There is a drying pattern called “Bernard cell”, which is particularly easy to present in solvent borne solution drying.
The above figure shows a coating pattern due to the formation of Bernard cell. It is often described as ‘orange peel’ as well, which is easy to present in solvent borne solution if the drying parameters are not suitable.
cause
During drying of a solvent borne coating, the evaporation of solvent(s) causes the differences in temperature, surface tension, solvent concentration and density within the film. These unbalances drive solvent flow (eddy current) to occur in the coating film as shown in the figure. Since the surface tension is higher at the edges of the cells than at their centers, coating material flows from regions of lower surface tension (centers) to regions of higher surface tension (edges), which further enhances the unevenness in the surface of the coating film. Thus-formed surface texture shows unevenness/not smooth/or orange peel as sometimes referred to. (Reference)
cure
To achieve good flow, (1) the surface tension must remain uniform over the complete surface of the coating layer while it is drying; (2) drying parameters (drying rate); (3) solvent mixture components; (4) suitable surfactants