Coating Technology

Functional coatings

Coatings are used for a variety of reasons, mainly for protection, decoration, and other " functional" reasons. Being non-wetting for e.g. water is often a favorable property, but a paint should also perfectly wet the object to be painted. This kind of properties can be determined by measuring surface tensions and wetting angles.

From the shape of a pendent drop the surface tension can be deduced (the lower the surface tension, the more the droplet will be stretched vertically, i.e. it will deviate from the spherical shape). The ease of wetting can be analysed and quantified from a microscopic picture of the droplet on a plate. In the picture shown the wetting is fairly poor.

Depending on the surface structure (roughness; composition heterogeneity) a surface that is not well wetted by a paint stays wetted once it has been wetted. This phenomenon can be studied by watching the shape of a droplet that is forced to walk over a plate by tilting the plate. In such a case the microscope image shows that the wetting angle in front of the walking droplet is much larger than the angle behind the droplet.