Phoenix-MT(M)
Contact angle
The contact angle (θ) is the fundamental parameter for describing the wettability of a solid surface by a liquid. It forms at the interface between the solid, liquid, and vapor phases and is defined by Young’s equation, which relates the three interfacial tensions involved. An angle less than 90 degrees indicates a hydrophilic surface, easily wetted by the liquid; an angle greater than 90 degrees identifies a hydrophobic surface or one with low surface energy. The measurement is typically performed using the sessile drop method: a drop of liquid is placed on the sample, and its image is captured by a digital camera. The drop profile is analyzed via mathematical fitting of the Young-Laplace equation to determine the contact angle with high precision. By using at least two test liquids with different polarities, it is possible to calculate the surface free energy (SFE) of the solid using established theoretical models such as Owens-Wendt or D-base. Complementary to the sessile method is the pendant drop method, in which the drop hangs from the syringe needle and its profile, analyzed using the Young-Laplace equation, provides the surface or interfacial tension value of the liquid.