The effect of sample viscoelastic properties and cantilever amplitudes on maximum repulsive force, indentation, and energy dissipation in bimodal AFM

Abstract

Bimodal atomic force microscopy (AFM) uses two eigenfrequencies to map nanomechanical properties with high spatial and temporal resolution. To reliably map surface properties and to understand the links between experimental observables, energy dissipation, and viscoelastic properties considering the effects of nonconservative interaction forces is essential. To avoid damaging the sample, the maximum force between the tip and the surface and the maximum indentation of the tip into the sample needs to be controlled. In this work, we use both experiments and simulations to study how viscoelastic properties affect the cantilever response in bimodal AFM. We simulate the tip-sample interaction force, indentation, and energy dissipation for samples with different viscous properties. Under the tested operating conditions, we observe that more energy is dissipated in the higher eigenmode. The larger higher eigenmode free amplitude increases the energy dissipation in both eigenmodes. The larger energy dissipation increases the contrast of the bimodal AFM dissipation map. The simulations are cross-compared with experiments and similar trends are observed. This work is important for understanding and optimizing bimodal AFM measurements on samples with significant viscoelastic responses, such as cells, tissues, and polymers.