Solvents evaporating from a coating film cause vortex flows in the respective film. These arise because the more solvent-rich materials from the lower layers of the film must be continuously transported to the surface. This leads to the formation of a macroscopically visible cell structure (Bénard cells). Differences in density, temperature and, especially, surface tension are the driving forces of these movement processes. Various surface defects can ultimately result from these Bénard cells: floating, leveling problems, air draft sensitivity.
In a pigmented coating system, the pigments also participate in the vortex motion, and if the various pigments differ in their mobility, the flow processes can cause them to separate from one another. The pigments are then no longer homogeneously distributed within the coating surface (floating). On horizontal surfaces, the cell structure itself is visible; on vertical surfaces striation can be seen (silking).
