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Friction 2022, 10(3): 360-373 https://doi.org/10.1007/s40544-020-0458-0
Research Article | Open Access | Issue | Published: 23 November 2020

Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

Show Author's Information Wenrui LIU1,2,  Rok SIMIČ2( ),  Yuhong LIU1( ),  Nicholas D. SPENCER2
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
Keywords:
superlubricity, aqueous lubrication, hydrogels, friction coefficient (COF), contact geometry
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LIU W, SIMIČ R, LIU Y, et al. Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures. Friction, 2022, 10(3): 360-373. https://doi.org/10.1007/s40544-020-0458-0
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Abstract Full text About this article

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.


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About this article

Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

Show Author's information Wenrui LIU1,2, Rok SIMIČ2( ), Yuhong LIU1( ), Nicholas D. SPENCER2
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland

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: superlubricity, aqueous lubrication, hydrogels, friction coefficient (COF), contact geometry

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Publication history
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Acknowledgements
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Publication history

Received: 27 July 2020
Revised: 14 September 2020
Accepted: 25 September 2020
Published: 23 November 2020
Issue date: March 2022

Copyright

© The author(s) 2020

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51875303) and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No 669562). Wenrui Liu wishes to acknowledge the Chinese Scholarship Council (CSC) for its support. The authors would also like to express their gratitude to Dr. André Brem for help with the rheometer setup.

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