Tribological conditions for contact lenses have very low contact pressures in the range 3-5kPa and sliding speeds around 12 cm/s. Using a microtribometer a series of experiments was run on commercially available contact lenses made from Etafilcon-A. These tests were run using 10 to 50 mN of normal load at speeds from 63 to 6,280 µm/s using a 1 mm radius glass sphere as a pin. The resulting contact pressures are believed to be nearly an order of magnitude larger than the targeted 3-5kPa. It is hypothesized that the viscoelastic nature of the hydrogel, viscous shearing of the packaging solution, and interfacial shear between the glass sphere and the contact lens all contribute to the friction forces. A model that includes all three of these contributors is developed and compared to the experimental data. The experimental friction coefficients vary from mu=0.025 to 0.075. The calculated fluid film thicknesses were between 1 and 30 nm. The average surface roughness of the lens and the glass sphere are Ra=15 nm and Ra=8 nm, respectively, suggesting that the contact is not in full elastohydrodynamic lubrication. Finally, the largest contributors to the friction force in these experiments were found to be viscous dissipation within the hydrogel and interfacial shear within the contact zone.

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Tribology Letters
vol. 18 (2005) pp 499-504