Swelling and evaporation determine surface morphology of grafted hydrogel thin films

TL;DR

This study investigates how swelling and evaporation influence surface pattern formation in grafted hydrogel thin films, revealing that initial swelling sets the pattern wavelength while evaporation alters the surface morphology.

Contribution

It provides experimental insights into the interplay between swelling and evaporation in determining hydrogel surface patterns, highlighting the role of elastocapillary length as a threshold.

Findings

Pattern wavelength scales with initial film thickness.

Evaporation significantly modifies surface morphology.

Elastocapillary length determines pattern formation threshold.

Abstract

We experimentally study the formation of surface patterns in grafted hydrogel films of nanometer-to-micrometer thicknesses 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.