Reconfigurable structured liquids

The past decades have witnessed the discovery and rapid development of adaptable, reconfigurable all-liquid systems where, by the interfacial assembly and jamming of nanoparticles (NPs), a liquid-liquid system can be structured, i.e., locked into a highly non-equilibrium spatial arrangement, maintaining all the inherent characteristics of the liquids but having the spatial organization and structural stability of a solid. Due to the nature of the interfacial jamming, external stimuli can be used to re-shape the liquids, imparting a responsiveness and malleability to the constructs. In general, the binding energy of a NP to the interface is small, so that compressive forces associated with the reduction in the interfacial area to minimize energy, are sufficient to expel the NPs from the interface. By generating nanoparticle surfactants (NPSs), where ligands dissolved in one liquid can interfacially interact with NPs dispersed in a second immiscible liquid, the binding energy of the NP is massively increased, enabling an interfacial jamming of the NPSs and an arresting of shape changes. Since the jamming is immune to the nature of the NPs, advantage can be taken of the inherent functionality of the NPs, e.g., catalytic, enzymatic, optical, electronic or magnetic, while chemical functionality, including selective functional group recognition, can be introduced through the ligands comprising the NPSs. NPS assemblies can be realized at the interface between any two immiscible liquids (oil-water, oil-oil, and water-water), underscoring the generality of this concept and the range of potential applications. Here, we review some of the basic principles underlying the formation of NPSs and the structuring of liquids, using examples drawn from work in our laboratories and elsewhere, and we provide a perspective on where challenges and applications of this unique state of matter exist.