Capillary forces in tapping mode

It is known that for an asymmetric back-and-forth motion of an oscillating tip, dissipation occurs during an oscillation cycle. Such an asymmetric motion can be due to the adhesion force when the tip touches the surface that gives a backward oscillating path different from the forward path. Therefore the adhesion leads to a loss of energy either in tapping or noncontact AFM mode. When operated in an ambient condition, a nanomeniscus may form between the tip and the surface at each oscillation and modify the imaging process and mechanical information, especially with soft objects. This is more detrimental than in contact mode, for which the presence of the meniscus is less problematic. 

The influence of capillary forces in tapping mode was investigated through the measurements of amplitude A and phase p curves as a function of the tip-surface distance. While these two values are directly available with the AFM instruments, another important quantity, that is, the average dissipated power, can be deduced 

Because the variations of the amplitude and of the phase depend both on the conservative and dissipative parts, the tapping mode is a more complex mode to use compared to the frequency modulation (FM) mode. In particular, the tapping oscillating behavior depends on the initial experimental condition chosen, oscillation amplitude A, and value of the frequency forcing co. 

For instance, for a cantilever oscillating at its resonance frequency, below an amplitude threshold, the oscillator may only experience the attractive regime at proximity of the surface (curves A in Fig. 9.8). The amplitude and phase signals are superimposed for approach and retraction curves, while when the amplitude is increased above this threshold, which depends on the RH and hydrophilicity of the tip, a h eresis shows up both in amplitude and phase signals. At given tip-sample distances, the system switches from attractive to repulsive interaction.  

More recently, Sahagun et al. extended these investigations to the dissipated energy, which is a crucial issue if one intends to use the dissipation as characteristics of the wetting properties of the sample and therefore map its hydrophobicity. This is of particular interest for biological applications for which the recognition of 

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L. Zitzler, S. Herminghaus, and F. Mugele, Capillary forces in tapping mode atomic force microscopy, Phys. Rev. B, 66,155436 (2002). 

Furthermore, height anomalies have been found in the measuronent of the size of biomolecules using tapping mode AFM. When images are obtained in air, the adhesive and capillary forces, due to the presence of a water layer on the surface. 

Soft biopolymers such as DNA and RNA are prone to tip-induced damage, often resulting from the relatively large capillary forces (when imaging in air) or friction forces experienced in contact mode, hi this latter case, tapping mode atomic force microscopy (TM-AFM) may be used [40-42]. We will briefly discuss the most commonly employed modes of operation of AFM that are routinely used to obtain sample topography. The most common modes are contact mode AFM (CM-AFM) [2,43] and TM-AFM [44,45]. 

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