Pickering emulsions have been successfully used as media for catalysis and separation. However, simultaneous reaction and separation cannot be performed in a continuous mode in these systems, because reagents cannot be readily loaded into or recovered from the dispersed phase. Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are continuous throughout the structure, have potential as media for simultaneous reaction and separation in a continuous mode. In this work, we take a major step toward realizing this vision by demonstrating the ability of bijels to be used in reactive separation performed in a batch fashion. To perform effectively, bijels must maintain their morphology and interfacial mass transfer properties during reaction. To strengthen the bijels, we modify the solvent transfer-induced phase separation (STRIPS) method to make bijels resistant to mechanical stresses and prevent detachment of nanoparticles from the oil/water interface due to pH changes by chemically fusing the interfacial nanoparticles. The reinforced bijel is successfully tested in base-catalyzed hydrolysis of esters and remains robust under these challenging conditions.

Bicontinuous interfacially jammed emulsion gels (bijels) comprise networks of interpenetrating domains of two immiscible liquids stabilized by nanoparticles jammed at the fluid–fluid interface. Bijels were first reported in simulation, which subsequently spurred experimentalists to develop fabrication approaches including thermal quenching, cosolvent removal, and direct mixing to explore bijels in applications that exploit their unique properties and morphology. Here, we comprehensively review recent developments in bijel fabrication and compare various bijel fabrication techniques. We also share our perspective on potential future directions and applications.

Studies of particle-stabilized biphasic structures have greatly expanded the possibilities of multiphasic systems by producing liquid composites with unique morphologies and properties. The solvent transfer-induced phase separation (STRIPS) method was previously introduced to prepare bicontinuous interfacially jammed emulsion gels (bijels), a unique class of particle-stabilized biphasic structures. Although STRIPS enables continuous processing, the requirement of an external aqueous phase may limit its application. In this work, a new method to produce a variety of three-dimensional multiphasic structures including bijels via vapor-induced phase separation (VIPS) is demonstrated. VIPS enables the fabrication of films and coatings by spreading or spraying a particle-laden suspension onto a surface without the need of an outer aqueous phase. Intriguingly, secondary nucleation of structures occurs within phase separated domains, leading to hierarchically bicontinuous biphasic structures. The dimensions of phase separated domains can be controlled by the rate of cosolvent removal. Moreover, a different path to bicontinuous morphology is identified, which does not occur via spinodal decomposition but rather relies on the formation of percolating structures of partially coalesced particle-covered droplets. The VIPS process provides a scalable path to produce bicontinuous biphasic composites with hierarchical structures for advanced coatings and membranes applications.

This chapter introduces the formation of bijels via solvent transfer-induced phase separation (STrIPS). STrIPS bijels are generated by triggering spinodal decomposition of a ternary liquid mixture composed of oil, water, solvent and surface-active nanoparticles. Bicontinuous oil/water channel networks are stabilized by interfacial jamming of the nanoparticles. Continuous production of STrIPS bijel fibers, planar films and microparticles is realized via a scalable route amenable to mass production. STrIPS bijels have asymmetric internal structures, with small domains on the surface and larger internal domains. We discuss the dependency of STrIPS bijel domain organization and size based on variation of nanoparticle concentration and surface functionalization. Moreover, we introduce a microfluidic technique to measure the fracture strength of STrIPS bijels. Mechanical reinforcement of STrIPS bijels is realized by interfacial nanoparticle cross-linking and sol–gel chemistry. Furthermore, applications of STrIPS bijels in reactive separations and as nanocomposite ultrafiltration membranes are introduced. The chapter concludes with an outlook and future perspectives on fundamental research, as well as applications for STrIPS bijels.

Solvent transfer-induced phase separation bijels with oils of different polarity are fabricated by using a mixture of hydrophilic and hydrophobic silica nanoparticles, further expanding their potential applications in separation and catalysis.