Contact angles provide information about the wettability of solid samples. Dynamic contact angles can be determined using a force-based tensiometer and the Wilhelmy equation. Imaginary contact angles are mathematical solutions to the Wilhelmy equation, which are particularly important for highly wettable and rough surfaces.
Fundamentals: The contact angle at the three-phase contact line
Before we turn our attention to imaginary contact angles, we need to clarify a few basic concepts. The contact angle θ is one of the most important parameters for describing the situation at the interface between different materials. The contact angle can be measured by placing a drop of liquid on a solid surface. Three different phases – solid, liquid and gas – now meet at the three-phase contact line.
When the system is at rest, there is a balance of tangential forces at the three-phase contact line. In this state, the static contact angle θC forms at the three-phase contact line. When the system is in motion, for example when the solid surface is tilted, the dynamic contact angles θAdv and θRec can be determined.
Contact angles provide information about the wetting of the solid material
Measuring the contact angle of a liquid on a solid provides information about the wetting behavior of this combination. The desired wetting behavior depends on the specific application.
At a contact angle of 180°, the drop lies on the surface in a spherical shape and touches the solid only at one point. In this case, we speak of complete wetting. At a contact angle of 0°, the drop is completely spread out or spread across the solid surface. The drop forms a thin film of liquid on the solid surface. In this case, we refer to complete wetting. However, with exceptionally wettable samples, it is possible to achieve better wetting than with a contact angle of 0°. This is referred to as imaginary contact angles.
What are dynamic contact angles?
The static contact angle describes the state of equilibrium, i.e. at rest. In addition to the static contact angle, dynamic contact angles can also be determined. They describe a state in which the three-phase contact line is in motion.
There are two types of dynamic contact angles. The advancing contact angle θAdv describes how a liquid wets a solid. Specifically, it provides insights into how a dry solid is wetted by a liquid. The receding contact angle θRec describes how the liquid detaches from the solid. The receding contact angle therefore characterizes how the liquid detaches from a wet solid.