Importance

Understanding the mechanics of underwater contact between soft materials and hard surfaces is of paramount importance due to its ubiquity and practical relevance. From biological creatures climbing flooded surfaces and human figures clinging to damp surfaces to adhesives sealing an underwater wound, performance depends on water drainage to achieve contact. We report a counterintuitive relationship between adhesion and water drainage. Surfaces with stronger thermodynamic adhesion work show slower discharge rates due to water entrapment in isolated puddles. Interestingly, the evacuation rates accelerate to the zero point of underwater grip work. This knowledge paves the way for the development of materials to achieve adhesion / adhesion underwater and prevent liquid entrapment through the appropriate combination of chemistry and surface structure.

Abstract

Thermodynamics tells us to expect that underwater contact between two hydrophobic surfaces will result in stronger adhesion compared to two hydrophilic surfaces. However, the presence of water not only modifies the energetics but also the dynamic process of reaching a final state, which combines solid deformation and liquid evacuation. These dynamics can create challenges to achieve strong grip / friction underwater, which affects a variety of areas including soft robotics, biolocomotion, and tire traction. Further investigation, requiring sufficiently precise resolution of film discharge while simultaneously monitoring surface wettability, was lacking. We perform high-resolution frustrated total internal reflection imaging in situ to follow the evolution of underwater contact between soft-elastic hemispheres of varying stiffness and smooth-hard surfaces of varying wettability. Surprisingly, we find that the exponential rate of water discharge from the hydrophobic-hydrophobic (adhesive) contact is three orders of magnitude lower than that from the hydrophobic-hydrophilic (non-adhesive) contact. The trend of decreasing rate with decreasing wettability of the glass changes sharply around a point where thermodynamic adhesion crosses zero, suggesting a transition in the vent mode, which is illuminated by spatio-height maps. three-dimensional temporal. Adhesive contact is characterized by early localization of sealed puddles, while non-adhesive contact remains smooth, with filmic release from a central puddle. Measurements with a human thumb and an alternately hydrophobic / hydrophilic glass surface demonstrate the practical consequences of the same dynamic: adhesive interactions cause instability in the valleys and lead to a more trapped water state and less solid-solid contact. respondent. These results offer an interpretation of the patterned texture observed in underwater biolomotor adaptations as well as an overview of the technological implementation.

Footnotes

    • Accepted August 17, 2021.
  • Author contributions: research designed by MS, NK, AD and HK; MS and NK have done research; MS, NK, AD and HK analyzed the data; and NK wrote the paper.

  • The authors declare no competing interests.

  • This article is a direct PNAS submission.

  • This article contains additional information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2104975118/-/DCSupplemental.

Data availability

All study data is included in the article and / or additional information. Additional data related to this article is available at https://zenodo.org/record/5518167 (45).


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