Growth kinetics of a nanoprotuberance under the action of an oscillating nanotip

The atomic force microscope is a versatile tool that allows many routes to be used for investigating the mechanical properties of soft materials on th e nanometer scale. In the present work, experiments were performed on polys tyrene polymer films of various molecular weight by approaching a vibrating nanotip towards the surface. The variation of the oscillating amplitude of the cantilever is interpreted as the result of the growth process of a nan oprotuberance. The growth rate is found to be dependent of the magnitude of the oscillating amplitude and of the molecular weight. A model is develope d describing in a very simple way the action of the tip and a viscoelastic response of the polymer. The numerical simulation-helps in understanding th e nonlinear relation between the growth rate and the vibrating amplitude of the microlever and describes qualitatively most of the experimental featur es. For the softer material, experimental situations are found that allow t he experimental results to be amenable with an analytical solution. The ana lytical solution provides a fruitful comparison with the experimental resul ts showing that some of the nanoprotuberance evolution cannot be explained with the approximation used. The presents results show that there exists a new and fascinating route to better understand the mechanical response at t he local scale. [S0163-1829(99)03003-9].