(34d) Human Tear-Film Evaporation Rate from Simultaneous Ocular-Surface Temperature and Tear-Breakup Area | AIChE

(34d) Human Tear-Film Evaporation Rate from Simultaneous Ocular-Surface Temperature and Tear-Breakup Area

Authors 

Radke, C., University of California-Berkeley
Tear-film evaporation rate is critical to eye health. If the 3-µm human tear film evaporates at close to the rate of pure water, the tear film completely disappears by the end of a blink. Fortunately, human tear film is covered by a 100-nm lipid film that is thought to retard tear-evaporation rates by approximately a factor of 10 [1,2]. Transient infrared ocular surface temperature (OST) provides a convenient, non-invasive, and simple in-situ method to gage tear evaporation rates. Current analyses of OST results, however, suggest little reduction of the tear evaporation rate compared to that of pure water.

For the first time, we devise a straightforward methodology to assess quantitative in-vivo tear-evaporation rates (TERs) from simultaneous measurement of transient infrared ocular surface temperature (OST) and tear-breakup area (TBA) on 9 human subjects. Specifically, we analyze simultaneously measured transient OST and transient TBA based on an extended heat-transfer model. Key to the model is explicit recognition of expanding tear breakup spots [3] that are proposed to be lipid free and, accordingly, exhibit higher evaporation rate than that for lipid-covered tear. Transient tear-breakup areas follow from image analysis. Remaining parameters include an overall heat transfer coefficient to capture natural-convective and radiative heat loss to the environment, and a temperature-dependent evaporation rate of pure water. These two parameters are obtained by independent in-vitro measurement of simultaneous temperature decline and mass loss for small volumes of heated water or agar gels undergoing cooling via natural convection. In all cases (9 subjects), model-predicted in-vivo OSTs agree well with experiment. Percent reductions in lipid-covered tear evaporation rate range from 62 to 95% of that for pure water, in agreement with accepted literature values [1-3]. Our new methodology for obtaining in-vivo human tear evaporation rate is simple and obtains realistic TERs.

1.  Liat Rosenfeld, Colin Cerretani, Danielle L. Leiske, Michael F. Toney, Clayton J. Radke, and Gerald G. Fuller, “Structural and Rheological Properties of Meibomian Lipid,” Investigative Ophthalmology and Vision Science, 54(4), 2720-2732 (2013).

2. C. Cerretani, N. Ho, and C. J. Radke, “Water-Evaporation Reduction by Duplex Films: Application to the Human Tear Film,” Advances in Colloid and Interface Science, 197-198, 33-57 (2013).

3. C.C. Peng, C. Cerretani, R.J. Braun, and C.J. Radke, “Evaporation-Driven  Instability of  the Precorneal Tear Film,” Advances in Colloid and Interface Science, Velarde Honorary Issue, 206, 250-264 (2014).