The Triple Line and Contact Angles

In the presence of three phases, the three interfaces can intersect along a line known as the triple line (cf. Fig. 1.3). 

If one of the phases is solid, the contact line lies in the solid interface and the angle with the liquid interface is called the contact angle 9. If the three phases present are solid/liquid/air, the equation describing the balance of forces projected in the plane of the solid surface, known as Young s relation, is  

If the solid is in contact with two liquids, A and B, Young s relation becomes 7sb = 7ab cos + 7sa  

The vertical component is balanced by the elastic force due to deformation of the solid, whose amplitude is microscopic (of the order of A), li 9 is not equal to 9e Young s forces are unbalanced and its contact line moves (Sect. 1.2.7). is called the dynamic contact angle. 

If all three phases are fluid (La, Lb, and air), the condition for mechanical equilibrium, known as the Neumann s triangle construction, can be written in vector form 

---