Cohesion, work

Usually the cohesion work is expressed in terms of the surface energy of the solid 7s by... 

The terms "cohesion work" and "adhesion work" can be defined as follows. When the liquid and the solid are brought into contact with each other a new interface is formed (index 1 corresponds to the liquid, index 2 to the solid phase, index 12 to the interface). The total decrease of Free Energy of this process is given by... 

In the last equation 7z -7,2 is the so-called wetting tension b. The cohesion work of the liquid is twice the surface tension value, equal to 27,. The difference between adhesion and cohesion work is the spreading coefficient, which now reads... 

Work of Cohesion, Work of Adhesion and Spreading Coefficient... 

The spreadability of liquid A on the surface of another phase B (liquid or sohd substance) can be also expressed using Harkins dimensionless spreading coefficient (S), which is the difference between adhesive and cohesive work ... 

Derive the expression (in terms of the appropriate works of adhesion and cohesion) for the spreading coefficient for a substance C at the interface between two liquids A and B. 

Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

The statement was made that the work of adhesion between two dissimilar substances should be larger than the work of cohesion of the weaker one. Demonstrate a basis on which this statement is correct and a basis on which it could be argued that the statement is incorrect. 

Because the work of cohesion, of the adhesive is 2 CJ, equation 3 follows. 

Surface energies are also associated with failure of an adhesive bond, because failure involves forming new surfaces and the appropriate surface energies have to be provided. The surface energy term may be the work of adhesion, VTa, or the work of cohesion, VTcoh. depending on whether the failure is adhesive or cohesive. For phases 1 and 2, these are defined as follows [lOj ... 

Usually tj/ is very much larger than Fq. This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than works of adhesion or cohesion. However, a modest increase in Fq may result in a large increase in adhesion as and Fo are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive. Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls the fracture energy of an adhesive joint, even if failure occurs far from the interface. This is demonstrated in Table 5, which shows the static toughness of a series of wedge test specimens with a range of adherend surface treatments. All of these samples failed cohesively within the resin, yet show a range of static toughness values of over 600%. 

A prospective location may appear to be right in all other respects but fails because of the manpower resource. This refers to not only the economic aspects such as the availability of labor and skills but also to the qualitative ones of being able to understand the local culture and customs, appreciation of people s attitudes and values. It also refers to the confidence in the ability to blend these with the culture and aspirations of the company to form a cohesive production unit that will work. A successful processing concept in one cultural environment may not be a success in another. 

Surface energy or surface tension, y, has been an important parameter widely used for characterizing adhesion. It is dehned as half of the work needed to separate two bodies of unit area from contact with each other to an infinite distance, as schematically shown in Fig. 2. If the contact pairs are of the same material, the surface energy is identical to the cohesive energy. 

Rubber swelling modifies the liquid/solid work of adhesion. Wo, because in addition to the initial liquid/solid interactions, liquid diffusion into the solid produces supplementary liquid/liquid interactions, hquid molecules having passed through the liquid/solid interface. Therefore, to the initial work of adhesion in the absence of swelling, Wq, an additional term corresponding to a fraction of the cohesion energy of the liquid, 2y, should be added. If / is the time of diffusion, the work of adhesion at /, Wo(t), can then be expressed as... 

---