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Thomas E. Angelini, Alison C. Dunn, Juan M. Uruena, Howard A. Ketelson, W G. Sawyer; Stress Induced Frictional Transitions in Cross-Linked Surface Gels. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6113.
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To determine the contact pressure limits on lubricity for a lightly crosslinked gel surface layer as a function of cross-link density. This study examines the friction response of the gel layer under varied loads and sliding speeds and correlates the changes in lubricity to contact pressure and the mechanics of the gel surface.
A series of experimental and minimally-crosslinked 2-3 um thick gradient gel layers (5) were created on silicone hydrogel contact lens surfaces (DAILIES TOTAL1®). Friction measurements were carried out on a custom micro-tribometer under low sliding speeds of v=20 um/s and very low pressures down to single kPa. Instrumented indentation experiments were performed using a micro-indentation platform, and the elastic constants of the gels were determined from these experiments.
The majority of samples showed a clear transition between dynamic stick-slip friction (mu~0.25 with fluctuations of ~0.25) to extremely smooth and low friction (mu~0.03 with no measurable fluctuations) at forces ranging 230-490 mN. Based on the contact mechanics experiments performed on these lenses this corresponds to contact pressures between 9-13 kPa. There was a clear and monotonic trend indicating that gel stiffness and the transition pressure increase with increasing crosslinked density. All gels transitioned at surface deformations on the order of their thicknesses, thus it is hypothesized that the mechanisms of these frictional transitions is due a collapse of the fluid retaining network.
The crosslinked density and resulting elasticity define the critical transition pressure between stick-slip and smooth friction. This transition deformation is comparable to gel thickness. At pressures greater than the threshold, the surface gel layer collapses locally, causing undesirable stick-slip friction. The conditions under which a soft surface gel layer on a bulk soft contact lens provides smooth sliding have been identified.
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