Interfacial Effects on Underwater Suction-Based Adhesion: Role of Surface Roughness

Octopuses, squids, and cuttlefish use suction to attach to underwater surfaces, capture prey, and maneuver in dynamic environments. Their remarkable and versatile suction capabilities have inspired biomimetic applications in wet biomedical patches, tissue engineering, and underwater soft robotics. However, the physical parameters governing suction adhesion, particularly the forces contributing to adhesion stability and failure mechanisms, remain debated. For instance, the role of van der Waals forces is widely contested. Here, we employ a simple experimental setup with a suction cup on a rough, soft substrate, revealing an instability transition in adhesion, with a shift from slow to rapid detachment on rough surfaces. Using confocal microscopy imaging and temporal mean square displacement (tMSD) analysis, we uncover the underlying mechanism of this transition, linking it to the percolation of interfacial fluid flow. Confocal imaging reveals water entrapment between the suction cup lip and the rough substrate, highlighting regions of direct solid-solid contact. Through tMSD analysis of the particle trajectories between two water entrapment regions, we identify a local transition from diffusive to super-diffusive behavior of the fluid flow, indicating a sudden detachment at the solid-solid contact regions. Our findings suggest that van der Waals forces significantly contribute to underwater suction adhesion, particularly on hydrophobic surfaces, enhancing adhesion longevity. To support our conclusion, we show that adhesion time is extended on substrates with higher root mean square roughness amplitude (h_rms) and higher peak material volume (Vmp) compared to those with lower h_rms and Vmp.