Observations of Adhesion

The problem of extrapolating down to molecular dimensions from macroscopic experience can be seen in two simple cases. First, consider the macroscopic task of sieving pebbles in the garden, as shown in Fig. 3.4(a). 

In the same way, consider the problem of getting a molecular size key into its receptor lock (Fig. 3.5(a)). The key sticks prematurely to the lock before it can enter, so the mechanism cannot work, as shown in Fig. 3.5(b), unless the molecular adhesion is reduced, by coating the receptor with contaminant atoms. 

A second example of the difference between macroscopic and molecular behavior is seen in the designing of machines. We know how to design machines like cars. To make a clutch mechanism between two rotating shafts, we have to press the two shafts together to get them both to rotate synchronously, as illustrated in Fig. 3.6(a). Alternatively, we could glue the shafts together to make a solid joint between the two shafts. 

The case of two rotating shafts in a molecular, nanoscale machine is quite the opposite. The shafts adhere spontaneously, leaping into contact, and provide 

Having defined the main adhesion fallacies, and determined where adhesion can always be found and predicted at the nanometer level, it is now possible to summarize the laws of adhesion which we believe to be universal. 

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Durig, U., Ziiger, O., and Pohl, D. W. (1988). Force sensing in scanning tunneling microscopy Observation of adhesion forces on clean metal surfaces. J. Microscopy. 152, Part 1, 259-267. 

The experimental observation of adhesion frequently employs peel and lap shear tests (88,89) see Figure 12.32 (90). The fracture energy, is obtained from the peel force, P, and the peel angle, 0, through the relation... 

It is perhaps worthwhile observing at this point that it is quite feasible to couple a chemical onto the surface of a material such as glass to reduce wetting and hence the level of adhesion by a polymer by incorporating some group incompatible with or even repellent to the polymer. 

Fig. 21. Patterns of rate dependency of adhesion energy observed in contact mechanical measurements (Eqs. 63 and 64) [117].

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Changes observed in the composition of the rubber/brass interphase correlated well with results of adhesion tests carried out on brass-plated steel wires embedded in blocks of rubber [46]. The force required to pull the wires out of the blocks decreased steadily as vulcanization temperature increased. This effect was especially pronounced when the specimens were aged at elevated temperature and humidity for several days before the wires were pulled out of the rubber blocks. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

The formation of these polar groups contributes increased adhesion. Observation of disappearing vinyl groups in the silane coupling agent and of the formation of polystyrene in the silica by FTIR analysis (Fig. 15) have confirmed the occurrence of a reaction between the polymer and the silane coupling agent [77]. 

The better interaction observed with the unmodifled clay was also explained in terms of surface energy. The values of surface energy of the fluoroelastomer and the clays, along with work of adhesion, spreading coefficient and interfacial tension are reported in Table 2.4. 

Majumder and Bhowmick [381] have investigated the influence of the concentration of TMPTA on the surface properties of EPDM mbber, modified in the presence of EB. The surface energy of the TMPTA-modified EPDM mbber has been observed to increase as compared to the unmodified one. Table 31.4 displays the contact angles and the work of adhesion of water, for the control and the modified EPDM surfaces. 

Eastman and Zhn [52] performed a study of SisN4 tips, subsequently coated with gold and finally with paraffin thin films, adhering to mica. The study was purely qualitative, because they did not have accnrate valnes for either their lever stiffnesses or their tip radii. By enclosing their AEM in a sealed container and controlling the humidity they found that hydrophobic tips show a lower adhesion with a surface expected to have a thin water film, as also observed by Jarvis and Pethica [51]. Surprisingly they did not see any dependence of adhesion on hnmidity for any of their tips. 

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