Brownian fluctuation spectroscopy using atomic force microscopes

We examine the use of the thermal fluctuations of the cantilever of an atom ic force microscope to study the microrheological behavior of fluids near a solid/liquid interface. A model-independent approach is used far the analy sis of power spectral densities and to extract frequency-dependent dissipat ive and induced-mass contributions of the fluid to the force experienced by the cantilever. The approach provides a framework for the calibration of A FM cantilevers using thermal fluctuations in viscous fluids and for extract ing the loss and storage moduli of viscoelastic fluids. The results show th at in viscous fluids the excess zero-frequency viscous dissipation (i.e., r elative to the magnitude in the bulk) caused by a nearby surface scales inv ersely with the distance between the cantilever and the surface, in contras t to the inverse cubic scaling assumed in the literature. Interestingly, th e observed scaling is practically identical to what is expected for the inc rease in the hydrodynamic drag on a sphere descending normally toward a fla t surface at low Reynolds numbers.