Janus Emulsions Research Articles

Particle-stabilized Janus emulsions that exhibit pH-tunable stability.

By developing a deeper understanding of the formation mechanism and the origin of the stability, we report a simple and large-scale fabrication approach to create Janus emulsions that can be controlled in size, geometry and stability.

Temperature-triggered reversible breakdown of polymer-stabilized olive-silicone oil Janus emulsions.

A one-step moderate energy vibrational emulsification method was successfully employed to produce thermo-responsive olive/silicone-based Janus emulsions stabilized by poly(N,N-diethylacrylamide) carrying 0.7 mol% oleoyl side chains. Completely engulfed emulsion droplets remained stable at room temperature and could be destabilized on demand upon heating to the transition temperature of the polymeric stabilizer. Time-dependent light micrographs demonstrate the temperature-induced breakdown of the Janus droplets, which opens new aspects of application, for instance in biocatalysis.

Photoinduced Reconfiguration of Complex Emulsions Using a Photoresponsive Surfactant.

Photoresponsive complex emulsions are prepared in a three-phase system consisting of two oils: hexane (H) and perfluorooctane (F). An aqueous solution of a mixed surfactant of fluorosurfactant, F(CF2) x(CH2CH2O) yH (Zonyl FS-300), and a synthesized light-responsive surfactant, 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (C4AZOC2TAB) was employed as the continuous phase. Complex emulsions with various geometries were prepared by one-step vortex mixing and a temperature-induced phase-separation method. It was noticed that the topology of the complex emulsion was highly dependent on the mass ratio of Zonyl FS-300/C4AZOC2TAB. Light microscopy images showed that phase inversion from an H/F/W- to an F/H/W-type double emulsion via a Janus emulsion was achieved by gradually increasing the mass ratio of C4AZOC2TAB/Zonyl FS-300. Upon UV/blue light irradiation, the topology of complex emulsions was turned to switch from an F/H/W double emulsion to a Janus emulsion to an entirely inverted H/F/W double emulsion. Dynamic interfacial tension measurements showed that UV irradiation of the interface between an aqueous trans-C4AZOC2TAB solution and hexane brings about an increase in the interfacial tension, suggesting the nature of photoinduced morphological changes in complex emulsions. The reconfiguration process of complex emulsions was illustrated by the Marangoni effect based on heterogeneity in the interfacial tension at the complex emulsion surface induced by controlling the molecular conversion of C4AZOC2TAB using light irradiation. Finally, we used the complex emulsions structure to form an on-off switch to start and shut off the evaporation of one volatile phase to achieve process monitoring. This could be used to initiate and quench a reaction, which offers a novel idea for achieving switchable and reversible reaction control in multiple-phase reactions.

A hybrid modular microfluidic device for emulsion generation

We report a novel modular plug-and-play microfluidic device for versatile emulsion generation, which consists of three parts: a top module for the dispersed phase supply, a glass capillary for emulsion creation and a bottom module for the continuous phase supply. By combining different modules and tapered glass capillaries, single emulsions, Janus emulsions and double emulsions have been successfully produced. The hybrid strategy allows us to produce smaller droplets through the tapered glass nozzles compared to current fully 3D-printed devices. On the other hand, it provides a simple and plug-and-play assembly manner compared to conventional microfluidic devices. Screw-thread plus gasket strategy has been proved to successfully seal the device and separate different liquid phases. Finally, magnetically responsive microparticles are synthesized based on the droplet templates produced in our device, which can be potentially applied in sensor and actuator fields.

PH-responsive magnetic Pickering Janus emulsions

We report ultrasonically generated pH-responsive Pickering Janus emulsions of olive oil and silicone oil with controllable droplet size and engulfment. Chitosan was used as a pH-responsive emulsifier. The increase of pH from 2 to 6 leads to a transition from completely engulfed double emulsion droplets to dumbbell-shaped Janus droplets accompanied by a significant decrease of droplet diameter and a more homogeneous size distribution. The results can be elucidated by the conformational change of chitosan from a more extended form at pH 2 to a more flexible form at pH 4–5. Magnetic responsiveness to the emulsion was attributed by dispersing superparamagnetic nanoparticles (Fe3O4 with diameter of 13 ± 2 nm) in the olive oil phase before preparing the Janus emulsion. Incorporation of magnetic nanoparticles leads to superior emulsion stability, drastically reduced droplet diameters, and opened the way to control movement and orientation of the Janus droplets according to an external magnetic field.

Microfluidic preparation of flexible micro-grippers with precise delivery function.

We used microfluidic technology for preparing gas-liquid Janus emulsions; we firstly proposed a one-step preparation method of micro-grippers and then characterized the function of oriented and precise delivery behavior. Because of the enrichment of Fe3O4 nanoparticles, the micro-gripper can reach a speed of 1.5 mm s-1 driven by a magnetic field. The micro-gripper's body is made of a poly(N-isopropylacrylamide) hydrogel, a reversible temperature-responsive polymer. The thermo-sensitivity of hydrogels offers the function of grasping, to closely integrate with the target carried, ensuring the stability of the carrying process. The reversible variation of the hydrogel allows the micro-gripper to be reusable and have a long shelf life.

Pickering Janus emulsions and polyelectrolyte complex-stabilized Janus gels

Janus emulsions, containing olive oil (OO) and silicone oil (SiO), were formed in presence of polyelectrolyte complex particles, i.e., gelatin-sodium polyacrylate (NaPAA) complexes. The diameter of completely engulfed Janus droplets can be tuned between 50 and 200μm by varying the gelatin/NaPAA complex particle size between 200 and 400nm. The gelatin/NaPAA complex particles adsorbed at the olive oil interface decrease the interfacial tension and stabilize the resulting completely engulfed Pickering Janus emulsions.Long-term stable Janus gels can be synthesized in presence of gelatin/sodium carboxymethylcellulose (NaCMC) mixtures. In that case Coulombic forces are of relevance with regard to the stabilization of the Janus droplets embedded in a gelatin/NaCMC gel matrix. Janus gels show elastic reological behavior and thixotropic properties.

From core-shell to Janus: Microfluidic preparation and morphology transition of Gas/Oil/Water emulsions

In this article, gas-liquid-liquid emulsion drops were firstly generated by one-step method in a coaxial microfluidic device. Based on that, the effects of several parameters on the morphology transition from core-shell to Janus droplets was studied systematically. By tuning flow rates, interfacial tensions and viscosity, we can change the size and morphology of droplets. Furthermore, non-spherical microparticles were obtained by using photo-responsive reagent through solidification. The developed approach offers a novel and simple strategy on the synthesis of Janus emulsions and anisotropic microparticles with controlled morphologies.

Janus Particles Templated by Janus Emulsions and Application as a Pickering Emulsifier.

One-step vibrational mixing has afforded the batch-scale preparation of a Janus emulsion. The fabrication of Janus particles (JPs) templated by Janus emulsions was motivated by the topology and composition of the Janus droplets being highly tunable and controllable. Two immiscible polymerizable monomers were introduced as inner phases of the Janus emulsion. The advanced geometry of the resultant JPs was easily and precisely controlled from "snowman" to "dumbbell" by adjusting the mass ratio of two oils in the initial emulsion. The surface coverage of one lobe to the other was tuned by adjusting the mass ratio of mixed surfactants. Moreover, the size of JPs was able to be extended continuously from hundreds of micrometers to a few hundred nanometers while their morphologies remained within this wide size range. The proposed strategy is a universal technique in the synthesis of a family of composite polymeric JPs with both chemical and shape anisotropy. In addition, the as-generated chemically biphasic JPs were applied as emulsifiers to stabilize Pickering emulsions, and more attractively, emulsion inversion was readily achieved by choosing JPs with different morphologies.

Janus Emulsions for the Detection of Bacteria.

Janus emulsion assays that rely on carbohydrate–lectin binding for the detection of Escherichia coli bacteria are described. Surfactants containing mannose are self-assembled at the surface of Janus droplets to produce particles with lectin binding sites. Janus droplets orient in a vertical direction as a result of the difference in densities between the hydrocarbon and fluorocarbon solvents. Binding of lectin to mannose(s) causes agglutination and a tilted geometry. The distinct optical difference between naturally aligned and agglutinated Janus droplets produces signals that can be detected quantitatively. The Janus emulsion assay sensitively and selectively binds to E. coli at 104 cfu/mL and can be easily prepared with long-time stability. It provides the basis for the development of inexpensive portable devices for fast, on-site pathogen detection.

Fabrication of hollow polymer particles using emulsions of hydrocarbon oil/fluorocarbon oil/aqueous surfactant solution

We have examined the influence of sodium dodecylsulfate (SDS) on the formation of emulsions consisting of hydrocarbon oil/fluorocarbon oil/water and the fabrication of hollow polymer particles by simple radical polymerization using these emulsions. Emulsions consisting of ternary mixtures of styrene/perfluoro-n-octane (PFO)/aqueous SDS solution are prepared at 80°C. Light and fluorescence microscopy observations indicate that the ternary mixtures are O/W (oil-in-water)-type emulsions. It is found that PFO droplets are present on the surface of styrene droplets. In other words, the oil droplets show Janus-type morphologies consisting of styrene and PFO. When the ternary mixtures of styrene/PFO/aqueous SDS solution are polymerized at 80°C, subsequent SEM observations indicate that the hollow cup-type polystyrene particles with a single hole on the surface are fabricated by polymerization using the Janus emulsion droplets. The hole size decreases with increasing polymerization time. These results suggest that morphologies of emulsions of ternary mixtures of hydrocarbon oil, fluorocarbon oil, and aqueous SDS solution contribute to those of polymer particles fabricated by the radical polymerization. Submicron capsules containing aqueous solutions of a fluorescent material are prepared through the hollow cup-type particles.

Catalytic Emulsion Based on Janus Nanosheets for Ultra-Deep Desulfurization.

Catalytic Janus nanosheets were synthesized by using an anion-exchange reaction between heteropolyacids (HPAs) and the modified ionic-liquid (IL) moieties of Janus nanosheets. Their morphology and surface properties were characterized by using SEM, energy-dispersive spectroscopy (EDS), FTIR spectroscopy, and X-ray photoelectron spectroscopy (XPS) studies. Because of their inherent Janus structure, the nanosheets exhibited good amphipathic character with ILs and oil to form a stable ILs-in-oil emulsion. Therefore, these Janus nanosheets can be used as both emulsifiers and catalysts to perform emulsive desulfurization. During this process, sulfur-containing compounds at the interface could be easily oxidized and efficiently removed from a model oil. Application of this Janus emulsion brings an efficient, useful, and green procedure to the desulfurization process. Compared with the desulfurization catalyzed by using HPAs in a conventional two-phase system, the sulfur removal of dibenzothiophene (DBT) achieved in a Janus emulsion system was improved from 68 to 97 % within 1.5 h. Moreover, this emulsion system could be demulsified easily by simple centrifugation to recover both the nanosheets and the ILs. Owing to the good structural stability of the Janus nanosheets, the sulfur removal efficiency of DBT could still reach 99.9 % after the catalytic nanosheets had been recycled at least six times.

Single, Janus, and Cerberus emulsions from the vibrational emulsification of oils with significant mutual solubility.

Single, Janus, and Cerberus emulsions are prepared in one system consisting of three oils: silicone (SO), fluorocarbon (FO) and ethoxylated trimethylolpropane triacrylate (ETPTA) with mutual solubility. An aqueous solution of Pluronic F127, which is an poly(ethylene oxide)/poly(propylene oxide) co-polymer of average composition EO97PO68EO97, was employed as the continuous phase. The three-dimensional phase diagram of the oils was determined, and different oil compositions within the various regions of the phase diagram were emulsified by one-step vortex mixing with an F127 aqueous solution. The result showed single, Janus, and Cerberus emulsions within the different regions of the phase diagram; i.e. the emulsions reflected the equilibrium system. The topology of the Cerberus droplets is to an overwhelming extent linear-singlet and exclusively lobe order of EF/FO/SF. Since the results indicate a significant effect of the equilibrium interfacial tensions on the drop topology, thermodynamic calculations were made using the experimentally determined interfacial tensions. The results, as expected, show that the Cerberus emulsions are thermodynamically preferred over separate drops of the individual oils. In addition, the calculations demonstrate that the order of lobes within a drop is thermodynamically favored.

Coalescence of Janus droplets prepared by one-step vibrational mixing

Janus emulsions are prepared by one-step vibrational mixing of two immiscible oils, silicone oil, SO, with density of 0.964 g/cm3 and sunflower oil saturated with dimethyl phthalate, VO (DMP), with density of 0.991 g/cm3, and an aqueous solution of sodium dodecyl sulfate as continuous phase. The destabilization of Janus emulsion is followed by the change of layer dimension as well as the microscope image, illustrating the two main aspects of creaming and coalescence. The fundamental impasse of the final separation of the Janus drop oils against unfavorable free energy change is resolved by a numerical analysis on a model emulsion with realistic interfacial tension values to show a thermodynamically favored path to the final destabilization process.

Interfacial Tension; a Stabilizing Factor for Janus Emulsions of Silicone Bixa Orellana Oils

AbstractThe destabilization process was investigated for a Janus emulsion of silicone and Bixa Orellana oils stabilized by polyoxyethylene sorbitan monooleate (Tw 80) and carboxymethyl cellulose. The emulsion stabilized with Tw 80 showed significant and fast creaming, a process that was prevented by the addition of the polymer. During the extensive coalescence of the emulsions stabilized by Tw 80, the Janus topology was retained for months of storage until, finally, separation of the oils occurred. This result strongly indicates an unexpected stabilizing action of the i nterfacial free energy. This conclusion was supported by a calculation for a realistic model system of the interfacial energy difference between two cases of coalescence. In the first case, the two coalescing Janus drops united into a larger Janus drop, while in the second case two drops formed, each with only one oil. The first case gave a spontaneous reaction (reduced interfacial energy), while the second one meant an increase of energy, i.e. it cannot happen without adding energy. The authors are aware that this stabilization is a new phenomenon in emulsion science with potential ramifications in future emulsion technology. However, it is essential to realize that the stabilization is of temporary occurrence in the destabilization process, and the free energy to give a final emulsion state with separated oils is overwhelmingly dominant. In short, Janus emulsions will, in the end, separate into layers of the liquids, like all emulsions.

Janus Emulsions of Bixa Orellana Oil

Janus emulsions of the oil from the Bixa orellana (BO) nut, silicone oil (SO), and an aqueous solution of Tween 80 were prepared by intense mixing and optical microscopy images were obtained of the drops. The results showed the expected disordered emulsion of large SO drops with a number of attached smaller drops of the vegetable oil, whose images were transformed to regular Janus drops by the shear, when the cover glass was applied. The drops with a large BO/SO volume ratio did not immediately form well-defined and topologically equal Janus drops due to kinetic factors. The microscopy image was used to evaluate the correct angles and radii by a mathematical rotation to form an image with a straight-line contact line.

Dynamics and rheology of Janus drops in a steady shear flow

The behavior and rheology of a dispersion of Janus drops (or Janus emulsion) under a steady shear flow are explored in the infinite dilution limit. To achieve analytical progress, the Janus drops are assumed to consist of a pair of fluids bounded to hemispherical domains of equal radii. At ‘freely’ suspended conditions the Janus drops undergo periodic orbits in a shear flow that are intermediate to that of a solid sphere and a disk that depend on the viscosities of the internal fluids. Non-Newtonian behavior is found for this system on account of the anisotropic hydrodynamics of the Janus drops. The viscosity of the Janus emulsion that corresponds to the minimum energy of dissipation is analogous to that derived by Taylor (1932) for a dispersion of simple drops. It is also found that an external force can induce the Janus drops to adopt a preferential orientation in a shear flow. Interestingly, a neutrally buoyant Janus drop with a displaced center of gravity can migrate lateral to the undisturbed shear flow; it is inferred that this phenomenon can lead to spatial-dependent rheology in pressure-driven flows.

A “perfect Janus emulsion”: Thermodynamic factors

ABSTRACTThe topology and interfacial thermodynamics of “perfect Janus emulsions” are considered, using two cases to illustrate the range of topological features. Initially, the correlation is established between lobe volumes and the location of the contact line, followed by a description of the essential angles involved and finally the requirements for interfacial tensions.

Janus emulsion mediated porous scaffold bio-fabrication

A three dimensional biopolymer network structure with incorporated nano-porous calcium phosphate (CaP) balls was fabricated by using gelatin-chitosan (GC) polymer blend and GC stabilized olive/silicone oil Janus emulsions, respectively. The emulsions were freeze-dried, and the oil droplets were washed out in order to prepare porous scaffolds with larger surface area. The morphology, pore size, chemical composition, thermal and swelling behavior was studied by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and micro-Differential Scanning Calorimetry (micro-DSC). Microscopic analysis confirmed that the pore size of the GC based sponges after freeze-drying may be drastically reduced by using Janus emulsions. Besides, the incorporation of nanoporous calcium phosphate balls is also lowering the pore size and enhancing thermal stability.

Completely engulfed olive/silicone oil Janus emulsions with gelatin and chitosan

Janus emulsions, formed by mixing two oil components (i.e., olive oil (OO) and silicone oil (SiO)) with water in the presence of two surface active biopolymers, i.e., gelatin and chitosan, are investigated in more detail. The stability of Janus droplets formed strongly depends on the polymer components used. The mixture of both biopolymers represents an extraordinary effect which can be related to the complex formation of gelatin and chitosan. Taken into account that under the given pH conditions, in the acidic pH range between 4 and 6, below the isoelectric point of gelatin, both polymers are polycations, one can conclude that non-Coulombic interactions are of relevance for the enhanced surface activity of the complexes. Dynamic interfacial tension (γ) measurements by using the drop profile analysis tensiometry (PAT) indicate a strong adsorption of the polymer complexes at the olive oil/water interface in contrast to the silicone/water interface. In a first step, the polymer complexes are adsorbed at the interface, and in a second step, a more rigid skin-like polymer layer is formed. This first example of a polymer-stabilized Janus emulsion opens new perspectives for the application, e.g., in food emulsions or for making scaffold materials.