Processable Composites with Extreme Material Capacities: Toward Designer High Internal Phase Emulsions and Foams

Abstract

High internal phase emulsions (HIPEs) and foams are ubiquitous in our daily lives. They are considered fluid composites with an extremely high-volume fraction of fluid fillers. As all other composite materials do, HIPEs and foams exhibit enhanced properties compared with the simple base fluids without liquid/gas fillers. They can disperse a huge amount of immiscible fluids in other fluids, and fluid fillers remarkably change their physicochemical properties, especially their rheological properties. Their unique structural and compositional features make this material special: A large amount of a dispersed phase is in a very small amount of a continuous phase, and they have an unusual flow behavior as soft glassy materials. Because their fluid nature allows many other materials (e.g., small molecules, polymers, colloids) to be easily dispersed inside, they have a huge potential to be transformed into functional materials that cannot easily be obtained with other hard materials. Here, we summarize the recent progress on HIPEs and foams, particularly in terms of four themes: (1) fabrication techniques, (2) stabilizers and their requirements, (3) flow behavior relating to rheological properties, and (4) porous materials templated from HIPEs and foams. We also discuss future directions and what needs to be done for each theme. Finally, although there have been numerous studies on HIPEs and foams, we present the general outlook for HIPEs and foams to better understand them so that they can be applied to new and unprecedented applications.