Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures Wenrui LIU1,2, Rok SIMIČ2,*, Yuhong LIU1,*, Nicholas D. SPENCER2 1
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
2
Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
Received: 27 July 2020 / Revised: 14 September 2020 / Accepted: 25 September 2020
© The author(s) 2020. Abstract: Polyacrylamide (PAAm) hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries: sphere-on-flat and flat-pin-on-flat. Irrespective of the contact geometry, the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of friction µ (COF) compared to the crosslinked surfaces, even achieving superlubricity (µ < 0.01). In general, a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds. However, after normalizing the friction force by the contact area, the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds, indicating that hydrogel friction is unaffected by the contact geometry at these speeds. At high sliding speeds, the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration. This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact, which presumably enhances the water supply in the contact, promotes rehydration, and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact. Keywords: hydrogels; contact geometry; aqueous lubrication; friction coefficient (COF); superlubricity
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Introduction
Hydrogels are soft materials consisting of a threedimensional, cross-linked polymer network containing a large amount of water. The high water content (> 90 wt%) in combination with good lubricating properties of hydrogels makes them good materials to construct polymeric analogues of articular cartilage [1–3] or other tissues subjected to rubbing, including the trachea [4], skin [5, 6], and blood vessels [7]. In order to develop hydrogels that would closely mimic natural lubrication systems and be used for medical applications, it is of great importance to understand their lubrication mechanisms. Numerous
studies have been conducted to investigate the tribological properties of hydrogels. However, the dissipation mechanisms of hydrogel friction are not yet completely understood, which impedes the achievement of any improvement in their lubrication performance. Currently, hydrogel friction is found to largely depend on the normal load, the relative sliding speed, and the surface structure of hydrogels.
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