Reconfigurable complex emulsions, which consist of multiphase droplets that can change morphology between encapsulated and Janus shapes, have become of recent fundamental and applied interest due to their unique stimuli-responsive characteristics. The newfound ability to dynamically change the structure and interfaces of droplets provides exciting opportunities for exploiting the properties and applications of fluids in ways not before possible, such as to create tunable lenses or droplet-based sensors. Droplet morphological reconfiguration, which is easily induced upon alteration of the balance of interfacial tensions, can be triggered in response to stimuli including pH, light, enzymes, temperature, and surfactants. This review describes recent advances involving reconfigurable complex droplet design, properties, and applications, highlighting both the opportunities and challenges associated with harnessing complex emulsions as responsive materials. We focus primarily on emulsions in which all droplet phases are immiscible with each other and the continuous phase, such as multiphase oil droplets dispersed in water or multiphase aqueous droplets dispersed in oil. The ability to manipulate the ordering of liquid interfaces in an emulsion while maintaining droplet stability has greatly enabled new directions for emulsion research and applications. Harnessing the dynamic structure and properties of reconfigurable complex emulsions presents a new frontier in the design of responsive materials relevant to optics, sensing, and active matter.

Micellar solubilization is a transport process occurring in surfactant-stabilized emulsions that can lead to Marangoni flow and droplet motility. Active droplets exhibit self-propulsion and pairwise repulsion due to solubilization processes and/or solubilization products raising the droplet's interfacial tension. Here, we report emulsions with the opposite behavior, wherein solubilization decreases the interfacial tension and causes droplets to attract. We characterize the influence of oil chemical structure, nonionic surfactant structure, and surfactant concentration on the interfacial tensions and Marangoni flows of solubilizing oil-in-water drops. Three regimes corresponding to droplet "attraction", "repulsion" or "inactivity" are identified. We believe these studies contribute to a fundamental understanding of solubilization processes in emulsions and provide guidance as to how chemical parameters can influence the dynamics and chemotactic interactions between active droplets.

Solubilization is a spontaneous transport process occurring in surfactant-stabilized emulsions that can lead to Marangoni flow and droplet motility. Conventionally, active droplets exhibit self-propulsion and pairwise repulsion due to solubilization processes and/or solubilization products raising the droplet’s interfacial tension. Here, we report emulsions with the opposite behavior, wherein solubilization and/or its products decrease the interfacial tension and cause droplets to attract. We systematically characterize the influence of oil structure, nonionic surfactant structure, and surfactant concentration on the interfacial tensions and Marangoni flows of solubilizing oil-in-water drops. Three regimes corresponding to droplet “attraction”, “repulsion” or “inactivity” are identified and the chemical trends leading to these behaviors are discussed. Notably, droplet inactivity, wherein no convective flow is observed, can still occur even when the droplet is solubilizing at appreciable rates. Droplets that are inactive can be induced to become active through doping of the surfactant continuous phase with another oil type. We believe these studies contribute to a new fundamental understanding of solubilization processes in emulsions and provide guidance as to how chemical parameters can be used to influence the dynamics and chemotactic interactions between active droplets.

Emulsion droplets, such as oil-in-water droplets stabilized by surfactant, are ubiquitous in products ranging from food to pharmaceuticals to paints. However, emulsion droplets are often thermodynamically unstable and thus persist under non-equilibrium conditions for extended times. As such, equilibrium properties like partition coefficients or interfacial tensions may be inadequate to describe the properties of an out-of-equilibrium droplet that can potentially experience conditions not accessible at equilibrium. Here, the partitioning of nonionic surfactants between microscale oil droplets and water is investigated under non-equilibrium conditions wherein the droplets are shrinking in volume over time via solubilization. Quantitative mass spectrometry is used to analyze the composition of individual micro-droplets as a function of time under conditions of varying droplet diameter, surfactant molecular structure and concentration, and oil molecular structure. We find that common nonionic surfactants partition into the oil droplets over a timescale of minutes and reach a non-equilibrium steady state; this steady state concentration can be orders of magnitude higher than the aqueous phase surfactant concentration and higher than what is accessible under equilibrium partitioning conditions. Using kinetic data and steady state apparent partition coefficients, we describe the surfactant distribution between the water and droplet using a mass transfer model. Over longer timescales of hours, the droplet sheds accumulated surfactant back into the water, creating transiently high concentrations of oil and surfactant near the droplet interface which leads to the evolution of ultralow interfacial tension. Introduction of an ionic surfactant that forms mixed micelles with the nonionic surfactant reduces the nonionic surfactant transfer into oil; based on this observation, we use stimuli-responsive ionic surfactants to trigger phase separation and mixing inside droplets via modulation of the nonionic surfactant partitioning. This study thus reveals generalizable non-equilibrium states and conditions experienced by solubilizing droplets which govern emulsion properties.

Polyelectrolyte capsules stabilize biphasic oil droplets while preserving droplet reconfigurability in the presence of surfactants.