Typical approach-retract curve

Typical Approach-Retract Curve. All the results presented for the following are focused in Figure 2b part b, at the spot where the oscillation amplitude just starts to change from the free one. Figure 10a shows typical results obtained on a hard surface (PS high molecular weight (M =284000) sample). 

Figure 9.14 shows a typical approach force curve along with schematic drawings of the relative positions of the SPM tip and the sample surface, as related to the force curve. At the start of the experiment, i.e., position A on the right-hand side of the figure, the tip is above the surface of the sample. As it approaches the surface the Z value decreases until at position B the tip contacts the surface. With further downward movement of the piezo the cantilever starts to be deflected by the force imposed on it by the surface. If the surface is much stiffer than the cantilever, we get a straight line with a slope of — 1, i.e., for every 1 nm of Z travel we get 1 nm of deflection (Une BC in Figure 9.14). If the surface has stiffness similar to that of the cantilever, the tip wUl penetrate the surface and we get a nonlinear curve with a decreased slope (line BD in Figure 9.14). The horizontal distance between the curve BD and the line BC is equal to the penetration at any given cantilever deflection or force. The piezo continues downward until a preset cantilever deflection is reached, the so-called trigger. The piezo is then retracted a predetermined distance, beyond the point at which the tip separates from the sample.

Another contact technique that can be used to analyze multiphase polymer films (i.e., local variations in the elastic properties of the surface) is force-distance spectroscopy. In this mode, the force-distance curves are plots of distance dependent on the forces that act on the tip in the vicinity of the surface. They are registered when the tip approaches the surface or is retracted from it (Maver et al. 2013). Typical approach/retract force-distance curves and their stages are presented in Figure 8.4 (1) the cantilever starts to approach the surface (2) the tip approaches the surface ... 

Figure 7 A typical force curve acquired by monitoring cantilever deflection as the piezoelectric raises and withdraws the sample surface from the tip. The approach and retract curves are dissimilar when strong chemical and/or physical attraction between the tip and the surface exists. Illustrations of the cantilever bending at various scanner positions are depicted above and below the force curve to assist the reader in interpreting the force curve.

Figure 14. LEFT typical force curve (black - approach curve, red - retract curve) RIGHT t5 pical plot of the extracted parameters as a function of measurement duration (approach labels were removed to increase plainness of the scheme). Reproduced with permission of the Royal Society of Chemistry from [57].

The data in Fig. 11 show an offset between the force and stiffness minima in the approach and retraction curves. The explanation for this is shown in Fig. 12, which illustrates the relationship between potential, force and interaction stiffness. These curves provide a basis for determining where the contact point with the surface is located. One definition of contact is the position on the curve where the repulsive force can first be detected (see 24), typically identified by a change in curvature of the force-displacement data (3). Therefore, the force gradient (stiffness-displacement data) reveals more clearly the attractive to repulsive transition. The initiation of repulsive contact is thus found from the minimum of the stiffness approach curve (marked at 0 nm), which marks the maximum attractive interaction stiffness. The stiffness data represent a convolution of force gradient and contact stiffness and is... 

Figure 1 Typical force-time curve acquired for microgel particle showing the approach (blue), dwell (black), and retract (red) portions. Adapted from Raz, N. Li, J. K. Fiddes, L. K. Tumarkin, E. Walker, G. C. Kumacheva, E. Macromoteco/es2010,43,...

AFM tips were held in contact with the PDMS samples for different dwell times to study the viscous relaxation. Figure 14 gives the typical force plots of 705 DP PDMS at different dwell times at a constant ramp rate of 0.1 Hz. When there is no delay, the retraction force curve starts at a more negative force. A longer dwell time causes a larger gap between the end of the approach curve and the beginning of the retraction curve. 

Figure 13(b) depicts typical force curves that were obtained when the interaction between neutral polymer films (both tip and substrate biased at —0.25 V versus polished silver reference electrode) and oxidized polymer films [E = -F0.30 V) was measured. Statistical analysis of over 100 consecutive approach and retract plots yielded an average adhesive force of... 

Figure 2b. A schematic of a typical AFM force-distance plot using a PEO modified tip. At (a), there is no interaction between tip and surface. As the chains begin to compress (b) a repulsive steric exclusion force is observed. At (c), the chains are compressed even more producing an even larger repulsive force that dominates the attractive van der Waals force. At (d), the chains arc so much compressed that the cantilever spring constant is much weaker than the spring constant of the PEO chains and the cantilever continues to bend upward the same amount as the sample has been moved due to the large repulsive force gradient. Upon retraction, no adhesion is observed (provided there is no bridging) and the curve coincides with the approach curve.

Figure 6. A typical force curve. When approaching the surface, the cantilever is in an equilibrium position (1) and the curve is flat. As the tip approaches the surface (2), the cantilever is pushed up to the surface -being deflected upwards, which is seen as a sharp increase in the measured force (3). Once the tip starts retracting, the deflection starts to decrease and passes its equilibrium position at (4). As we start moving away from fhe surface the tip snaps in due to interaction with the surface, and the cantilever is deflected downwards (5). Once the tip-sample interactions are terminated due to increased distance, the tip snaps out, and returns to its equilibrium position (6). The image was reproduced with permission by C. Roduit [5].

Fig. 7 a Transmission electron microscopy images of core-sheU polystyrene-siUca colloids after the polystyrene core was removed by heating the samples at 500 °C for 3.5 h. Inset TEM image of a shell at high magnification. The dark area representing the shell shows uniform thickness, b A typical force curve of a hollow silica sphere with a shell of 50 nm and a diameter of 1.9 p.m. The open spheres denote the approach, and the filled spheres denote the retract part of the curve. Hardly any hysteresis is visible, i.e., the deformation is elastic, c Deformation of hollow silica spheres as a function of applied load. The straight line shows a linear fit to the data... 

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