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Thermodynamics tells us to expect underwater contact between two hydrophobicsurfaces to result in stronger adhesion compared to two hydrophilic surfaces.However, presence of water changes not only energetics, but also the dynamicprocess of reaching a final state, which couples solid deformation and liquidevacuation. These dynamics can create challenges for achieving strongunderwater adhesion/friction, which affects diverse fields including softrobotics, bio-locomotion and tire traction. Closer investigation, requiringsufficiently precise resolution of film evacuation while simultaneouslycontrolling surface wettability has been lacking. We perform high resolutionin-situ frustrated total internal reflection imaging to track underwatercontact evolution between soft-elastic hemispheres of varying stiffness andsmooth-hard surfaces of varying wettability. Surprisingly, we find exponentialrate of water evacuation from hydrophobic-hydrophobic (adhesive) contact is 3orders of magnitude lower than that from hydrophobic-hydrophilic (non-adhesive)contact. The trend of decreasing rate with decreasing wettability of glasssharply changes about a point where thermodynamic adhesion crosses zero,suggesting a transition in mode of evacuation, which is illuminated by3-Dimensional spatiotemporal heightmaps. Adhesive contact is characterized bythe early localization of sealed puddles, whereas non-adhesive contact remainssmooth, with film-wise evacuation from one central puddle. Measurements with ahuman thumb and alternatively hydrophobic/hydrophilic glass surface demonstratepractical consequences of the same dynamics: adhesive interactions causeinstability in valleys and lead to a state of more trapped water and lessintimate solid-solid contact.

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