When a gecko moves on a ceiling it makes use of adhesion and stiction. Stiction—static friction—is experienced on microscopic and macroscopic scales and is related to adhesion and sliding friction1. Although important for most locomotive processes, the concepts of adhesion, stiction and sliding friction are often only empirically correlated. A more detailed understanding of these concepts will, for example, help to improve the design of increasingly smaller devices such as micro- and nanoelectromechanical switches2. Here we show how stiction and adhesion are related for a liquid drop on a hexagonal boron nitride monolayer on rhodium3, by measuring dynamic contact angles in two distinct states of the solid–liquid interface: a corrugated state in the absence of hydrogen intercalation and an intercalation-induced flat state. Stiction and adhesion can be reversibly switched by applying different electrochemical potentials to the sample, causing atomic hydrogen to be intercalated or not. We ascribe the change in adhesion to a change in lateral electric field of in-plane two-nanometre dipole rings4, because it cannot be explained by the change in surface roughness known from the Wenzel model5. Although the change in adhesion can be calculated for the system we study6, it is not yet possible to determine the stiction at such a solid–liquid interface using ab initio methods. The inorganic hybrid of hexagonal boron nitride and rhodium is very stable and represents a new class of switchable surfaces with the potential for application in the study of adhesion, friction and lubrication.
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We acknowledge financial support by the Swiss National Science Foundation within the funding instrument ‘Sinergia’. S.F.L.M. acknowledges the receipt of an FP7 Marie Curie European reintegration grant, ERC grant OxideSurfaces and, together with S.D.F., support by FWO–Vlaanderen. We thank M. Schmid for help with the depiction of the STM images, G. Schütz for discussions and A. P. Seitsonen for the artwork in Fig. 1g.
The authors declare no competing financial interests.
Extended data figures and tables
a, h-BN/Rh(111) thin-film sample and sample holder ready for the desorption experiment in the ultrahigh-vacuum chamber. b, Thermal desorption experiment with temperature and power data shown in the top panel and pressure data in the bottom panel. The purple data are pyrometer readings, which were calibrated to a previous thermocouple measurement using a Rh crystal. The simulated temperature is a solution of equations (1) and (2) with the applied power and a starting temperature of 25 °C as input variables.
The substrate potential during scanning (image scanned from bottom to top) was switched from E1 = 0 V (deintercalated, yellow arrow) to E2 = −0.25 V (intercalated, light blue arrow) at about one-fifth of the way from the lower edge of the STM image. On the timescale of imaging, intercalation-induced flattening of the surface is instantaneous, in sharp contrast to deintercalation (compare with Fig. 1e). Image size, 66 nm × 66 nm; tunnelling current, 0.1 nA; tip potential fixed at −0.45 V.
Advancing, followed by receding electrolyte drop on h-BN/Rh(111), at an applied substrate potential E = +0.1 V
Advancing, followed by receding electrolyte drop on h-BN/Rh(111), at an applied substrate potential E = +0.1 V, where the nanomesh occurs in its normal corrugated state. The video shows all images captured between t = 68 s and t = 80 s in Figure 3a. (MOV 4472 kb)
Advancing, followed by receding electrolyte drop on h-BN/Rh(111), at an applied substrate potential E = –0.35 V
Advancing, followed by receding electrolyte drop on h-BN/Rh(111), at an applied substrate potential E = –0.35 V, where hydrogen intercalation has caused flattening of the nanomesh. The video shows all images captured between between t = 95 s and t = 115 s in Figure 3b. (MOV 7657 kb)
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Mertens, S., Hemmi, A., Muff, S. et al. Switching stiction and adhesion of a liquid on a solid. Nature 534, 676–679 (2016). https://doi.org/10.1038/nature18275
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