The electrostatic force that deflects the cantilever is similar to the
force on the plates of a capacitor, given by the equation shown above.
It is proportional to the square of the voltage between the probe and
sample, and the spatial derivative of the total capacitance.  In the SKPM,
the total voltage consists of externally applied voltages and the work
function difference (WFD) of the materials.  The cantilever is deflected
via an applied AC voltage, which is detected with an interferometer.  The
magnitude of deflection is proportional to the capacitance, the peak AC
voltage, and the WFD.  The magnitude may be minimized by applying a DC
voltage equal to the WFD.  The probe interacts with multiple sites on the
sample, so the resulting DC voltage becomes an average of many WFDs and
capacitances.