Attractive forces slow contact formation between the deformable bodies underwater

Adhesion and friction between surfaces that touch are known to depend sensitively on gaps of near contact down to the nanoscale. While chemical affinity, compliance and roughness of the surfaces determine the thermodynamic state of contact, the actual state also depends on dynamics of its formation, which can depend on interplay between both surface properties and those of the mediating fluid. Presence of water changes the adhesive interaction between surfaces, while its resistance to evacuate the contact region frustrates its formation by leaving behind nanoscopic puddles. By high-resolution, in-situ, Frustrated Total Internal Reflection (FTIR) imaging, we track the evolution of contact between soft-elastic hemispheres of varying stiffness and smooth–hard surfaces of varying wettability. We find the exponential rate of water evacuation from hydrophobic–hydrophobic (adhesive) surfaces is 3 orders of magnitude smaller than that from hydrophobic–hydrophilic (non-adhesive) contact. This counterintuitive result comes from adhesive surfaces to more tightly sealing puddles of trapped water. Similar observation for a human thumb touching alternatively hydrophobic/hydrophilic glass surfaces, demonstrates generality of the mechanism and points to practical consequences.