Confinement Induced Alteration in Interfacial Energy

Hypothesis: When a liquid is confined between two parallel plates, the pressure field at its bulk alters, because of the concave meniscus at the air-water interface. For an aqueous surfactant solution in contact with an oil, like an oil-in-water emulsion, this effect can alter the surfactant concentration at the interface of the two phases thereby changing the interfacial energy. Alteration of interfacial energy is expected to affect morphology of a complex system consisting of three immiscible phases. Experiments: A drop of crosslinkable silicone liquid was placed on a glass slide and was immobilized by partial crosslinking. An aqueous solution of surfactant, with SiO2 microspheres dispersed in it, was dispensed on this drop to create a liquid pool. It was then confined between two parallel plates to create a meniscus. Finding: Consequently, the particles got embedded at the oil-water interface to different extent, depending on the degree of confinement. Force balance along tangents to different interfaces showed that the interfacial energy of silicone-water (aqCTAB 0.3 mM) interface increases from 33.3 ± 0.5 mJ/m2 for an unconfined system to 45.4 ± 2.2 mJ/m2 for the confined one. This effect then elucidates how confinement alters complex emulsion morphologies in multiphase systems.