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Elastohydrodynamic friction of robotic and human fingers on soft micropatterned substrates

Abstract

Frictional sliding between patterned surfaces is of fundamental and practical importance in the haptic engineering of soft materials. In emerging applications such as remote surgery and soft robotics, thin fluid films between solid surfaces lead to a multiphysics coupling between solid deformation and fluid dissipation. Here, we report a scaling law that governs the peak friction values of elastohydrodynamic lubrication on patterned surfaces. These peaks, absent in smooth tribopairs, arise due to a separation of length scales in the lubricant flow. The framework is generated by varying the geometry, elasticity and fluid properties of soft tribopairs and measuring the lubricated friction with a triborheometer. The model correctly predicts the elastohydrodynamic lubrication friction of a bioinspired robotic fingertip and human fingers. Its broad applicability can inform the future design of robotic hands or grippers in realistic conditions, and open up new ways of encoding friction into haptic signals.

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Fig. 1: Experimental set-ups and Stribeck curves for flat and patterned soft materials.
Fig. 2: EHL lubrication film thickness on patterned surfaces.
Fig. 3: Modelling the critical EHL transitions for patterned geometries.
Fig. 4: Material- and geometry-based framework for the transition EHL friction coefficient.

Data availability

Source data are provided with this paper. All other data that support the results in this study are available from the corresponding author on reasonable request.

Code availability

The MATLAB codes for solving Supplementary equation (S10) are available at https://doi.org/10.6084/m9.figshare.14233238.

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Acknowledgements

The authors thank R. Ewoldt, J. F. Brady, R. G. Larson, J. Frechette and A. Dunn for discussions. Y.P., C.M.S., C.N.H. and L.C.H. were supported in part by the National Science Foundation (NSF) through award no. CBET-2042635 and the AAAS Marion Milligan Mason Award. K.G. was funded by the Eugene V. Cota-Robles Fellowship from the University of California, Los Angeles. Y.V. was supported by the NSF through award no. 1751348.

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Contributions

Y.P. and L.C.H. designed the study and validated the theory. Y.P., C.M.S. and C.N.H. produced the micropatterned substrates. Y.P., A.K., Y.S. and Y.V. conducted the human finger experiments. Y.P., K.G. and V.J.S. conducted the robotic finger experiments. All authors reviewed the data and wrote the paper.

Corresponding author

Correspondence to Lilian C. Hsiao.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–12 and Tables 1–4.

Supplementary Video 1

Demonstration of sliding experiments between a robotic finger and soft patterned substrates.

Supplementary Video 2

Demonstration of sliding experiments between a human finger and soft patterned substrates.

Source data

Source Data Fig. 1

Raw data for Fig. 1i.

Source Data Fig. 2

Raw data for Fig. 2b.

Source Data Fig. 3

Raw data for Fig. 3.

Source Data Fig. 4

Raw data for Fig. 4.

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Peng, Y., Serfass, C.M., Kawazoe, A. et al. Elastohydrodynamic friction of robotic and human fingers on soft micropatterned substrates. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-00990-9

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