Adhesion mapping

Erom the previous sections it is clear that there are a number of different possible models that can be applied to the contact of an elastic sphere and a flat surface. Depending on the scale of the objects, their elasticity and the load to which they are subjected, one particular model can be more suitably applied than the others. The evaluation of the combination of relevant parameters can be made via two nondimensional coordinates X and P [16]. The former can be interpreted as the ratio of elastic deformation resulting from adhesion to the effective range of the surface forces. The second parameter, P, is the load parameter and corresponds to the ratio of the applied load to the adhesive puU-off force. An adhesion map of model zones can be seen in Figure 2. 

Fig. 4 Map of the validity regimes of the several contact mechanics models (adhesion map). Pad/P denotes the ratio between the adhesive component of the load and the total one. Si is the elastic compression, whereas Sad is the deformation due to adhesion. h0 is the effective range of action of adhesive forces (h0=Q.97z0, whereby z0 denotes the equilibrium interatomic distance). Adapted from [108]...

Compositional contrast, as well as modulus, can also be assessed by laterally resolved imaging of normal or lateral forces in the contact mode AFM. In the corresponding mapping [121] (see also Chap. 4), force-displacement curves are recorded for each pixel. Subsequently, the pull-off forces, as a measure for adhesion, and the indentation part of the loading curves, to extract/fit the elastic modulus, are evaluated for each pixel. In particular for adhesion mapping, the use of chemically functionalized AFM probe tips [122], has been shown to be a suitable approach to map chemical composition and functional group distributions down to the sub-50 nm scale [123]. The mapping of adhesion, friction, and surface mechanical properties will be treated in more detail in Chap. 4. 

Practical adhesion mapping can be performed in the so-called force-volume (FV) mode. In this mode, f-d curves are acquired for each pixel. Thus, information is obtained on attractive forces before the tip contacts the surface, indentation in the contact region (see Sect. 4.3), adhesive interactions, and the dissipated energy (as area under the f-d curve, compare Fig. 4.3). 

Advanced off-fine analysis software can help to access the statistical and the lateral distribution of the pull-off forces (panels c and d, respectively). The corresponding data, e.g., magnitude of the pull-off force, displayed in a 2D adhesion map (Fig. 4.4d) show that the border region is indeed characterized by a gradually changing, mixed chemical composition. The corresponding composition gradient can thus be mapped. Similarly, the statistically treated data yield a bimodal distribution of the pull-off forces (Fig. 4.4c). 

Finally, Nie and co-workers reported an increase in surface energy of mechanically scratched structures on biaxially oriented PP based on adhesion mapping and friction force imaging data using a conventional Si3N4 (244). 

The distributions of filler particles in carbon-black- and silica-filled elastomers were detected by intermittent contact mode SFM phase imaging, as well as adhesion mapping by Trifonova and co-workers (264). In the detailed particle size analysis evidence for the observation of primary filler particles was found. 

If we set out to unravel surface chemical functionalities with high spatial resolution down to atomic detail, we also encounter various practical (technical) problems. It is fair to say that the technique development for direct space analysis (again, we exclude Fourier space methods) is still lagging much behind. Chemical force microscopy can be considered as a first step in the direction of a true description of surface chemical functionalities with high spatial resolution in polymers, primarily based on the chemically sensitive analysis of AFM data via adhesion mapping. At this point the detailed theory for force spectroscopy is not developed beyond the description of London forces. The consideration of the effect of polar functional groups in force spectroscopy (similar to difficulties with solubihty parameter and surface tension approaches for polar forces, as well as specific interactions) is still in its infancy. Instead, one must still rely on continuiun contact mechanics to couple measured forces and surface free energies. 

Adhesion map obtained in water with a -CH3 tip is shown in Fig. 7. This image shows higher average adhesion force than in Fig. 5. The areas of small adhesion forces can be interpreted as areas where there is some Tinuvin 770 remaining on the surface. 

Fig. 7 Adhesion map obtained on stabilized polypropylene, after rinsing with chloroform and dichloromethane, with a -CH] terminated tip in water. Dark colour indicates high adhesion and bright colour indicates low adhesion.

This washed surface was then allowed to age in air and we have measured again adhesion forces after 2 months (Fig. 6). The behaviour of this aged surface becomes again hydrophilic and is very similar to the one observed for the PP6s surface. It means that an outermost layer of Tinuvin 770 has reappeared on the surface. The additives contained in the bulk of the material have migrated toward the surface. The adhesion map obtained in water with a -CH3 tip (cf Fig. 8) shows a homogeneous distribution of low adhesion forces, i.e. of Tinuvin 770. 

Fig. 46 Data obtained on PS/PSox patterned surfaces produced by photolithography and oxygen plasma oxidation of polystyrene (a) XPS mapping of the O Is peak intensity (obtained with a Kratos Axis Ultra spectrometer) (b) adhesion map obtained by AFM in water with a sUicon probe (vertical scale = 25 nN), revealing the hydrophobicity contrast in the pattern (c) ToF-SIMS image recorded with the signal of CNO ions on a patterned surface conditioned with a solution of flbronectin and Plutonic F68, revealing the selective adsorption of the extracellular matrix protein on the oxidized tracks and (d) micrograph of rat hepatocytes on a patterned substrate conditioned with a solution of type I collagen and Plutonic F68, showing the selective adhesion of the cells on the oxidized tracks. Adapted from Refs, 25 and 312...

Figure 173 Adhesion map. Johnson [20]. Reproduced with permission of Elsevier.

Applying the appropriate contact mechanics toP-S relation, one can obtain the mechanical properties of elastic surface. The adhesion map indicates that even very stiff polymeric materials lie far away from the DMT zone, while most other types of polymeric materials, from rubber and gel to materials of intermediate stiffness, reside in the JKR zone. One of the reasons why sometimes the DMT model is still used regardless of this fact may be due to mathematical handiness, as expressed in Equation 2.4. The AFM cannot directly observe the contact area a hence, the form of the JKR model, which does not have an explicit solution for P(S), is evaded. 

Johnson KL, Greenwood JA. An adhesion map for the contact of elastic spheres. J Colloid Interface Sci 1997 192 326-333. 

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