Viscoelastic spectra of soft polymer interfaces obtained by noise analysisof AFM cantilevers

The dynamic properties of the environment of an atomic force microscopy (AF M) tip can be probed by observing the cantilever's mechanical noise, In the absence of viscoelastic dispersion, the power spectral density is well app roximated by the resonance of a simple harmonic oscillator. By fitting reso nance curves to the data, the friction coefficient xi, the effective mass n l and the effective spring constant kappa are obtained. The variation of th ese parameters with the cantilever-substrate distance reflects the hydrodyn amic and viscoelastic interactions between the cantilever and the surface, For cantilevers approaching soft polymer brushes, one finds an elastic inte raction extending to about the brush height. The viscous interaction, on th e other hand, is more long-ranged. For tips deeply immersed into the brush the noise spectra are no longer described by a simple harmonic oscillator. There is an excess noise at low frequencies that we attribute to viscoelast ic dispersion. Hydrodynamic interactions, which should also induce deviatio ns from the pure resonance behaviour should be largely screened by the poly mer chains. By use of the fluctuation dissipation theorem, the viscoelastic spectrum xi\omega\ can be derived explicitly from the noise power spectrum . Copyright (C) 1999 John Wiley & Sons, Ltd.