Force versus Indentation Curves
Force versus indentation curves are calculated by the subtraction from the measured data a reference deflection, measured on stiff and compliant surfaces (typically, only approach part of force curve is analyzed). When a stiff material (not easily deformable, typically silicon or glass) is investigated, the deflection reflects the position of the sample. In such case, force curve is a straight- sloped line and is usually employed as a reference line that is needed for the force calibration (red line in Fig. 4.11a). For compliant samples, like cells, cantilever deflections are much smaller and the resulting force curve has a non-linear character (blue line in Fig. 4.11b).
The difference between these two curves determines the deformation of the sample surface. For a given load force Fi, the indentation Az,- is calculated as a difference between zxi positions:
where zi = F;/kcantilever As a result, the force versus indentation curve is determined (Fig. 4.11b). It describes the mechanical response to the applied load force and it is characteristic for each material.
Figure 4.11 (a) The idea of the force-indentation dependence determination, (b) the resulting, exemplary force-indentation curve, calculated for a living keratinocyte.