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Lecture : Water Content and Polymer Mesh Size Control Hydrogel Lubricity
8 december, 2016, 16:00 - 17:00Gratis
Permeable aqueous materials are ubiquitous in biology. All moist epithelia (including the cornea) are covered in a protective mucus barrier, which is a biopolymer-based hydrogel that has a mesh size on the order of hundreds of nanometers. The purpose of the work presented here is to determine the relationship between friction (lubricity) and mesh size, which is also directly related to water content, and to what degree lubricity is controlled by the mesh size. This study examines the friction behavior of a suite of very soft high water content hydrogels as a function mesh size and sliding speed over a range from hundreds of nanometers per second to blink speed (100 mm/s).
Lubricity experiments were conducted on a custom high-speed microtribometer at 2mN load over a range in sliding speeds from 200 nm/s to 100 mm/s. The configuration used a spherical probe and a flate plate both made of the same hydrogel formula with a wide range in mesh size, which was achieved by varying the crosslink concentration. The mesh size was measured using small angle x-ray scattering. Five samples that spanned a range in mesh size of nearly an order of magnitude (1.3±0.1 nm to 9.4±1.1 nm) were selected for testing.
The lubrication behavior of hydrogels was found to be different from the classical engineering Stribeck curve in several major ways. At slow sliding speeds hydrogel friction is low, even in the absence of fluid lubrication and a speed-independent regime was observed below a sliding speed of ~1 mm/s. Interestingly, above this transition speed the friction coefficient rises in proportion to the square root of the velocity (V½). Additionally, both the low friction coefficient at low speeds and the transition in friction coefficient at high speeds were found to be controlled by the hydrogel mesh size. Thus, hydrogels made of identical chemistry could span an order of magnitude in friction coefficient (µ0.10) depending on the mesh size. Such profound changes in behavior are thought to arise from the shearing of solvent through the surface mesh, and thereby changing the lubricity through a viscous dissipation mechanism.