Surface-pattern evolution in a swelling gel under a geometrical constraint: Direct observation of fold structure and its coarsening dynamics

We demonstrate here the direct observation of the side view of surface folds in a swelling gel, which are induced by elastic instability. The fold structure along the thickness direction has been clarified, which is a prerequisite for understanding its singular nature. The elementary coarsening process of the fold pattern is a discrete process and no transverse motion of folds along the surface direction is observed. The fold pattern grows self-similarly; however, the shape characteristics of the fold pattern are dependent on the initial thickness of the gel. This indicates that the shape of the fold is dependent on the strength of the constraint from the bottom surface.