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.
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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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Wenrui LIU. He received his bachelor degree from the University of Science and Technology Beijing, China, in 2016. He is currently a Ph.D. student in the State Key Laboratory of Tribology at Tsinghua University, China. From August 2019 to June 2020, he worked with Prof. Nicholas D. Spencer as a visiting Ph.D. student at the Laboratory of Surface Science and Technology at ETH Zurich, Switzerland. His research interests include hydrogel friction, liquid superlubricity, and surface & interface analysis.
Rok SIMIČ. He obtained his bachelor degree in physics in 2009 and a Ph.D. degree in mechanical engineering in 2013, both at the University of Ljubljana, Slovenia. During his Ph.D. period, he worked in the group of Prof. Mitjan Kalin studying lubrication of diamond-like carbon coatings. Since 2015, he has been a postdoctoral researcher in the group of Prof. Nicholas D. Spencer at ETH Zurich. The focus of his current work lies on soft matter, in particular friction, surfaces, and mechanics of hydrogels. His research spans from the fundamental science of hydrogels to biomedical applications such as contact lenses. He has published 20 peer-reviewed papers, supervised several postgraduate and Ph.D. students, and held training courses and coordinated projects.
Yuhong LIU. She received her Ph.D. degree in CAS Key Laboratory of Molecular Nanostructure & Nanotechnology Institute of Chemistry, Chinese Academy of Sciences, Beijing, China, in 2005. She joined the State Key Laboratory of Tribology at Tsinghua University in 2005. Her current position is an associate professor. Her research areas cover nanotribology, nanostructure & nanotechnology of surface and interface, chemical-mechanical planarization, and water-based lubrication.
Nicholas D. SPENCER. He is a professor of surface science and technology, ETH Zurich, obtained his bachelor and Ph.D. degrees at the University of Cambridge, before working as a postdoc at the University of California, Berkeley. After a decade in the US chemical industry, he joined the ETH Zurich in 1993, where he has served as both the head of the Department of Materials and the President of the Research Commission. He is co-founder of Swiss Tribology and the founder and Chair of the International Tribology Forum. He is also co-founding Editor and Editor in Chief of the journal Tribology Letters. He is a member of the Swiss Academy of Engineering Sciences, a fellow of both the Royal Society of Chemistry (UK) and the Society for Tribologists and Lubrication Engineers, and was awarded the 2018 Tribology Gold Medal. He has published over 440 papers in academic journals, edited several monographs, and holds over a dozen patents.
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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 10, 360–373 (2022). https://doi.org/10.1007/s40544-020-0458-0
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DOI: https://doi.org/10.1007/s40544-020-0458-0