Indentation of a microparticle into an oil-coated, soft silicone surface

Small scale contact between a soft, liquid-coated layer and a stiff surface is found 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 usually possess uncrosslinked molecules (i.e. liquid silicone oil) when the modulus is low. The presence of liquid near the contact line can cause capillary forces on the particle. We consider a similar situation in which uncrosslinked liquid molecules are already placed at a surface prior to contact. More specifically, we investigate how the thickness of an oil layer relates to the indentation depth of a glass microsphere into a lightly crosslinked PDMS network. A simple model that balances the capillary force of the oil layer with the elastic force from the substrate is proposed to predict the position of the particle. Interestingly, the vertical force associated with a thin layer of liquid on a soft substrate appears to govern the indentation depth of a microsphere into the polymer network.