ABSTRACT
The extent a liquid drop wets a solid surface is quantified in terms of the equilibrium apparent solid-liquid-air three-phase contact angle θ made by the drop once on the surfaces of the solid. As such, θ is used to discuss wetting of both solid surfaces and powdered particles. Initially, the origin of θ and its relationship with wetting are given. This is followed by a detailed description of its measurement on solid surfaces and powdered particles where methods like the telescope-goniometer, tilting plate, captive bubble, Wilhelmy balance, capillary rise at a vertical plate, capillary tube, drop shape analysis, axisymmetric drop shape analysis and capillary penetration for powders and granules are discussed. Thereafter, contact angle hysteresis and wetting (i.e. Young, Wenzel and Cassie-Baxter) states are discussed. In addition, the interfacial energy of solid surfaces, also known as surface energy, and methods of measuring it were discussed. These methods include the Berthelot’s geometric mean combining rule and Antonow’s rule, Bangham and Razouk model, Zisman approach, Good and Girifalco approach, Fowkes and Zettlemoyer approaches, Owens and Wendt and Wu approaches, Ward and Neumann approach and van Oss-Chaudhury-Good approach. Lastly, natural and bioinspired hydrophilic, oleophilic, hydrophobic, oleophobic, omniphobic, superhydrophobic, superoleophobic and superomniphobic surfaces and powdered particles were discussed with specific examples.
