Contact Mechanics and Adhesion

The adhesion force between two objects can arise from a combination of different contributions such as the van der Waals force, electrostatic force, chemical bonding, and hydrogen bonding forces, capillary forces, and others (e.g., bridging or steric forces on polymer-coated surfaces)  

The adhesion force between two materials may therefore depend not only on the materials themselves but also on the ambient conditions. For micro- and 

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It has recently become common to use the JKR theory (Johnson, Kendall Roberts, 1971) to extract the surface and inteifacial energies of polymeric materials from adhesion tests with micro-probe instruments such as the Surface Force Apparatus and the Atomic Force Microscope. However the JKR theory strictly applies only to perfectly elastic solids. The paper will review progress in extending the JKR theory to the contact mechanics and adhesion of linear viscoelastic spheres. The observed effects of adhesion hysteresis and rate-dependent adhesion are predicted by the extended eory. 

The derived force versus tip separation curve is shown in Fig. 3.27B. Why is this not the same as the curves in Fig. 3.25 and 3.26 On the scale used, the forces in the noncontact regime before jump-in are very small, and since the surface is ideally rigid, the repulsive force curve is vertical and not merely steep. With a real specimen, the surface forces may be large enough to lead to deformation during the jump-in. At smaller separations, if the contact pressure in the repulsive regime exceeds the yield strength of the surface, then plastic deformation will occur. This is one of several possible types of force curves. They depend on many factors such as the nature of the interaction, surface forces, the deformability of the surface, and the medium between the tip and surface (air, liquid or vacuum) [116,117]. Force ciu es can be used to study and verify fundamental theories of contact mechanics and adhesion on polymer surfaces [118]. 

Due to the high practical relevance of adhesion forces in industrial and everyday applications, a broad spectrum of experimental methods to measure adhesion forces has been established and there are, for example, standardized procedures such as peel tests for adhesive tapes or tack tests for pressure-sensitive adhesives. We will focus here on some representative examples of experimental work targeted toward a fundamental understanding of contact mechanics and adhesion phenomena. 

Another method to study the contact mechanics and adhesion behavior of soft solids is the so-called JKR test using elastomeric poly(dimethylsiloxane) (PDMS) lenses that are brought in contact with flat surfaces or with each other [884]. The soft PDMS ensures almost ideal JKR behavior of the contacting surfaces. Apphed load, indentation and contact radius, and neck shape can be determined simultaneously, which allows comparison with the JKR predictions. The surfaces of the lenses can easily be modified by treatment with an oxygen plasma to induce a silica-like surface that can then be modified using silane chemistry. As long as these layers are kept thin, the mechanical properties will still be dominated by the bulk PDMS. This type of experiments have been used extensively to study the influence of separation rate on adhesion (for a review, see Ref. [885]). 

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