Dynamics of rigid microparticles at the interface of co-flowing immiscible liquids in a microchannel

We report the dynamical migration behavior of rigid polystyrene microparticles at an interface of co-flowing streams of primary CP₁ (aqueous) and secondary CP₂ (oils) immiscible phases at low Reynolds numbers (Re) in a microchannel. The microparticles initially suspended in the CP₁ either continue to flow in the bulk CP₁ or migrate across the interface into CP₂, when the stream width of the CP₁ approaches the diameter of the microparticles. Experiments were performed with different secondary phases and it is found that the migration criterion depends on the sign of the spreading parameter S and the presence of surfactant at the interface. To substantiate the migration criterion, experiments were also carried out by suspending the microparticles in CP₂ (oil phase). Our study reveals that in case of aqueous-silicone oil combination, the microparticles get attached to the interface since S<0 and the three phase contact angle, θ>90°. For complete detachment of microparticles from the interface into the secondary phase, additional energy ΔG is needed. We discuss the role of interfacial perturbation, which causes detachment of microparticles from the interface. In case of mineral and olive oils, the surfactants present at the interface prevents attachment of the microparticles to the interface due to the repulsive disjoining pressure. Finally, using a aqueous-silicone oil system, we demonstrate size based sorting of microparticles of size 25μm and 15μm respectively from that of 15μm and 10μm and study the variation of separation efficiency η with the ratio of the width of the aqueous stream to the diameter of the microparticles ρ.