Interfacial Viscoelasticity and the Moving Contact Line: Consequences On Tear Film Stability

Meibomian lipids are essential to the stability of the tear film. These excretions are a mixture of nonpolar species such as long-chain wax esters, cholesterol esters and fatty acids. They are traditionally thought to form ~100nm thick blanket over the aqueous part of the tear film. The spreading of this layer and tear film dewetting are important variables influencing tear film stability. The ability of aqueous layers coated with monolayers of meibomian lipids to wet substrates is expected to affect respreading processes necessary to coat and protect the cornea. Interfacial rheological measurements of meibomian lipids spread at the air-water interface reveal that these films are remarkably viscoelastic. In this study we have examined the consequence of surface viscoelasticity on the dynamics of the moving contact line as a drop translates across a surface. The dynamic, advancing contact angle was measured as a function of the translational velocity of the drop. Monolayer coatings of a fatty alcohol lacking surface elasticity were compared against viscoelastic meibomian lipid monolayers. Whereas the fatty alcohol was found not to change the behavior of the moving contact line from its classical response, the interfacial viscoelasticity of the meibomian lipids induced a strong “stick-slip” motion. It is demonstrated that surface rheology can have a strong influence on wetting phenomena and hence malformed meibomian lipids could influence lipid layer spreading, as well as tear film dewetting.