When a film is applied to a corner, surface tension, which tends to minimize the surface area of the film, may cause the film thickness to decrease or increase at the corner, as shown in Figure 2.9b and Figure 2.9d, respectively. In the case of coated object edges, an increase in thickness has been observed. This phenomenon is related to the change of surface tension with solvent concentration. 40 In a newly formed film, the decrease in film thickness at the edges is caused by the surface tension of the film. As a result, the solvent evaporates faster at the edge of the film because there is a greater surface area per unit volume of fluid near the edge (Fig. 2.10a). As more solvent (which typically has a lower surface tension than the polymer) evaporates, a higher surface tension exists at the edge, thus causing material to transport from region 2 towards the edge to region 1 (Fig. 2.10b). Due to the exposure of the underlying material, the newly formed surface in region 2 will have a lower surface tension,
Figure 2.9 (a) Freshly applied thick film at the corner. (b) Film thickness decreases at the corners due to surface tension. (c) Freshly applied film at the corner. (d) Film thickness increases at the corners due to surface tension.
solvent evaporation
Fig. 2.10 (a) Newly formed film near the edge. (b) Material flows from zone 2 to zone 1. (c) Material flows further from zone 2 to the surrounding environment.
The solvent concentration is high. Therefore, more material is transported from region 2 to the surrounding regions (regions 1 and 3) due to the surface tension gradient across the regions
