  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.
