Mesh Size Control of Polymer Fluctuation Lubrication in Gemini Hydrogels

Polyacrylamide (PAAm) hydrogels are excellent synthetic materials for in vitro biotribology studies. Recent work with hydrogels sliding in a Gemini contact has revealed unique friction behavior at low speed that is contrary to the classic Stribeck curve. In these interfaces the friction coefficients are minimum at low speeds and appear to be speed-independent. In this report, we investigate the role of mesh size, ξ, on the low friction regime, termed thermal fluctuation lubrication, and we also explore the origins of a transition from this behavior at higher speeds to polymer relaxation lubrication. PAAm hydrogels of varying concentration were prepared and tested in a Gemini configuration using a pin-on-disk microtribometer with an applied load of 2 mN and over a range of sliding speeds from 0.03 mm/s to 100 mm/s. We found that increasing mesh size or decreasing polymer concentration promotes lower friction coefficients. Many samples underwent a transition from a low friction behavior to an increasing friction coefficient with increasing sliding speed that scaled with speed to the 1/2 power. This transition speed was found to correlate with the mesh size and relaxation time of the polymer network.