In 1805, Thomas Young studied the mechanical equilibrium at the solid/liquid/gas three-phase contact line (the balance of forces acting on the contact line formed by the intersection of the liquid-gas interface and the solid surface), and introduced the macroscopic concept of “contact angle” and Young’s equation. Based on the assumptions of an isotropic, homogeneous and smooth surface, Young’s equation gives the relationship between the intrinsic contact angle of the solid surface and the interfacial free energy at the solid/liquid/gas three-phase contact line.
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Source: Phys.org