(184b) Black Spots in the Human Tear Film: An Evaporation-Driven Instability | AIChE

(184b) Black Spots in the Human Tear Film: An Evaporation-Driven Instability

Authors 

Radke, C. - Presenter, University of California-Berkeley
Peng, C. C., University of California, Berkeley
Cerretani, C., University of California, Berkeley



Tear-film instability is  a major signature of eye health. When an interblink is prolonged, randomly distributed ruptures occur in the tear film. “black spots” and/or “black streaks” appear in 15 to 40 s for normal individuals. For people who suffer dry eye, tear-film breakup time can be less than a few seconds. In spite of decades of effort, there currently is no satisfactory quantitative explanation for the origin of tear rupture. Based on the experiments of Liu et al.,1 we hypothesize that tear-film breakup is due to local tear-film lipid layer rupture and resulting locally elevated evaporation that drives tear-film rupture. 

        As observed in experiment, the 100-nm thick lipid layer covering the tear film initially ruptures giving a larger local evaporation rate than that in the surrounding lipid-covered tear film.   Increased evaporative flux drives a hole in the tear film. As the hole deepens, local salinity increases. The growing hole is suppressed by curvature-driven healing flow and by osmotic-suction from the cornea due to the local salinity increase. Tear rupture occurs only when the locally high evaporative flux outweighs the two healing flows. The new proposed rupture mechanism leads to significant increased salinities at the bottom of the rupture spot (or streak) that we coin salinity “hot spots”. Local salinity hot spots appear even when a growing instability does not rupture during a 5-s interblink.

       Coupled nonlinear evolution equations for thin-film flow, evaporative heat transfer, and salt concentration are solved numerically. Tear-film instability and corresponding local increase of salinity is confirmed for higher, but yet reasonable, evaporation rates. Predicted roles of environmental conditions such as wind speed and relative humidity on tear-film stability agree with clinical observations. Most importantly, locally elevated evaporation leads to hyperosmolar spots in the tear film and, hence, vulnerability to epithelial irritation and dry-eye symptoms. Tear-film rupture is more likely with contact-lens wear because initial tear-film thickness is reduced.

        This study provides the first (and only) physically consistent, quantitative explanation for observed black streaks and/or spots in the human tear film during interblink. Importantly, this study explains the formation of “hot spots” of locally high concentrated solutes in the tear film, such as salts, proteins, mucins. A tear film repeatedly peppered with salinity hot spots is vulnerable to dry eye even when tear-film breakup is not observed.

1.Liu H, Begley CG, Chalmers R, Wilson G, Srinivas SP, Wilkinson JA. Optom Vis Sci. 2006; 
  83:723-30.