Adhesive forces inhibit underwater contact formation for a soft-hard collision

TL;DR

This study investigates how water evacuation dynamics and surface wettability influence underwater contact formation and adhesion between soft and hard surfaces, revealing that hydrophobic-hydrophobic contacts significantly slow water evacuation and affect contact stability.

Contribution

The paper provides high-resolution in-situ imaging of underwater contact evolution, uncovering the impact of wettability on water evacuation rates and contact stability, with implications for soft robotics and bio-locomotion.

Findings

Water evacuation from hydrophobic-hydrophobic contact is three orders of magnitude slower than from hydrophobic-hydrophilic contact.

A transition in evacuation mode occurs where thermodynamic adhesion crosses zero, indicating a change in contact behavior.

Adhesive contacts trap more water and exhibit localized puddles, reducing solid-solid contact quality.

Abstract

Thermodynamics tells us to expect underwater contact between two hydrophobic surfaces to result in stronger adhesion compared to two hydrophilic surfaces. However, presence of water changes not only energetics, but also the dynamic process of reaching a final state, which couples solid deformation and liquid evacuation. These dynamics can create challenges for achieving strong underwater adhesion/friction, which affects diverse fields including soft robotics, bio-locomotion and tire traction. Closer investigation, requiring sufficiently precise resolution of film evacuation while simultaneously controlling surface wettability has been lacking. We perform high resolution in-situ frustrated total internal reflection imaging to track underwater contact evolution between soft-elastic hemispheres of varying stiffness and smooth-hard surfaces of varying wettability. Surprisingly, we find exponential rate of water evacuation from hydrophobic-hydrophobic (adhesive) contact is 3 orders of magnitude lower than that from hydrophobic-hydrophilic (non-adhesive) contact. The trend of decreasing rate with decreasing wettability of glass sharply changes about a point where thermodynamic adhesion crosses zero, suggesting a transition in mode of evacuation, which is illuminated by 3-Dimensional spatiotemporal heightmaps. Adhesive contact is characterized by the early localization of sealed puddles, whereas non-adhesive contact remains smooth, with film-wise evacuation from one central puddle. Measurements with a human thumb and alternatively hydrophobic/hydrophilic glass surface demonstrate practical consequences of the same dynamics: adhesive interactions cause instability in valleys and lead to a state of more trapped water and less intimate solid-solid contact.