Shape-Recovering Liquids

The segregation of particles to the interface between two immiscible liquids to reduce the interfacial energy is a fundamental principle that underpins emulsification and the phase behavior of a wealth of multicomponent composite systems. Breaking this textbook rule, we find that the strong binding and two-dimensional assembly of ferromagnetic particles at a liquid-liquid interface not only suppresses emulsification, but also increases the interfacial tension, as found when the interface between two immiscible liquids assumes the shape of a Grecian urn. This shape is rapidly and reproducibly recovered after being destroyed by vigorous agitation. The suppression of emulsification, the rapid formation of a stable, non-planar equilibrium shape of the interface, and the increase in interfacial tension have their origins in the attractive in-plane dipolar magnetic interactions between the particles.