Abstract
Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces. Since the real contact area of sliding solids is much smaller than the apparent contact area, the nanobridges formed on the distant asperities can contribute significantly to the overall friction. Therefore, it is essential to understand how the water nanobridges mediate the ‘noncontact' friction, which helps narrow the gap between our knowledge of friction on the microscopic and macroscopic scales. Here we show, by using noncontact dynamic force spectroscopy, the single capillary bridge generates noncontact friction via its shear interaction. The pinning–depinning dynamics of the nanobridge's contact line produces nonviscous damping, which occurs even without normal load and dominates the capillary-induced hydrodynamic damping. The novel nanofriction mechanism may provide a deeper microscopic view of macroscopic friction in air where numerous asperities exist.
Highlights
Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces
The water bridge can be formed in the nanometric gap between two distant asperities[17], with its nanometric height maintained constant, and results in the noncontact friction, unlike in dry condition, even in the absence of normal load on the solid–solid contact
The liquid bridge has been usually considered to add an additional normal load, increasing the microscopic real contact area and thereby enhancing the overall friction[10,11,18]. Such an indirect effect does not describe the friction that the bridges may produce directly between contact as well as noncontact nano-asperities. This direct contribution is important, in particular, because the distant asperities consist of the large part of the apparent contact area beyond the real contact area (Fig. 1a)[19,20] and because the water bridge can be formed in the gap of distant asperities with separation less than B10 nm in ambient condition[21]
Summary
Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces. The liquid bridge has been usually considered to add an additional normal load, increasing the microscopic real contact area and thereby enhancing the overall friction[10,11,18] Such an indirect effect does not describe the friction that the bridges may produce directly between contact as well as noncontact nano-asperities. This direct contribution is important, in particular, because the distant asperities consist of the large part of the apparent contact area beyond the real contact area (Fig. 1a)[19,20] and because the water bridge can be formed in the gap of distant asperities with separation less than B10 nm in ambient condition[21] (see Supplementary Note 1).
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