Nonlinear response in nanoshear modulation force microscopy: Fourier analysis

Fourier analysis of oscillating forces at a shear-modulated tip provides new insight into static-to-kinetic friction transitions (stiction). In addition to contrast in conventional friction force microscopy, layers of autophobically dewetted PVA films exhibit remarkable differences in stiction. These differences relate to strong adsorption of first layer to mica substrate and concomitant conformational arrest, as compared to bulk-like behavior in the second layer. The third Fourier harmonic is found to be a sensitive gauge to variable degrees of sliding as a function of both drive amplitude and normal load (tensile to compressive). For a nanoscale drive, it is discovered that a largely static contact at compressive loads becomes a largely sliding contact at tensile loads. This finding has implications for the analysis of shear modulation force microscopy of polymers in the context of contact mechanics models, and for studies under variable sample compliance (temperature or solvent induced).