Particles are known to adsorb at the oil/water interface in small molecule systems, when both of the fluids wet the particles partially. Particles adsorb at an equilibrium contact angle, which depends on their relative wettability towards oil and water. In principle, partially-wettable particles should also adsorb at the interface between two immiscible polymers. However, polymers - even highly immiscible ones - do not usually show large chemical differences, and hence realizing particles with partial wettability is expected to be challenging.

We have tested the interfacial activity of a variety of particles such as iron, iron oxyhydroxide, titanium dioixide, teflon, and hydrophobic silica in two different polymer pairs: (1) polyisoprene (PI) and polydimethylsiloxane (PDMS), and (2) PI and polyisobutylene (PIB). Surprisingly, all of these particles readily adsorb at the interface between both these polymer pairs. Particle adsorption at the PI/PIB interface is especially remarkable since these two polymers are chemically quite similar to each other.

We attempted to explain the adsorption of particles using Young's equation for the equilibrium contact angle. The surface energy of the particle was estimated by a “float-sink” test, and the solid/polymer interfacial energy was then calculated using Griffalco-Good theory. We were not able to correctly predict whether the particles will go to the interface of given two fluids. Thus Griffalco-Good theory is unable to predict why particles adsorb at the interface of PI/PDMS and PI/PIB.