Optical Characterization of Underwater Contact Mechanics

For survival in extreme environments, organisms have evolved adhesive mechanisms and materials that out-compete those of human technology. While the chemistry of underwater bio-adhesion is a source of valuable insight, the mechanics by which surfaces expel water and come in contact underlies the critical understanding of natural solutions and evaluating biomimetic analogs. Hydrophobicity of an adhesive surface has been shown to be crucial in removing water from a hydrophilic substrate, but the resulting contact is typically heterogeneous, with patches of unevacuated water. Thermodynamically, the hydrophobic part of any adhesive or surface drives the water out in presence of any other akin moiety.
Here, we present an FTIR-based imaging technique to spatially resolve and quantify the thickness of puddles formed between two solids in contact underwater. The technique is validated by comparing measured air gap thickness between a glass lens and glass prism with Hertzian contact theory and then applied to characterize the formation and drainage of patches of water as a soft PDMS lens approaches a functionalized and pristine glass surface. The work paves the way for a better understanding of the contact interfaces involved in the field of tribology, adhesion, and soft coatings.