work [@Zwo08a]. Electrostatic fitting in the range of distances $1-4\;\mu m$ and voltage interval $\pm (3-4.5)\;V$ yielded the cantilever stiffness $k=8.55\pm 0.38\;N/m$ (with the sphere attached) and contact potential $V_0=10\pm 10\; mV$. After calibration, the dispersive force was measured and averaged (using 40 force curves at 20 different locations yielding an average of 800 curves). The result is shown in Fig. \[fig3\] together with the theoretical curves (see Sec. \[Sec3\]) without (red) or with (black) adsorbed water layer. Each experimental point (circles) is defined with rather large uncertainty in distance, which are shown by the horizontal bars for some points. Indeed, the main uncertainty $\Delta F$ in the dispersive force at short distances comes from the uncertainty $\Delta d =1\; nm$ in the separation upon contact $d_0=7.5\pm 1\; nm$. The error in the force due to error in $d_0$ can be estimated as $(\Delta F/F) \approx c(\Delta d_0/d_0) $, where $c\simeq 2.5$ in the investigated range of distances. Other sources of errors include the error of the cantilever spring constant $\Delta k/k\approx 4\%$ and the error in the radius of the sphere $\Delta R/R=2 \%$. These errors propagated to the force are negligible in comparison with that arising from $\Delta 