The Effect of Tear Film on Ocular Surface Temperature: A Thermodynamic Study

The thermodynamic effect of tear film on ocular surface remains ambiguous in literature, though in general it is considered to warm the surface in blinking phase, and cool it at later stage. In this investigation, we modeled the temporal variation in ocular surface temperature (OST) on the basis of Newton's law of cooling to look into the tear film's thermodynamic effect. We assumed, the OST varied periodically and the OST just before a blink was equal to the OST observed when the surface was in equilibrium. Then, this model was incorporated into a thermodynamic equation, describing the heat exchange at the ocular surface. Simulations were subsequently performed to determine the moment when the tear film evaporated the exactly amount of energy it had just brought to the surface by blinking in an interblink period. It was found that, for tear film to possibly warm the ocular surface both in the blinking phase and the entire interblink period, its thickness had to be above 32 μm, assuming the rate of spontaneous blinking was once per 6 s. However, according to literature, tear film thickness is likely to be 3 μm, which in this case, the tear film would evaporate the tear-transferred heat just within a second, and the warming effect by tear film would be minimal; cooling should be the dominating effect.