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Danielle L. Leiske, Liat Rosenfeld, Cecile Monteux, Michelle Senchyna, Howard A. Ketelson, Gerald G. Fuller; Non-ideality in the Dynamic Wetting Behavior of Meibum Covered Droplets. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3733.
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Lipid layer spreading and tear film dewetting are important variables influencing tear film stability. The viscoelasticity of meibomian lipids (ML) may govern tear film wetting dynamics. Furthermore, changes in ML composition that may accompany dry eye could alter melt temperature and viscoelastic properties, which may ultimately influence tear film spreading and dewetting. The purpose of this work was to gain insight into the effects of surface elasticity on the dynamic wetting of ML droplets.
Multiple collections of ML from a healthy individual were pooled, dried, weighed and reconstituted in chloroform. Arachidyl alcohol (AA), which forms a Newtonian monolayer, was used as a control. AA was also dissolved in chloroform. A clean glass plate was placed on a linear actuator capable of moving at speeds between 0.01-10 mm/s. Water droplets (35 µL) were placed on the plate in contact with a Teflon block, surfactant solutions were spread on the droplet surface. The actuator moved at constant velocities such that the droplet was held in place against the block, contact angle was observed with a CCD camera.
For a single plate velocity, the contact angles of AA covered droplets were constant. The equilibrium contact angle increased with increasing plate velocity such that the results followed the Cox-Voinov law, which predicts ideal behavior in Newtonian systems with constant surface tension. Although the contact angle was constant for ML covered droplets above 5 mm/s, the contact angle did not follow the Cox-Voinov law. At low velocities 0.01-0.5 mm/s we observed a stick/slip regime of the contact line where the contact angle would gradually increase then suddenly drop at regular intervals (period of ~2mm).
As the dynamic wetting of AA covered droplets followed predicted behavior, we conclude that insoluble surfactants do not necessarily result in non-ideality. We attribute the stick/slip and irregular behavior at high velocities of ML covered drops to interfacial elasticity. This deviation could influence lipid layer spreading, as well as tear film dewetting in dry eye patients.
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