Morphological transition in the instability of surface-attached hydrogel films*

Hydrogels may swell drastically when brought in contact with solvent, as driven by gradients of osmotic pressure, resulting in a change in volume that depends on the swelling ratio. For poly-N-isopropylacrylamide gels (PNIPAM), the ratio of wet-to-dry volume is usually a few hundred percent. However, grafting hydrogel films onto a rigid substrate geometrically constrains the swelling. The formation of a surface pattern can result of swelling-induced in-plane stresses.

We study the formation of patterns observed at the surface of PNIPAM-hydrogel films grafted onto silicon wafers, with dry thickness varying from 10 nm to 5 µm. After crosslinking, swelling in a good solvent, combined with subsequent drying in ambient air, gives rise to a plethora of morphologies. Among them, we can see creases and more complex shapes that depend on the wet/dry state of the sample. We observe that both the dry and wet wavelengths of the pattern scale with the initial dry thickness, with a logarithmic correction involving a relevant length scale. The choice of this length scale may involve the pore size or the elasto-capillary length, depending on thickness. The agreement between the resulting correction and the experimental data gives clues for explaining the underlying mechanism of pattern formation.