Conditions of Equilibrium where Several Surfaces Intersect

4 Conditions of Equilibrium where Several Surfaces Intersect 

At equilibrium, (dUs)Wni and (dGs)RXnj must be a minimum, and from this requirement we can derive a relation for the angles of contact at the triple line  

This equation is equivalent to the static force balance where each of the y terms is considered as the force applied to the vertex. This condition of equilibrium is often called a Neumann triangle. Equation (233) is the basis of Young s equation, which is used in the contact angle determination of liquid drops on flat substrates (see Sections 5.5 and 9.1). 

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