Experimental Studies of Contact and Adhesion

The previously enunciated basic concept of adhesion as atomic bonding (Section 12.2.2) is intellectually credible but rather remote in the overtly behavioristic sense. There are very few experiments which directly demonstrate the relation between adhesion of surfaces on contact and interatomic or intermolecular forces. Extraneous complications introduced by the manipulations intrinsically connected with the experimentation obscure the results. The work of Bailey and Kay [17] is especially noteworthy. Using a double cantilever technique, they partially split a thin sheet of mica. The specific surface energy of the process was calculated from the force required to pull the split portion 

The jump technique, used by Tabor tt at. [18,19], measures the adhesion ascribable to van der Waals forces when two thin cylindrical sheets of mica oriented at right angles approach each other. Approach is controlled by a piezoelectric transducer and the separation of the two sheets is measured by interferometric fringes of equal chromatic order. The specimen not driven by the piezoeletric transducer is held in position by a sensitive cantilever spring. At some critical distance of approach the attractive van der Waals dispersion forces increase faster than the counterforce of the deflected spring and the two specimens flick into contact, a process which is monitored by the interferometric fringes. No external forces are applied to establish contact. 

The nature of the contact was followed by means of the electrical resistance. If the contact area has a radius K, then to a close approximation the formula for contact resistance R is given by 

The tensile pull-off force T for clean polycrystalline copper was found to be proportional to the two-thirds power of the normal load in the range 0.5 to 50 mN 

This means that for a clean ductile metal with a smooth surface, the area of adhesive contact is substantially equal to the area of elastic deformation. 

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