Adhesion mechanical theory

The most-often cited theoretical underpinning for a relationship between practical adhesion energy and the work of adhesion is the generalized fracture mechanics theory of Gent and coworkers [23-25] and contributed to by Andrews and Kinloch [26-29]. This defines a linear relationship between the mechanical work of separation, kj, , and the thermodynamic work of adhesion ... 

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. 

The mechanisms of adhesion are explained by four main theories mechanical theory, adsorption theory, diffusion theory, and electrostatic theory. 

TABLE 1 Single Chemical Bond Forces (in pN) for Every Tip-Substrate Combination, Calculated on the Basis of the JKR Theory of Adhesion Mechanics ... 

Physical adsorption is a universal phenomena, producing some, if not the major, contribution to almost every adhesive contact. It is dependent for its strength upon the van der Waals attraction between individual molecules of the adhesive and those of the substrate. Van der Waals attraction quantitatively expresses the London dispersion force between molecules that is brought about by the rapidly fluctuating dipole moment within an individual molecule polarizing, and thus attracting, other molecules. Grimley (1973) has treated the current quantum mechanical theories involved in simplified mathematical terms as they apply to adhesive interactions. 

Finally, we note that several other contact mechanics theories have been put forward, which are not described in detail in this contribution. The most important ones of these theories for AFM applications include the Derjaguin-Muller-Toporov (DMT), the Bumham-Colton-Pollock (BCP), and the Maguis mechanics [11, 12 ]. These theories differ in the assumptions (and limitations) and yield different expressions for the pull-off force. For example, the DMT theory, which assumes that long-range surface forces act only outside the contact area (as opposed to JKR, where adhesion forces only inside the contact area are assumed), predicts a pull-off force of —2 tRW. 

The introduction to the concept of static and kinetic friction in Chapter 7, Section 2 is admittedly simplistic. Familiarity with the experimental details of measuring friction leads to a more realistic view in behavioristic terms and also to some theoretical questions. In particular, the theory of stick-slip friction requires that be greater than and distinct from and indeed Fig. 7-5a shows a discontinuity between static and kinetic friction. But the model for the fundamental adhesive mechanism of friction does not predict such a discontinuity. 

To explain the pronounced effect of additives on tool wear in metal cutting, Dorinson [74] developed a theory of the inhibition of junction growth at contacting asperities based on the concept of dynamic competition between asperity adhesion and the quenching of such adhesion by additive reaction. The adhesion mechanism involves the following sequence migration of metal on the chip side of the contact interface to the tool side,... 

The fracture-based approach derives from continuum fracture mechanics theory, which claims the strength of most real solids is governed by flaws within the material [2]. To help predict this type of behavior, many test methods have been developed to determine fracture properties of adhesives. These tests are used to characterize the mode I, II, and III fracture properties of many types of material systems. In this study, the focus will be on the mode I and II characteristics of bonded joints for automotive applications. 

A classic instance of the mechanical theory of adhesion is where one phase is keyed into the other. Here the adhesion is enhanced above the increase proportional... 

The adhesion properties of all types of polyolefins are not easy to explain because these properties are affected by different phenomena. Using of a single theory or mechanisms based on the physical and chemical adhesion manifestations is difiicult for the description of interdisciplinary nature and diversity. There is considerable information to discuss each of the adhesion mechanisms. Therefore, it is not possible to select only the thermodynamic theory of adhesion that is the best to describe the surface free energy of the polyolefin. All mechanisms and adhesion theories are implied by the diversity of polymer systems, which are embraced in combination with research for the analyses of adhesion properties. The physical and chemical composition in the first atomic layers dictates the adhesion and some other properties of the polymer materials. This layer represents underneath layer and this subsurface partially controls the outer layers. The double bonds and cross-linked stmctures limit the mobility macromolecules of polyolefins in the subsurface layers, which results in the functional group stabilization on the surface. Other basic research is necessary for an examination of the polymer subsurface layer and explanation of its effect changes of the surface properties. Moreover, for the improvement of quantitative measurements of adhesion, additional investigation is required. 

A number of adhesion theories have been proposed to identify the formation of adhesive forces. The contributed adhesion mechanisms are (1) chemical bonding such as chemisorption theory (2) physical interaction such as polarization, electrostatic, and diffusion theory (3) thermodynamical interpretation such as adsorption theory and (4) mechanical interlocking. No single theory exists to explain the entire property of adhesion oti various substrates and adhesives. However, those theories may provide a guideline to understand the principle of the adhesion as the following details (Fig. 2). 

The macroscopic mechanism looks simple, because we grip the film and pull it off by applying a force (Fig. 3.9(a)). However, it is obvious that the applied force is not acting directly at the contact. The film has to bend and apply leverage to the adhesive region. So there is a complex mechanical system operating. To understand this we need mechanics. In particular, we apply the continuum mechanics theory in Chapter 14. 

Bradley had read Tomlinson s paper and developed an improved method of measuring the adhesion, together with a better theory based on London s wave mechanics theory of the forces between molecules. By adding up the London forces for all the molecules in two rigid sphaes, Bradley came to the conclusion that the adhesive force required to separate them should be proportional to the sphere diameter, as shown in Fig. 4.11. He also showed that the force should be proportional to the work of adhesion W of the spheres, that is the energy required to separate one square meter of intaface reversibly. Thus he produced his famous equation for adhesion of spheres shown in Fig. 4.11. 

The numbers of these two types of doublets may be derived, at equilibrium for dilute suspensions with low adhesion, using statistical mechanics theory to calculate the ratio of doublets to singlets... 

Die compaction of agglomerates has been simulated in a computer sphere model by Thornton et When the particles are in a liquid paste, the liquid is usually expelled through pores in the mold, and the partieles form a filter cake. The mechanism by which this occurs has been modeled by Woodcock et The application of forces to powder beds, sometimes immersed in liquids, is the subject of soil mechanics. " This considers particles to interact via Coulomb s law, Equation (11.16), but also takes into account the hydrodynamic forces acting on the individual particles. Unfortunately, as we have seen. Coulomb s law is not correct for particles which experience molecular adhesion, so the friction coefficients found in soil mechanics theories seem to vary. Friction seems to increase as the particles get smaller because smaller particles adhere more strongly. Soil mechanics is therefore a difficult science. 

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