INSTRUMENTATION AND EXPERIMENTATION - Nanoindentation with Biological Applications - page 70

3.4.6. Adhesion
Adhesion between the AFM cantilever probe surface and the sample
surface can be measured for pN- to nN-scale forces, and has been
leveraged to infer chemical specificity of biological sample surfaces. 18
As the focus of this chapter is the use of AFMs as indenters that apply
primarily compressive contact, such applications of AFM to infer intra-
and intermolecular rupture forces under tension will not be discussed in
detail, but are summarized in reviews specific to biological cells and
molecules. 18,19
In the context of instrumented indentation analogues, the sensitivity
of AFM cantilevered probes to adhesive interactions is inherently high,
such that elastic properties of contact samples can be readily inferred
from JKR and similar adhesion models. 20 In fact, both the force required
to pull the probe onto the biomaterial (the so-called “snap to contact”
upon approach of the surface) and the force required to pull the probe
from the biomaterial (the so-called “rupture force” or “adhesion force”
upon retraction from the surface) can be quantified directly from the P-h
responses (see Chapter 4 for more detail). In order to maximize
quantitative interpretation of such probe-surface interactions, it is
preferable to employ colloidal ( i.e. , spherical) probes of well-
characterized materials, rather than to estimate contact areas of
nominally sharp Si or Si 3 N 4 fabricated cones. Further, it is important to
note that the rate of loading and unloading strongly affects the measured
force and work of adhesion, and this rate cannot be controlled directly
via AFM cantilevers. Only the rate of piezoactuated cantilever base
displacement is controlled directly, and the actual unloading rate must be
calculated from the load-time slope just prior to rupture of the probe
from the surface.
3.4.7 . Pixellated mapping of mechanical response
In all of the above cases, one apparent advantage of AFM-based
indentation over instrumented indentation is that data can be acquired for
every pixel within an image area. For example, a square image of 512 ×
512 pixels comprises 262,144 data points, which may represent the