Particle Stabilization of Oil-Fluorocarbon Interfaces and Effects on Multiphase Oil-in-Water Complex Emulsion Morphology and Reconfigurability

Encapsulation of Hydrophobic-but-Not-Lipophilic Perfluoro Liquids Based on a Self-Assembled Double Emulsion Template via Solvent Evaporation Method

The facile encapsulation of perfluoro liquids that are hydrophobic but not lipophilic into liposomes or microcapsules presents a significant challenge in the fields of biomedicine, dynamic optics, functional chemical applications, etc. This is due to their chemical inertness and physical immiscibility, particularly those with low boiling points. In this study, a novel strategy based on a double emulsion template via solvent evaporation is proposed after investigating the mechanism of three-phase emulsion systems. The perfluoro liquid droplets can be easily emulsified into a polymer solution as the second emulsion layer, where the polymer shell is formed during solvent evaporation in the continuum medium under proper processing controls. The morphology of particles is predictable and fits well with the linear model derived from Neumann's triangle in three-phase systems. Furthermore, a comprehensive study on the encapsulation of perfluoro ketone, which is widely used as a green fire extinguisher agent, is conducted as an example. The encapsulated perfluoro ketone showed instant thermal response upon heating while maintaining a good shelf life at room temperature. The remarkable fire suppression performance exhibited great potential for practical applications. This work offers more insight into the encapsulation of "naughty" perfluorinated chemicals and provides more possibilities for extended applications.

Chemical Programming of Solubilizing, Nonequilibrium Active Droplets

ConspectusThe multifunctionality and resilience of living systems has inspired an explosion of interest in creating materials with life-like properties. Just as life persists out-of-equilibrium, we too should try to design materials that are thermodynamically unstable but can be harnessed to achieve desirable, adaptive behaviors. Studying minimalistic chemical systems that exhibit relatively simple emergent behaviors, such as motility, communication, or self-organization, can provide insight into fundamental principles which may enable the design of more complex and life-like synthetic materials in the future.Emulsions, which are composed of liquid droplets dispersed in another immiscible fluid phase, have emerged as fascinating chemically minimal materials in which to study nonequilibrium, life-like properties. As covered in this Account, our group has focused on studying oil-in-water emulsions, specifically those which destabilize by solubilization, a process wherein oil is released into the continuous phase over time to create gradients of oil-filled micelles. These chemical gradients can create interfacial tension gradients that lead to droplet self-propulsion as well as mediate communication between neighboring oil droplets. As such, oil-in-water emulsions present an interesting platform for studying active matter. However, despite being chemically minimal with sometimes as few as three chemicals (oil, water, and a surfactant), emulsions present surprising complexity across the molecular to macroscale. Fundamental processes governing their active behavior, such as micelle-mediated interfacial transport, are still not well understood. This complexity is compounded by the challenges of studying systems out-of-equilibrium which typically require new analytical methods and may break our intuition derived from equilibrium thermodynamics.In this Account, we highlight our group's efforts toward developing chemical frameworks for understanding active and interactive oil-in-water emulsions. How do the chemical properties and physical spatial organization of the oil, water, and surfactant combine to yield colloidal-scale active properties? Our group tackles this question by employing systematic studies of active behavior working across the chemical space of oils and surfactants to link molecular structure to active behavior. The Account begins with an introduction to the self-propulsion of single, isolated droplets and how by applying biases, such as with a gravitational field or interfacially adsorbed particles, drop speeds can be manipulated. Next, we illustrate that some droplets can be attractive, as well as self-propulsive/repulsive, which does not fall in line with the current understanding of the impact of oil-filled micelle gradients on interfacial tensions. The mechanisms by which oil-filled micelles influence interfacial tensions of nonequilibrium interfaces is poorly understood and requires deeper molecular understanding. Regardless, we extend our knowledge of droplet motility to design emulsions with nonreciprocal predator-prey interactions and describe the dynamic self-organization that arises from the combination of reciprocal and nonreciprocal interactions between droplets. Finally, we highlight our group's progress toward answering key chemical questions surrounding nonequilibrium processes in emulsions that remain to be answered. We hope that our progress in understanding the chemical principles governing the dynamic nonequilibrium properties of oil-in-water droplets can help inform research in tangential research areas such as cell biology and origins of life.

Compound Droplet Generation by a Hybrid Microfluidic Device

Microfluidic technology based on a compound droplet plays an increasingly significant role in different disciplines, such as genetic detection, drug transportation, and cell culture. Low-cost, stable, and rapid methods to produce compound droplets are more and more in demand. In this paper, a hybrid 3D-printed microfluidic device was designed to realize efficient fabrication of multicore compound droplets, where a first oil phase (O1) is cut by a water phase (W) to form pure O1 droplets, and then the W phase containing O1 droplets is cut by a second oil phase (O2) to generate multicore compound droplets. A series of experiments were conducted to determine the influence of the flow rate and viscosity on the formation dynamics of compound droplets. It is found that the number of inner cores is mainly affected by the W and O2 phases, and a W phase with higher viscosity and a higher flow rate is more likely to produce compound droplets with more inner cores. This work provides new insights into the formation dynamics of compound droplets and can contribute to the optimization of emulsion production.

Multilevel Information Encryption Based on Thermochromic Perovskite Microcapsules via Orthogonal Photic and Thermal Stimuli Responses

Increasing modal variations of stimulus-responsive materials ensure the high capacity and confidentiality of information storage and encryption systems that are crucial to information security. Herein, thermochromic perovskite microcapsules (TPMs) with dual-variable and quadruple-modal reversible properties are designed and prepared on the original oil-in-fluorine (O/F) emulsion system. The TPMs respond to the orthogonal variations of external UV and thermal stimuli in four reversible switchable modes and exhibit excellent thermal, air, and water stability due to the protection of perovskites by the core-shell structure. Benefiting from the high-density information storage TPMs, multiple information encryptions and decryptions are demonstrated. Moreover, a set of devices are assembled for a multilevel information encryption system. By taking advantage of TPMs as a "private key" for decryption, the signal can be identified as the corresponding binary ASCII code and converted to the real message. The results demonstrate a breakthrough in high-density information storage materials as well as a multilevel information encryption system based on switchable quadruple-modal TPMs.

Evaluation of Interfacial Structure of Self-Assembled Nanoparticle Layers: Use of Standard Deviation between Calculated and Experimental Drop Profiles as a Novel Method

The self-assembly of nanoparticles (NPs) at interfaces is currently a topic of increasing interest due to numerous applications in food technology, pharmaceuticals, cosmetology, and oil recovery. It is possible to create tunable interfacial structures with desired characteristics using tailored nanoparticles that can be precisely controlled with respect to shape, size, and surface chemistry. To address these functionalities, it is essential to develop techniques to study the properties of the underlying structure. In this work, we propose an experimental approach utilizing the standard deviation of drop profiles calculated by the Laplace equation from experimental drop profiles (STD), as an alternative to the Langmuir trough or precise microscopic methods, to detect the initiation of closely packed conditions and the collapse of the adsorbed layers of CTAB-nanosilica complexes. The experiments consist of dynamic surface/interfacial tension measurements using drop profile analysis tensiometry (PAT) and large-amplitude drop surface area compression/expansion cycles. The results demonstrate significant changes in STD values at the onset of the closely packed state of nanoparticle-surfactant complexes and the monolayer collapse. The STD trend was explained in detail and shown to be a powerful tool for analyzing the adsorption and interfacial structuring of nanoparticles. Different collapse mechanisms were reported for NP monolayers at the liquid/liquid and air/liquid interfaces. We show that the interfacial tension (IFT) is solely dependent on the extent of interfacial coverage by nanoparticles, while the surfactants regulate only the hydrophobicity of the self-assembled complexes. Also, the irreversible adsorption of nanoparticles and the increasing number of adsorbed complexes after the collapse were observed by performing consecutive drop surface compression/expansion cycles. In addition to a qualitative characterization of adsorption layers, the potential of a quantitative calculation of the parameter STD such as the number of adsorbed nanoparticles at the interface and the distance between them at different states of the interfacial layer was discussed.

Ionic Liquid-in-Water Emulsions Stabilized by Molecular and Polymeric Surfactants

Ionic liquids have drawn notable attention for their unique solvent properties and use in applications such as batteries and chemical separations. While many ionic liquids are water-soluble, there are numerous examples of ionic liquids that are sufficiently hydrophobic to remain phase separated from water. However, relatively little is known about the stability and properties of ionic liquid-in-water emulsions. Here, we survey a series of ionic liquid-in-water emulsions stabilized by a range of ionic and nonionic molecular surfactants and polymers. To assess droplet stability and dynamics, we characterize the ionic liquid-surfactant interfacial tension, describe qualitative coarsening rates, and quantify droplet solubilization rate. In some instances, we observe unexpected spontaneous formation of complex double and triple emulsions. Our observations highlight approaches for ionic liquid emulsion formulation and provide insight into how to address challenges surrounding stabilization of ionic liquid-in-water droplets with molecular surfactants.

Complex Suspended Janus Droplets Constructed through Solvent Evaporation-Induced Phase Separation at the Air-Liquid Interface

Phase separation technology has attracted extensive scientific interest because of its intriguing structure changes during the phase separation process. Phase separation inside emulsion droplets in continuous surroundings has been well studied in recent years. Many investigations have also been conducted to study the droplet phase separation phenomena in noncontinuous surroundings. However, studies on the phase separation phenomena and the spreading behavior of suspended droplets at the air-liquid interface were rarely reported. In this study, PEGDA-glycerol suspended Janus droplets with a patchy structure were produced by utilizing solvent evaporation-induced droplet phase separation at the air-liquid interface. By altering the glycerol/PEGDA volume ratio, the initial proportion of ethanol, and the concentration of surfactants, suspended droplets with different morphologies can be achieved, which include filbert-shaped droplets (FSDs), half lotus seedpod single-phase Janus droplets (HLSDs), lotus seedpod single-phase Janus droplets (LSDs), lotus seedpod-shaped droplets (LSSDs), multiple-bulge droplets (MBDs), and half gourd-shaped droplets (HGSDs). A patchy structure was generated at the air-droplet interface, which was attributed to the Marangoni stresses induced by nonuniform evaporation. Furthermore, a modified spreading coefficient theory was constructed and verified to illustrate the phase separation at the air-droplet interface, which was the first research to predict the phase separation phenomena at the air-liquid interface via spreading coefficients theory. Moreover, we studied the factors that led to the droplets being able to float by designing the combined parameters, including three interfacial tensions and the equilibrium contact angles. Therefore, a simple and versatile strategy for creating suspended Janus droplets has been developed for the first time, which holds significant potential in a variety of applications for material synthesis, such as the electrospinning solution behavior when sprayed from the nozzle into the air.

Self-Organization of Active Droplets into Vortex-like Structures

Natural or artificial active objects can demonstrate mirror asymmetry of collective motion when they are moving coherently in a vortex. The majority of known cases related to the emergence of collective dynamical chirality are referred to as active objects with individual structure chirality and/or dynamical chirality. Here, we demonstrate that dynamically and structurally achiral active droplets can self-organize into vortex-like structures. Octane droplets dispersed in the aqueous solution of an anionic surfactant are activated with ammonia addition. The motion of droplets is due to the Marangoni flow emerging at the interfaces of the droplets. We found out that different modes of vortex motion of droplets in the emulsion can arise depending on the size of the region that confines the motion of the droplets and their number density and velocity.

Interfacially-adsorbed particles enhance the self-propulsion of oil droplets in aqueous surfactant

Understanding the chemo-mechanical mechanisms that direct the motion of self-propulsive colloids is important for the development of active materials and exploration of dynamic, collective phenomena. Here, we demonstrate that the adsorption of solid particles on the surface of solubilizing oil droplets can significantly enhance the droplets' self-propulsion speeds. We investigate the relationship between the self-propulsion of bromodecane oil droplets containing silica particles of varying concentration in Triton X-100 surfactant, noting up to order of magnitude increases in propulsion speeds. Using fluorescently labeled silica, we observe packing of the particles at the oil-water interfaces of the rear pole of the moving droplets. For bromodecane oil droplets in Triton X-100, the highest droplet speeds were achieved at approximately 40% particle surface coverage of the droplet interface. We find particle-assisted propulsion enhancement in ionic surfactants and different oil droplet compositions as well, demonstrating the breadth of this effect. While a precise mechanism for the propulsion enhancement remains unclear, the simple addition of silica particles to droplet oil-water interfaces provides a straightforward route to tune active droplet dynamics.

Crystallization of Active Emulsion

Active matter contains self-propelled units able to convert stored or ambient free energy into motion. Such systems demonstrate amazing features related to the phenomenon of self-organization and phase transitions and can be used for the development of artificial materials and machines that operate away from equilibrium. Significant advances in the fabrication of active matter were achieved when studying low-density gas and small crystallites. However, the technique of preparation of active matter, where one can observe the formation of stable crystals, is extremely challenging. Here, we describe the novel method to obtain a stable 2D crystal in the active octane-in-water emulsion during the process of heterogeneous crystallization. Active motion is driven by the Marangoni flow emerging at the interface of the droplet. It is established that the crystal volume increases linearly in time in the process of crystallization. Moreover, the dependence of the crystal growth rate on the average velocity of droplets motion in the emulsion has a maximum. The kinetics of crystal growth is defined by a competition between the processes of attachment and detachment of droplets from the crystal surface. Crystallization proceeds via condensation of droplets from the gas phase through the formation of liquid as an intermediate phase, which covers the crystal surface with a thin layer. Inside the liquid layer the bond-orientational order of droplets decreases from the crystal surface toward the gas phase. We anticipate our study to be a starting point for the development of new materials and technologies on the basis of nonequilibrium droplet systems.

Advances and Opportunities of Oil-in-Oil Emulsions

Emulsions are mixtures of two immiscible liquids in which droplets of one are dispersed in a continuous phase of the other. The most common emulsions are oil-water systems, which have found widespread use across a number of industries, for example, in the cosmetic and food industries, and are also of advanced scientific interest. In addition, the past decade has seen a significant increase in both the design and application of nonaqueous emulsions. This has been primarily driven by developments in understanding the mechanism of effective stabilization of oil-in-oil (o/o) systems, either using block copolymers (BCPs) or solid (Pickering) particles with appropriate surface functionality. These systems, as highlighted in this review, have enabled emergent applications in areas such as pharmaceutical delivery, energy storage, and materials design (e.g., polymerization, monolith, and porous polymer synthesis). These o/o emulsions complement traditional emulsions that utilize an aqueous phase and allow the use of materials incompatible with water. We assess recent advances in the preparation and stabilization of o/o emulsions, focusing on the identity of the stabilizer (BCP or particle), the interplay between stabilizer and oils, and highlighting applications and opportunities associated with o/o emulsions.