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 nonsoluble surfactants at an air/liquid interface may affect the dewetting properties of the aqueous layer. An important example is the tear film, which comprises an aqueous layer covered with a ∼100-nm-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 of central importance to understanding tear film stability. To better understand the role of surface viscoelasticity in tear film stability, we have developed a methodology to systematically control interfacial rheology of thin aqueous layers at the onset of dewetting events. The apparatus allows control over the surface pressure of the monolayer, which is a key feature since this variable controls the surface viscoelasticity. Three insoluble monolayer materials were used: newtonian arachidyl alcohol (AA), DPPC, a phospholipid that is slightly viscoelastic, and meibum, which produces a strongly viscoelastic monolayer. It is reported 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 film 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.