Contact angle

When an interface exists between a liquid and a solid, the angle between the surface of the liquid and the outline of the contact surface is described as the contact angle θ (lower case theta). The contact angle (wetting angle) is a measure of the wettability of a solid by a liquid.

Background

In the case of complete wetting (spreading), the contact angle is 0°. Between 0° and 90°, the solid is wettable and above 90° it is not wettable. In the case of ultrahydrophobic materials with the so-called lotus effect, the contact angle approaches the theoretical limit of 180°.

Figure: contact angle on different materials

According to Young's equation, there is a relationship between the contact angle θ, the surface tension of the liquid σl, the interfacial tension σsl between liquid and solid and the surface free energy σs of the solid:

Schematic diagram of contact angle

The contact angle with one or more liquids can be used to determine the surface free energy of a solid.

Significance

The contact angle is important wherever the intensity of the phase contact between liquid and solid substances needs to be checked or assessed: coating, painting, cleaning, printing, hydrophobic or hydrophilic coating, bonding, dispersing etc.

Measuring methods