Surface Instability of Bilayer Hydrogel Subjected to Both Compression and Solvent Absorption

Area of Science:

• Materials Science
• Soft Matter Physics
• Mechanical Engineering

Background:

• Bilayered structures, like hard films on soft substrates, exhibit instability leading to surface patterns (wrinkles, creases) under stimuli.
• For bilayer hydrogels, surface morphology is typically governed by solvent-induced swelling/shrinking and mechanical forces.

Purpose of the Study:

• To investigate the impact of upper layer thickness on bilayer hydrogel surface morphology under compression.
• To analyze the effect of pre-water absorption on the mechanical instability and resulting surface patterns of bilayer hydrogels.

Main Methods:

• Utilized finite element (FE) simulations to model hydrogel behavior.
• Employed theoretical analysis to complement FE simulation results.
• Examined two key factors: upper layer thickness and pre-water absorption.

Main Results:

• Increased upper layer thickness can lead to stable surface wrinkles without period doubling.
• Pre-absorption of water can induce fold formation or complex hierarchical wrinkles.
• Findings provide new insights into controlling hydrogel surface morphology.

Conclusions:

• The thickness of the upper hydrogel layer significantly affects pattern formation under compression.
• Water pre-absorption introduces novel morphological outcomes, including folds and hierarchical wrinkles.
• This research expands the understanding of bilayer hydrogel instability and offers pathways for experimental realization.