Electric Force Microscopy is analogous to standard MFM, except that gradients being sensed are due to electrostatic forces. In this method the cantilever is vibrated by a small piezoelectric element near its resonant frequency. The cantilever's resonant frequency changes in response to any additional force gradient. ¹

Attractive forces make the cantilever effectively "softer", reducing the cantilever resonant frequency. Conversely, repulsive forces make the cantilever effectively "stiffer", increasing the resonant frequency.¹

References

1. Scanning Probe Microscopy Training Notebook, Digital Instruments, Veeco Metrology Group
2. http://www.digitalinstruments.com/