The Young equation of contact angle

The component theories, which are presented in Chapter 3, are useful for estimating the interfacial tensions of solid-interfaces (solid-liquid, solid-solid) and for characterizing solid surfaces using the experimental data for the few properties that can be measiued (liquid-gas surface tension, liquid-liquid interfacial tension, contact angle). Important equations in this context arc the Young equation 

When a drop of liquid is placed in contact with a solid, we have three intalaces the solid/liquid, solid/vapour and liquid/vapour interfaces. Each of these has its own intcrfacial energy. For a drop that partially wets a solid, the total interfacial energy is minimal when the horizontal components of the interfadal tensions are in equilibrium. At this point, the contact angle between liquid and solid has a value that is determined by the 

Introduction to Applied Colloid and Surface Chemistry, First Edition. Georgios M. Kontogeorgis and S0ren KiiL 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

the Young equation is derived as a force balance across the x-axis (jiCosS + Ysi = Ys) and provides the relationship between the contact angle ( ) and surface tensions of solid and liquid (s, 1) and the liquid-solid interfacial tension (si)  

Equation 4.1 is correct if the spreading pressure is zero, which is almost always true for low energy surfaces like polymers. The spreading pressure (tts = Ys-Ysv) is the difference between the solid surface tension (in contact with vacuum or air) and the solid surface tension in presence with vapour coming from the liquid drop. 

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