Contact angle – Measuring surface tension
The contact angle is a measure of the ability of a liquid to wet the surface of a solid. The shape that a drop takes on a surface depends on the surface tension of the fluid and the nature of the surface. At the boundary between droplets and the gaseous environment, the surface tension causes a curved contour.
At the edge of the drop, where the contour merges into the bearing surface, the contact angle between the interface liquid / solid and the tangent to the interface liquid / gaseous forms.
Meaning of the angle
If the liquid runs evenly on the solid surface, complete wetting is present with a contact angle of 0 °. If the angle is between 0 ° and 90 °, the surface is wettable. The surface is called hydrophilic.
An angle between 90 ° and 180 ° means the surface is not wettable. It is hydrophobic.
If the angle clearly approaches the value of 180 °, it is an ultrahydrophobic surface that is completely liquid-repellent. This property is described as a lotus effect.
- Angle between 0 ° and 90 ° = surface wettable, hydrophilic
- Angle between 90 ° and 180 ° = surface not wettable, hydrophobic
- Angle is close to 180 ° = ultrahydrophobic surface, completely liquid-repellent, lotus effect
Contact angle as a basis for decision-making in practice
The knowledge of the contact angle is the basis for assessing the suitability of paints, varnishes and coolants that should wet the materials as well as possible. In the refinement of certain textiles and water-repellent building facades on the contrary, the lowest possible wetting should be achieved.
The wettability can be influenced by a suitable surface treatment. The measurement of the contact angle enables the planned development of optimal methods in this field.
The contact angle measurement takes place within the scope of the drop contour analysis, in which the shadow image of the drop is optically evaluated. In addition, the temperature dependence of the contact angle can be determined exactly with the heating microscope of Linseis.