Capillary-driven indentation of a small particle into a soft, oil-coated 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), especially for soft commercially available kits. The presence of liquid near the contact line can cause capillary forces on the particle. We consider a similar situation in where silicone oil is already coated onto the surface prior to making contact with a microparticle. More specifically, we investigate how the thickness of the oil layer relates to the indentation depth of a glass microsphere on a soft PDMS network. A simple model that balances the capillary force of the oil layer with the elastic and surface forces from the substrate is proposed to predict the position of the particle.