Crease formation in microscale friction on a soft, adhesive surface

Studies of friction started centuries ago, yet an understanding of small-scale static friction on soft materials remains incomplete. This is partially due to the diversity in material properties that exist in soft materials, as well as with challenges in experimentally visualizing and probing the mechanics. We employ an experimental approach that combines force and confocal microscopy to combine force quantification with cross-sectional imaging. In our experiments, a glass microparticle is put in contact with a soft, adhesive polydimethylsiloxane (PDMS) surface, which is then pulled laterally. Confocal imaging reveals the emergence of creases. Creasing appears on PDMS surface exposed to ambient light as well as UV-ozone treatment. Using finite element analysis, we study what parameters govern the creasing behavior. Our experimental and numerical results show that creases are self-contacting, and move through the contact zone in a Schallamach wave-like manner, like a ruck in a rug. Yet for this to be possible and remain self-contacting, the crease interface must be adhesive in the normal direction, while allowing for self-slip. Additionally, our study shows that the interfacial strength at the particle-PDMS interface must be sufficiently high for creasing to occur.