Scanning Force Microscopy (SFM) has become a standard tool to image and manipulate surfaces with nanometer resolution. In order to minimize sample deformations due to the tip interaction, in particular when dealing with soft biological samples, the SFM images are usually taken by using different dynamic operation modes [1]. Phase contrast images, obtained by simultaneously recording the phase lag of the cantilever oscillation relative to the driving signal, often provides significantly more contrast than the topographic image. At fixed feedback amplitude, phase shift variations are directly linked to energy dissipation processes [2][3][4]. However, most of the phase and energy dissipation images are purely qualitative, mainly due to the absence of simple relationships relating phase changes and energy dissipation with specific surface properties.As a general approach, power dissipation in AM-AFM is naturally considered synonymous of energy dissipated per cycle. Assuming that the dissipation takes place in each oscillation cycle, it would be proportional to the oscillation frequency, ν 0, i.e. P dis =ΔEν 0 , being ΔE the energy dissipated in the contact process. In striking contrast to this apparently natural argument, we show that the time-averaged dissipated power is not always proportional to ΔE due to a beating phenomenon where the interaction is occasionally dissipative [5,6].In air ambient condition, the phase contrast is strongly influenced by capillary forces [5]. When the tip approaches the sample, water condensation from the humidity can induce the formation of a nanometer-sized water bridge. In this work we analyze the energy dissipation process involved in the formation and rupture of a nanometer-sized capillary-condensed water bridge (see Fig. 1). With the help of numerical simulations, dissipation contrast in AM-AFM is shown to be a result of a non-trivial interplay between the energy dissipated in each rupture process and the bi-stable motion of the cantilever. In the repulsive regime (see Fig. 2), the dissipated power is approximately constant and independent of the amplitude as expected. In contrast, in the attractive regime, after the contact process, the cantilever, which has lost energy, will not reach the same amplitude as before the contact, and the tip may not hit the sample surface during the next swings. The power dissipation is then lower than expected.In the repulsive regime, the dissipated power is a function of the tip and sample contact angles being independent of the elastic properties of the system. Working in this regime, energy dissipation images in air can be regarded as surface hydrophobicity maps.

A method to calculate electrostatic forces in the context of atomic force microscopy that is useful for the calculation of the electrostatic forces when different length scales are included in the simulation is presented. The versatility of the method allowed for an analysis of the behavior of forces as a function of the tip apex geometry. For example, for flattened, worn out tips, the force at the point of contact with a dielectric sample could be 2.5 times larger than that of a sharp tip. A simple analytical approximation has been also developed for the local characterization of thin films at the nanoscale.

The sticking effect between hydrophilic surfaces occurring at increasing relative humidity (RH) is an everyday phenomenon with uncountable implications. Here experimental evidence is presented for a counterintuitive monotonous decrease of the capillary adhesion forces between hydrophilic surfaces with increasing RH for the whole humidity range. It is shown that this unexpected result is related to the actual shape of the asperity at the nanometer scale: a model based on macroscopic thermodynamics predicts this decrease in the adhesion force for a sharp object ending in an almost flat nanometer-sized apex, in full agreement with experiments. This anomalous decrease is due to the fact that a significant growth of the liquid meniscus formed at the contact region with increasing humidity is hindered for this geometry. These results are relevant in the analysis of the dynamical behavior of nanomenisci. They could also have an outstanding value in technological applications, since the undesirable sticking effect between surfaces occurring at increasing RH could be avoided by controlling the shape of the surface asperities at the nanometric scale.

The effects of adhesion hysteresis in the dynamic‐dissipation curves measured in amplitude‐modulation atomic force microscopy are discussed. Hysteresis in the interaction forces is shown to modify the dynamics of the cantilever leading to different power dissipation curves in the repulsive and attractive regimes. Experimental results together with numerical simulations show that power dissipation, as measured in force microscopy, is not always proportional to the energy dissipated in the tip–sample interaction process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Power dissipation, as measured in dynamic force microscopy, is usually considered to be proportional to the energy lost in the contact process through the oscillation period. In the presence of adhesion hysteresis, the dissipation is not triggered in each oscillation and only a fraction of the cycles contributes to the dissipated power. We derive a simple analytical closed expression for the dissipated power versus the oscillation amplitude which is in good agreement with the results of numerical simulations. Our theoretical and numerical results predict the existence of two possible dissipation regimes. In the first one, the power dissipated is linear with the energy dissipated by the substrate as expected. In the second regime, a beating pattern appears, causing the fraction of cycles undergoing hysteresis to be inversely proportional to the energy dissipated. As a consequence, the measured power dissipation is a function of a characteristic dissipation length and it is not proportional to the energy lost in a single adhesion process.