  Small scale contact between a soft, liquid-coated layer and a stiff surface
is common in many situations, from synovial fluid on articular cartilage to
adhesives in humid environments. Moreover, many model studies on soft adhesive
contacts are conducted with soft silicone elastomers, which possess
uncrosslinked liquid molecules (i.e. silicone oil) when the modulus is low. We
investigate how the thickness of a silicone oil layer on a soft substrate
relates to the indentation depth of glass microspheres in contact with
crosslinked PDMS, which have a modulus of <10 kPa. The particles indent into
the underlying substrate more as a function of decreasing oil layer thickness.
This is due to the presence of the liquid layer at the surface that causes
capillary forces to push down on the particle. A simple model that balances the
capillary force of the oil layer and the particle-substrate adhesion with the
elastic and surface tension forces from the substrate is proposed to predict
the particle indentation depth.
