Force curve cycles

Force Curve Cycles. Multiple force curve cycles have been done at one place of the sample, avoiding deliberately the compressive (part b in Figure 2a) were the tip is more likely to damage the sample surface. For the lowest molecular weights, the vertical displacement AZu necessary to separate the tip from the sample (arrow in Figure 2a) decreases with the number of cycles (or with time as a cycle duration was Is). 

Force curves recorded at different times and locations on a sample were reproducible, indicating homogeneous surface coverage. Considering the sphere diameter of 4—5 gm used in this study, more than 1000 PEG chains are expected to be compressed in the interaction area. The reproducibility of the force curves with time proved that the relaxation of the PEG chains after release of compression was a rapid process in comparison with the AFM cycle time of loading and unloading. ... 

The force curves, i.e., loading-unloading cycles, were recorded with a velocity of 0.4 /traJs. Hysteresis in the force-distance curves is known to occur under certain conditions of scan velocity. However, hysteresis was minimal for the velocity used in the present work. Following measurement, the tip displacement versus distance curves recorded with the AFM-LPM were transformed to force versus tip—sample separation curves, following the procedure described in refs 16 and 17. 

Figure 7 Force curve experiments on PS films normalized variation of the vertical displacement AZu needed to unstick the tip from the sample with time for three samples 1890 - 23000 - 284000. The variation with time is obtained by the number of cycles multiplied by the duration of one cycle. Whereas for the 284000 sample AZu is quite constant, its changes for the smaller PS its normalized variation can be fitted by an exponential law, enabling to extract a characteristic time for the smaller samples.

Figure 9.13b shows the expected force curves for two successive Gaussian defects with different forces. Depending on the defect strength, either a reversible modulation (as in the left part of the curve in Fig. 9.13b) or a hysteresis cycle (as in the right part of the curve in Fig. 9.13b) is observed. The two defects of Fig. 9.13a therefore correspond to weak and strong defects, respectively. 

Adhesion (pull-off force) data are calculated as the largest negative force detected during the retraction curve. In addition to directly extracted data on the maximum adhesive force, further data may be calculated from the force-displacement eurve. The area enclosed between the approach force curve and the retract force curve accounts for the dissipation of the energy per oscillation cycle. Finally, the maximum deformation of the sample is calculated as the difference in the piezo-displacement between the points of maximum and zero foree, measured along the approach curve, and corrected for the ehange in the deflection of the cantilever. The ealculated value includes both elastic and plastic contributions and reaches its maximum at the peak force. 

Representative AFM force curves for Sample 1 and Sample 2 are shown in Figures 4 and 5, respectively. The measured adhesive force between the silicon tip and the composite surface was similar for both samples, although substantial variation was noted in the measurement. These findings indicate that the AFM tip interacts primarily with the resin-rich composite surface on both samples. Force measurements for Sample 3 and Sample 4 indicated substantially greater adhesive forces between the AFM tip and the composite surfaces when compared to Samples 1 and 2. In fact, it was not possible to obtain reproducible force of adhesion measurements for Sample 3 or Sample 4 using the same conditions used for Sample 1 and Sample 2 because the tip remained imbedded in the composite surfaces for almost the entire retraction cycle. Substantial differences between the force curves for Sample 3 and Sample 4 were not observed. 

Just as important as the maximum active force a muscle can exert at various lengths, is the rate at which the muscle shortens as a function of the force load, i.e., the force-velocity curve. Both the length-tension curve and the force-velocity curve vary according to the degree of activation of a muscle. The rate at which crossbridges cycle is an inverse function of the load force (Figure 4). 

The mechanical behavior of the contractile apparatus of smooth muscle is also very similar to that of striated muscle. So that to the extent that the force-velocity curves reflect the interaction of mechanical force and the rate of enzymatic catalysis, the steps of the chemomechanical transduction cycles in the two muscles are apparently modulated in similar ways. Also relationships between the active isometric force and muscle length are very similar (except as noted above for shorter lengths). 

A process is forced by sinusoidal input m,). The output is a sine wave If these two signals arc connected to an x — y recorder, we get a Lissajous plot. Time is the parameter along the curve, which repeats itself with each cycle. The shape of the curve will change if the frequency is changed and will be different for different kinds of processes. 

Figure 7. Mechanical unfolding of RNA molecules (a, b) and proteins (c, d) using optical tweezers, (a) Experimental setup in RNA pulling experiments, (b) Pulling cycles in the homologous hairpin and force rip distributions during the unfolding and refolding at three pulling speeds. (C) Equivalent setup in proteins, (d) Force extension curve when pulUng the protein RNAseH. Panel (b) is from Ref. 86. Panels (a) and (d) are a courtesy from C. Cecconi [84]. (See color insert.)...

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