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How drops start sliding over solid surfaces


It has been known for more than 200 years that the maximum static friction force between two solid surfaces is usually greater than the kinetic friction force—the force that is required to maintain the relative motion of the surfaces once the static force has been overcome. But the forces that impede the lateral motion of a drop of liquid on a solid surface are not as well characterized, and there is a lack of understanding about liquid–solid friction in general. Here, we report that the lateral adhesion force between a liquid drop and a solid can also be divided into a static and a kinetic regime. This striking analogy with solid–solid friction is a generic phenomenon that holds for liquids of different polarities and surface tensions on smooth, rough and structured surfaces.

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Figure 1: Schematics of friction force measurements.
Figure 2: Lateral adhesion force experiment of a drop of ionic liquid (volume ≈1.5 μl) on a fluorinated silicon wafer.
Figure 3: Lateral adhesion forces for drops of different liquids on solid surfaces.
Figure 4: Velocity dependence of lateral adhesion forces.
Figure 5: Lateral adhesion force measurement of a water drop on a goose feather.


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We thank G. Auernhammer, M. Bonn, N. Encinas, M. Kappl, T. Kajiya, P. Papadopoulos, F. Schellenberger, W. Steffen and D. Wang for simulating discussions, and M. Bach, G. Glaser and G. Schäfer for technical support. This work was supported by the Collaborative Research Center 1194 (H.-J.B.), ERC advanced grant 340391 SUPRO (H.-J.B.), SPP 8173 (D.V.) and the EU Marie Sklodowska-Curie grant 722497 (D.V.). N.G. thanks the National Postdoctoral Science Foundation of China for the International Postdoctoral Fellowship, and S.W. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. D.W.P. is grateful for funding from the German National Academic Foundation.

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N.G. carried out the experiments and wrote the manuscript. D.W.P., N.G., R.B. and H.-J.B. designed and constructed the homebuilt set-up. F.G. and S.W. prepared the solid surfaces. R.B., D.V., N.G. and H.-J.B. contributed to the experimental planning, data analysis, and manuscript preparation. All authors reviewed and approved the manuscript.

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Correspondence to Nan Gao or Rüdiger Berger.

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The authors declare no competing financial interests.

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Gao, N., Geyer, F., Pilat, D. et al. How drops start sliding over solid surfaces. Nat. Phys. 14, 191–196 (2018).

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