surfaces) are separated by the smaller distance $d-2h$. As one can see the effect of water becomes very significant at separations $d$ below $10\;nm$, which are not accessible in this study. We presented only the forces between flat surfaces because at these small separations there is no a reliable way to estimate the roughness correction. The method developed in [@Mia05] can be applied only for $d\gg w+w_{sph}$; for the system under investigation this condition is $d\gg 2.3\;nm$. At distances where the theory is not applicable it was demonstrated experimentally [@Zwo08a] that the roughness correction increases very significantly, although it was proven for much rougher surfaces than those in the present study.

The effect of water layer on the dispersive force for experimentally investigated separations is shown in Fig. \[fig3\] by continuous and dashed black curves, where we included also the effect of roughness. As one can see a continuous layer of a thickness of $h=1.5\; nm$ is already excluded by the experiment on the level of one standard deviation. However, if there is a discontinuous layer of water containing 50% of air, it is still in agreement with the experiment. We can conclude that the effect of water layer on 