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We reveal the ultralow friction or superlubricity of water nanodroplets containing cations and anions on graphene substrates at high ion concentration by molecular dynamics simulations. When the ion concentration is higher than 7 wt.% and the nanodroplet diameter is larger than 10 nm, the friction coefficients of water nanodroplets are lower than 10−2, and can decrease to the order of 10−3 with increasing the ion concentration further. At a certain ion concentration, the optimal nanodroplet diameter of 17–20 nm exists at which the friction coefficient is the lowest. The ultralow friction behaviors of water nanodroplets containing cations and anions are mainly attributed to the opposite variation trends between the interfacial adhesion energy and surface energy of water nanodroplet with ion concentration, and the interfacial hydrophobicity sustained by high ion concentration. These results unveil the essential role of ions in achieving the superlubricity of water nanodroplets.

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Publication history

Received: 16 May 2022
Revised: 02 July 2022
Accepted: 17 July 2022
Published: 04 August 2022

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© Tsinghua University Press 2022

Acknowledgements

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Nos. 11972186, 11890674, and 51921003), the Western Light Project of the Chinese Academy of Sciences (CAS) (No. xbzg-zdsys-202118), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Email: nanores@tup.tsinghua.edu.cn

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