Identification of energy-dissipation processes at the nanoscale is demonstrated by using amplitude-modulation atomic force microscopy. The variation of the energy dissipated on a surface by a vibrating tip as a function of its oscillation amplitude has a shape that singles out the dissipative process occurring at the surface. The method is illustrated by calculating the energy-dissipation curves for surface energy hysteresis, long-range interfacial interactions and viscoelasticity. The method remains valid with independency of the amount of dissipated energy per cycle, from 0.1 to 50 eV. The agreement obtained between theory and experiments performed on silicon and polystyrene validates the method.

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The mechanical properties of nanoconfined water layers are still poorly understood and continue to create considerable controversy, despite their importance for biology and nanotechnology. Here, we report on dynamic nanomechanical measurements of water films compressed down to a few single molecular layers. We show that the mechanical properties of nanoconfined water layers change dramatically with their dynamic state. In particular, we observed a sharp transition from viscous to elastic response even at extremely slow compression rates, indicating that mechanical relaxation times increase dramatically once water is compressed to less than 3-4 molecular layers. Atomic Force Microscopy (AFM) and Surface Force Apparatus (SFA) measurements suggest that water layers confined between hydrophilic surfaces assume spontaneous order [4-6, 10, 11] and exhibit sharp increases in effective viscosity, relaxation times, and elasticity [4][5][6]. However, other measurements indicate that water under similar circumstances shows little change in effective viscosity [7]. It is also not clear if layering influences only the elastic response of the liquid or both the viscous and elastic response [5,[12][13][14][15]. Recent measurements have shown that nanoconfined liquids can exhibit sharp changes in viscoelastic properties in response to mild changes in their dynamical state [12,16,17]. To resolve these issues, it is therefore imperative to carefully measure the elastic and viscous response of nanoconfined water layers under different dynamic conditions. We used a small-amplitude (A=1Å) AFM technique [18], developed in our lab, to perform linear viscoelastic measurements of molecularly confined ultrapure water layers at extremely slow loading rates (Schematic see Fig. 1). Although we used ultrapure water, there could be a substantial amount of ions in solution originating from the freshly cleaved mica surface. Measurements were performed far below the resonance to ascertain well-behaved phase behavior of the cantilever motion. This ensures that phase changes corresponded to the dissipative behavior of the liquid and not the complicated phase behavior of the cantilever.The loading rate was controlled by the approach speed of an atomically flat mica surface towards a silicon AFM tip from 2Å/s to 14Å/s. At these speeds, the tip takes between 1.25 s to 0.18 s to traverse one molecular layer of water (width 2.5Å). This is extremely slow compared to molecular re-arrangement times. For the measurements, we immersed the cantilever and substrate in a liquid cell filled with pure water. We continuously measured the cantilever amplitude and phase using a very sensitive fiber interferometer while the sample was approached until contact with the mica surface occurred. zero, a strong static deflection of the cantilever, and a large change in the phase. From the phase and amplitude of the cantilever, we calculated the effective stiffness, according to, where k L is the cantilever stiffness, A 0 the drive piezo amplitude, A the measured c...

The authors demonstrate that the compositional sensitivity of an atomic force microscope is enhanced by the simultaneous excitation of its first two flexural eigenmodes. The coupling of those modes by the nonlinear probe-surface interactions enables to map compositional changes in several conjugated molecular materials with a phase shift sensitivity that is about one order of magnitude higher than the one achieved in amplitude modulation atomic force microscopy.

There has been a long-standing debate about the physical state and possible phase transformations of confined liquids. In this report, we show that a model-confined liquid can behave both as a Newtonian liquid with very little change in its dynamics and as a pseudosolid, depending solely on the rate of approach of the confining surfaces. Thus, the confined liquid does not exhibit any confinement-induced solidification in thermodynamic equilibrium. Instead, solidification is induced kinetically when the two confining surfaces are approached with a minimum critical rate. This critical rate is surprisingly slow (on the order of 6 Å/s), explaining the frequent observation of confinementinduced solidification.