Homogeneous microscopic bumps generated on λ-carrageenan cast film surface

Abstract

Here we investigate the phenomenon of bump formation on λ-carrageenan (CG). In this phenomenon, microscopic bumps (ca. 2.8 µm wide and ca. 0.4 µm high) form on the surface of λ-CG cast film created by evaporating a 1.0 wt% λ-CG solution in a PTFE petri dish at 60 ºC. We observe that the size of the bumps can be controlled by adjusting the concentration and volume of the λ-CG solution, and this is correlated with the film thickness. Specifically, as the film thickness increases, the size of the bumps also increases with strong correlation. We also investigate the mechanism underlying bump formation. We find that bumps form by complete drying and that the λ-CG film exhibits less-adhered inner layers with periodic curves resembling the bumps. These results suggest the curved layer structure is related to the formation mechanism. In other words, the drying shrinkage strain is relieved on each layer through curving, leading to the generation of bumps on the free surface. Therefore, we hypothesize that the fundamental mechanism behind the formation of these bumps lies in the self-organization of λ-CG, which results in the creation of an adhesion-less layered structure during the cast film formation process.