are shown to arise mainly from the experimental uncertainty in determining the separation upon contact due to nanoscale surface roughness.

Notably at short separations (comparable to the total sphere-plate rms roughness $w_{sph}+w$) the influence of surface roughness is also significant. The roughness can also strongly increase the force that complicates the situation to a significant degree. In any case, further experimental work is necessary in combination with smoother surfaces to minimize roughness contributions. Roughness is a rather strong barrier because the measurements we did already are at the limits of realistically possible sphere smoothness. The influence of the water layer will be weaker if it wets incompletely the metal surfaces. It can happen due to presence of hydrocarbons and other chemicals leading to a relatively large contact angles $80^{\circ}$ [@But05; @Zwo08d].

[**Acknowledgements**]{} The research was carried out under project number MC3.05242 in the framework of the Strategic Research programme of the Materials innovation institute M2i (the former Netherlands Institute for Metals Research (NIMR)). Financial support from the Materials innovation institute M2i is gratefully acknowledged.

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A. Cleland, [*Foundations of Nanomechanics*]{} (Springer, New York, 2003); P. Ball, Nature [**447**]{}, 772 (2007); F. Capasso, J. N. Munday, D. Iannuzzi, and H. B. 