Adhesive wetting-The Young-Dupre equation

Introducing into equation (1.45) the expressions (1.9) and (1.12) for ctlv and W° derived from the simple nearest-neighbour model described in Section 1.1, this equation can be re-written as a function of S/L and L/L pair energies  

Equation (1.46) shows that the contact angle results from the competition of two types of forces cohesion forces responsible for crLv (= Wc/2) and adhesion forces responsible for W . Depending on the strength of S/L and L/L interactions, different contact angles can be obtained (Table 1.1). 

As shown in the Table, wetting can be obtained not only in systems featuring strong L/S interactions (ionic, covalent, metallic or some mixture of them), but also in systems featuring weak L/S interactions (for instance van der Waals interactions) provided that L/L interactions are weak too. The nearly immiscible molten Cu/W system (6 = 10°, Table 5.2) is a typical example of strong L/L and 

S/L interactions, which are both metallic. No wetting is observed in the CU/AI2O3 system (0 = 128°, Table 6.1) which features strong L/L but weak S/L interactions. However, many organic liquids, whose cohesion is due to weak, physical, interactions, wet silica well despite the fact that their S/L interactions are rather weak (Johnson and Dettre 1993). 

From the above, it is obvious that good wetting does not necessarily mean a thermodynamically strong interface good wetting means that the interfacial bond is energetically nearly as strong as the cohesion bond of the liquid itself. 

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