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Force curves measured with silicon nitride tips on a cleaved mica surface immersed in deionized water. (a) The FFM is used to measure the full force curve (black curve) with a lever of stiffness k = 0.014 N/m. The vertical green bar marks the location of the jump to contact if the FFM protocol would be deactivated and a regular AFM approach curve would be performed. The blue and red curves (shifted upward for clarity) show that it is possible to track reversibly back and forth and with no hysteresis any portion of the attractive part of the interaction. (b) AFM conventional static mode with a lever of stiffness k = 0.028 N/m. As the tip approaches the surface, the uncontrolled jump to contact occurs when the gradient of the force exerted by the surface on the tip exceeds k. The characteristic hysteresis curve is well visible.
Block diagram of the FFM operation. Static mode (red loop): via the lever stiffness k gives directly the static force acting on the tip. Dynamic mode (blue loop): the tip oscillation is kept constant. The voltage and the phase applied to the piezoelement are measured quantities from which and are determined; d is the tip-surface distance.
Force curves measured by FFM in air on a hydrophilic silicon native oxide surface using a silicon tip and a lever of stiffness k = 0.35 N/m. When the tip-surface distance is around 10 nm, a capillary bridge suddenly forms and the force abruptly increases to about 10 nN. When the tip is further approached, the force keeps increasing up to 20 nN. On the return path, the hysteresis is well visible. The hysteresis is intrinsic to the capillary bridge properties and not related to the apparatus response. The inset shows the position of the tip measured during the experiment.
Force curves. Classical silicon nitride tip on a lever of stiffness k = 0.015 N/m. The tip interacts with the surface of freshly cleaved mica. The measurement is performed in deionized water at f = 1125 Hz. The tip oscillation is 0.2 nm. (a) Normalized piezoelectric excitation , where is the voltage applied to the piezoelement to keep the tip oscillation amplitude constant and as d is very large; (b) is the phase between and the tip oscillation. (c) determined using Eq. (1) applied to data (a) and (b); (d) damping determined using Eq. (2); (e) result of the numerical integration of , i.e., of the curve (c); and (f) the numerical integration of is compared to the measured force.
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