Swelling and Evaporation Determine Surface Morphology of Grafted Hydrogel Thin Films

We experimentally study the formation of surface patterns
in grafted
hydrogel films of nanometer-to-micrometer thickness during imbibition-driven
swelling followed by evaporation-driven shrinking. Creases are known
to form at the hydrogel surface during swelling; the wavelength of
the creasing pattern is proportional to the initial thickness of the
hydrogel film with a logarithmic correction that depends on microscopic
properties of the hydrogel. We find that, although the characteristic
wavelength of the pattern is determined during swelling, the surface
morphology can be significantly influenced by evaporation-induced
shrinking. We observe that the elastocapillary length based on swollen
mechanical properties gives a threshold thickness for a surface pattern
formation and consequently an important change in morphology.