Contacthoek

Contacthoek, also known as the contact angle, is the angle at which a liquid–vapor interface meets a solid surface, measured within the liquid. It is a principal measure of wettability: small angles indicate high wettability and that the liquid spreads readily, while large angles indicate poor wetting and limited spreading. In Dutch, the term is literally contacthoek.

The angle arises from interfacial tensions among the three phases involved: γ_lv for the liquid–vapor interface, γ_sv for the solid–vapor interface, and γ_sl for the solid–liquid interface. Young's equation relates these tensions to the contacthoek θ: γ_sv = γ_sl + γ_lv cos θ, so θ quantifies the balance of forces at the contact line.

Common methods to measure the contacthoek include the sessile drop method, where a liquid drop sits on a solid surface and its profile is analyzed, and the Wilhelmy plate or captive-bubble methods. Surfaces with roughness or chemical heterogeneity exhibit contacthoek hysteresis, producing distinct advancing (θ_A) and receding (θ_R) angles depending on whether the liquid is increasing or decreasing on the surface.

Applications span coatings, adhesion, inkjet printing, lubrication, and microfluidics. By tailoring surface chemistry and texture, engineers control θ to achieve desired spreading, coating uniformity, or capillary transport. Limitations include sensitivity to cleanliness, temperature, and time, as well as dynamic effects during rapid spreading; at small scales, additional factors such as line tension may also influence measurements.