meniscus radius, and initial and final film thicknesses. The calculated breakup time agrees with those observed experimentally.