Role of the adhesion between a nanotip and a soft material in tapping mode AFM

The present work investigates the role of the adhesion in the oscillation behavior of a tip cantilever system when soft materials are studied. To do so, the attractive interaction between a sphere and a concave surface is calculated with the aim of predicting the change in adhesion with indentation depth. The net result is an increase in adhesion as a function of the indentation depth. Then, the adhesion expression is included in equations describing intermittent contact situations. The computed variation of the oscillation amplitude differs significantly from that obtained when uniquely a sphere–plane interaction is considered. The predicted behavior is in good agreement, particularly at the bifurcation spot, with experimental results obtained on soft materials.