Pinning-Induced Folding-Unfolding Asymmetry in Adhesive Creases.

Area of Science:

• Soft Matter Physics
• Materials Science
• Biophysics

Background:

• Compression of soft elastic materials and biological tissues can induce creasing, a surface instability leading to self-contact.
• Unfolding after compression is often imperfect, leaving residual scars that act as nuclei for subsequent creases, indicating a form of surface memory.

Purpose of the Study:

• To investigate the mechanics of creasing and unfolding in soft elastic matter, specifically focusing on the role of surface tension and contact line dynamics.
• To elucidate the origins of folding memory in soft materials by examining the contact line region during creasing and unfolding.

Main Methods:

• Experiments were conducted on sticky polymer surfaces subjected to compression.
• Confocal microscopy was employed to visualize and resolve the contact line region during folding and unfolding.
• Analysis focused on the resulting crease morphologies and the dynamics of the contact line.

Main Results:

• Surface tension was observed to induce a secondary fold at the edge of the self-contact zone, creating singular elastic stress.
• Crease profiles exhibited self-similar morphologies.
• An intrinsic asymmetry between folding and unfolding was identified, attributed to contact line pinning, similar to wetting phenomena on imperfect solids.

Conclusions:

• Contact line pinning is a critical factor in the creasing instability of soft surfaces, inhibiting complete unfolding.
• This pinning mechanism is responsible for the observed folding memory in soft materials, influencing their mechanical response upon repeated compressions.