TLeye2005 - Tribology Letters, Vol. 18, No. 4, April 2005 (...

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Friction coef±cient of soft contact lenses: measurements and modeling A.C. Rennie, P.L. Dickrell and W.G. Sawyer* Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA Received 12 October 2004; accepted 16 January 2005 Tribological conditions for contact lenses have very low contact pressures in the range 3–5 kPa and sliding speeds around 12 cm/s. Using a microtribometer a series of experiments was run on commercially available contact lenses made from Eta±lcon-A. These tests were run using 10–50 mN of normal load at speeds from 63 to 6280 l m/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 3–5 kPa. 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 l = 0.025 to 0.075. The calculated fluid ±lm thicknesses were between 1 and 30 nm. The average surface roughness of the lens and the glass sphere are R a =15 nm and R a =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. KEY WORDS: contact lens, hydrogel friction, microtribology, microtribometer 1. Introduction Hydrogels are polymer matrices that contain 30–70 % water [1] and are the primary material for soft contact lenses. Although soft materials are well suited for the eye, a signi±cant fraction of people are unable to wear contact lenses because of extensive eye irritation. There is an existing clinical hypothesis that ocular comfort is related to friction. The mechanical properties of hydro- gels are extremely sensitive to water content and humidity, and contact pressure is known to cause redistribution and/or reduction of the water content of hydrogel materials [2]. Therefore, tribological testing of hydrogels must be done under low contact pressure conditions. Nairn and Jiang [3] reported the friction coefficients of polyhydroxy ethyl methacrylate (p-HEMA) based lenses to be in the range of l = 0.060–0.115 using a custom-built pin-on-disk tribometer. The contact pres- sure in their study was 3.5 kPa. Freeman et al . [4] reported the friction coefficient of flat disks of p-HEMA hydrogels to be in the range of l = 0.02–1.7 over a wide range of applied load, lubrication, hydrogel crosslink density, and degree of hydrogel hydration again using a special purpose tribometer.
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TLeye2005 - Tribology Letters, Vol. 18, No. 4, April 2005 (...

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