Energy of Adhesion and Contact Angles

The thermodynamics of thin liquid films and adhesion is well documented in the literature [15,17,100-103]. The first theoretical approaches were developed mainly for thin soap films. Most of the results of the thermodynamics of soap 

E(hg) is negative for adhesive systems and —E(he) is traditionally known as the free energy of adhesion. 

An important consequence resulting from the approach of the interfaces is that Yf, the surface energy of the film, is lowered by the energy of adhesion. When the interfaces are far apart, E(h) is equal to zero and y/ is simply equal to 2yint, where yint is the tension of a single interface. At equilibrium, the surface energy of the film is Yf = 2yint -F E(he). 

As the tension of the film is different from the tension of two single isolated interfaces, a contact angle, 0, is expected at the junction between adherent and free interfaces. This is depicted in Fig. 2.30. The mechanical equilibrium of the contact line at the junction between free and adherent interfaces dictates the value of the contact angle  

This relation is known as the Young-Dupr6 equation. It shows that the energy of adhesion can be determined by measuring the contact angle and the surface 

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