(499a) Consequences of Interfacial Viscoelasticity On Thin Film Stability

The phenomenon of dewetting is frequently observed in our everyday life. It is of central importance in many technological applications as well as in a variety of physical and biological systems. The presence of non-soluble surfactants at an air/liquid interface may affect the dewetting properties of the aqueous layer.

The tear film is comprised of an aqueous layer covered with a ~100nm thick blanket of lipids, known as the meibomian lipids. Interfacial rheological measurements of meibomian lipids reveal that these films are remarkably viscoelastic. Tear film dewetting is an extremely important parameter that influences tear film stability.

In this research, in order to better understand the role of the meibomian lipids in the tear film stability, we have developed a methodology to characterize dewetting of thin aqueous layers covered with insoluble surfactants. Meibum was used to form a viscoelastic monolayer. As comparison to meibum we have studied DPPC, a phospholipid which is also viscoelastic, and the Newtonian arachidyl alcohol (AA). The apparatus allows control of the surface pressure of the monolayer, which is a key feature since this variable controls the surface viscoelasticity.

We have found, both experimentally and theoretically, that monolayers of viscoelastic surfactants are able to stabilize thin films against spontaneous dewetting. As the surface pressure of these layers is increased, their effectiveness is enhanced. Moreover, these surfactants are able to reduce the critical thickness for dewetting. Meibum is particularly effective in stabilizing thin films. Our results suggest that the meibomian lipids play a vital role in maintaining tear film stability in addition to suppressing evaporation. Furthermore, changes in meibomian lipid composition that alter the lipid melt temperature and viscoelastic properties, may ultimately influence tear film stability, which in clinical cases may result in dry eye disease.