Dynamic force microscopy in fluid

Low-amplitude dynamic force microscopy can operate in a non-contact mode, sensing changes in liquid properties near a surface. Operation of the microscope in water at the higher amplitudes often required for stable imaging has been investigated. When driven by direct application of a force to the tip, the microscope is stable over a wide range of operating frequencies. At low frequency, the interfacial stiffness extracted from approach curves is found to be of the order of 1 N m^-1 on first contact, which is indicative of imaging via a compressed liquid layer. Measurements of the spectral response of the cantilever and numerical simulations confirm this and show that viscous damping at the surface also plays a role.