Work of adhesion - What is it and how to measure it?

Work of adhesion is defined as a work required to separate two phases from each other. When the two phases are in contact with each other, there is an interface between them with a certain interfacial energy, γ_AB. When the two phases are separated, this interface will disappear but two new are formed. These are the interface between phase A and air (γ_A) and the interface between phase B and air (γ_B). The work that is required to separate these is work of adhesion and described by Dupré as: W_AB = γ_A + γ_B – γ_AB.

Now, if the two phases are solid and liquid, the equation can be written as

W_SL = γ_S + γ_L – γ_SL

,where γ_S is the surface free energy of the solid, γ_L is the surface tension of a liquid and γ_SL is the interfacial tension between solid and the liquid.

Young - Dupré equation can be used to simplify the calculations

While the surface tension of the liquid is easy to measure, the other two are more cumbersome. To simplify the measurement of work of adhesion, the Dupré equation can be combined with the Young equation; γ_S = γ_SL + γ_Lcosθ_Y

which then gives Young-Dupré equation

W_SL = γ_L(cosθ + 1).

This equation makes it possible to easily access the work of adhesion with simple contact angle measurements.

For example, work of adhesion between water drop and a glass slide would be measured by measuring the contact angle of water on the glass surface. For surface tension of water, you can use a literature value or measure it. With this information, the work of adhesion can be calculated.

To read more about contact angle measurements, please download the white paper through the link below.