In Situ Nanoscale Dynamic Contact Mechanics of Compliant Materials

Studying surface interactions and mechanical responses of soft biological materials have been receiving much attention in recent years. As the scale of contacts between surfaces decreases below a micron, surface forces and adhesion become important. This presentation introduces a quantitative, in-situ dynamic measurement technique to examine nanoscale adhesive contacts to soft materials. In this method, the probe tip oscillates at a frequency near its resonance as it approaches the sample surface. Any change in dynamic stiffness due to tip-surface interaction results in a significant phase shift sensed by the probe tip and allowed examination of adhesive and repulsive interactions. Notably, the resulting measurement may be best termed an “interaction stiffness curve” as measured is a convolution of tip-surface interaction potential and not actual contact stiffness. We found frequency-dependent interaction stiffness for viscoelastic samples even before the tip makes contact with the surface. The interaction stiffness curve can be fit to deconvolute the mechanical properties, loss and storage moduli, adhesion energy, shear viscosity, and strain energy release rate for various soft compliant materials.