Janus Droplets Research Articles

Dynamic In Situ Monitoring of the Salt Counter-ion Effect on Surfactant Effectiveness Using Reconfigurable Janus Emulsions

A straightforward method for visualization and quantification of surfactant effectiveness within different electrolyte environments based on using reconfigurable Janus emulsions as novel optical probes is reported. More specifically, we investigated the effect of different types and concentrations of salt counter-ions on the surfactant surface excess of commercial ionic and non-ionic surfactants, namely sodium dodecyl sulfate (SDS) and Tween 80 via in situ monitoring the morphological reconfigurations of biphasic Janus emulsions comprising hydrocarbon and fluorocarbon oils. We find that significant variations in interfacial tensions of SDS-stabilized interfaces (up to 15 mN·m-1) can be evoked by titrating mono-, di-, and trivalent cationic counter-ions, which is coherent with the lyotropic (Hofmeister) series. In contrast, the salt counter-ion effect on the surfactant effectiveness was less pronounced for the non-ionic surfactant Tween 80 (∼3 mN·m-1). Our results reveal a facile in situ method for monitoring the central role of electrolyte type and concentration on surfactant effectiveness and, more broadly, illustrate that Janus emulsions serve as powerful optical probes to dynamically study the properties of surfactants at liquid interfaces. We demonstrate the utility of our findings for an electro-induced morphological reconfiguration of Janus droplet morphologies by dynamically tuning Cu2+ concentration in solution using an electrode setup. The latter provides a unique platform for liquid-phase, real-time, and continuous tuning of Janus droplet morphologies, e.g., for their application in sensing and dynamic optical device platforms.

AC-electric-field-controlled multi-component droplet coalescence at microscale.

Droplet coalescence with fast response, high controllability and monodispersity has been widely investigated in industrial production and bioengineering. Especially for droplets with multiple components, programmable manipulation of such droplets is crucial for practical applications. However, precise control of the dynamics can be challenging, owing to the complex boundaries and the interfacial and fluidic properties. AC electric fields, with their fast response and high flexibility, have attracted our interest. We design and fabricate an improved flow-focusing microchannel configuration together with a non-contact type of electrode featuring asymmetric geometries, based on which we conduct systematic investigations of the AC-electric-field-controlled coalescence of multi-component droplets at the microscale. Parameters such as flow rates, component ratio, surface tension, electric permittivity and conductivity were given our attention. The results show that droplet coalescence in different flow parameters can be achieved in milliseconds by adjusting the electrical conditions, which shows high controllability. Specifically, both the coalescence region and reaction time can be adjusted by a combination of applied voltage and frequency, and unique merging phenomena have appeared. One is contact coalescence with the approach of paired droplets, while the other is squeezing coalescence, which occurs in the start position and promotes the merging process. The fluid properties, such as the electric permittivity, conductivity and surface tension, present a significant influence on merging behavior. The increasing relative dielectric constant leads to a dramatic reduction of the start merging voltage from the original 250 V to 30 V. The range of effective voltage for coalescence decreases with the addition of surfactant, offering a stricter and yet higher selectivity on electrical conditions, about 1500 V. The conductivity presents a negative correlation with the start merging voltage due to the reduction of the dielectric stress, from 400 V to 1500 V. Finally, we achieve the precise fabrication process of the Janus droplet via implementation of the proposed method, where the components of the droplets and the coalescence conditions are well controlled. Our results can serve as a potent methodology to decipher the physics of multi-component droplet electro-coalescence and contribute to applications in chemical synthesis, bioassay and material synthesis.

Tunable Janus geometric morphology from aqueous two-phase systems on a superhydrophobic substrate

Precise manipulation of the morphologies of aqueous Janus droplets by leveraging the PEG/DEX ATPS on a superhydrophobic surface.

Numerical study on the engulfing behavior between immiscible droplets in a confined shear flow

This study investigates the engulfing behavior between two equal-sized immiscible droplets suspended in a confined shear flow. The finite element analysis is carried out, where a ternary diffuse-interface model and the Stokes equation are coupled. The numerical results of nearly 60 cases are studied to systematically examine the effect of shear rate, surface tension, and confinement on the engulfing behavior. In general, the partial and complete engulfing result in Janus and compound droplets, respectively, when the shear rate is low. It is found that the partial engulfing regime is significantly affected by the shear rate such that a proper condition for the Janus formation becomes narrower as the shear rate increases. We also note that the shell of a compound droplet can be broken by increasing the initial separation or confinement.

Preparation of Janus Droplets and Hydrogels with Controllable Morphologies by an Aqueous Two-Phase System on the Superamphiphobic Surface.

Janus particles, having the property integration of each component, have attracted increasing attention due to their considerable potential in the field of material engineering applications. However, organic solvents or sophisticated equipment during the fabrication processes is generally inevitable. Here, we report a facile route to prepare Janus droplets and hydrogels via aqueous two-phase systems (ATPS). Simply merging two polymers, i.e., polyethylene glycol (PEG) and dextran (DEX), as aqueous droplets on a superamphiphobic surface leads to phase separation, provided that their concentrations exceed the threshold in the mixed aqueous droplets, thus generating a Janus structure. Various morphologies of such Janus droplets can be well controlled by manipulating the locations of these two polymers' concentration on the phase diagram, and the evolution of the mixed droplets are deterministic on the basis of the kinetics of their phase separation and the degree of hydrophobicity of the substrate. Introducing monomers and/or nanoparticles, further, into a certain phase of the ATPS droplet followed by photopolymerizing enables Janus hydrogel particles with diverse functionalities to be obtained. The ease and green techniques with which the Janus balance and curvature between two phases of the Janus droplet can be finely tuned point to new directions in designing Janus particles and hold great promises in biological engineering.

Exploring multiphase liquid crystal polymeric droplets created by a partial phase-separation

HypothesisPolymerization of the monomers lowers the miscibility of the polymer. Therefore, a partial phase separation phenomenon can be induced depending on the polymerization conditions. Based on this, usage of anisotropic material has advantages to precisely control the internal structure as well as the external shape. ExperimentsJanus liquid crystal polymer (LCP) droplets and particles with unusual complex shapes were prepared via two steps, thermal- and photo-polymerization using the reactive mesogen(RM) mixture. The internal configuration of mesogens in the droplets and their assembly behaviors were controlled by varying surface condition. Janus LCP particles with complex shapes were formed by anisotropic deswelling and characterized via optical and electron microscopes. FindingsThe partial phase separation in the RM mixture induced the density gradient throughout the droplet, forming the Janus droplets with a certain nematic configurations in each compartment. LCP Janus droplets show not only easy-control of colloidal assembly with surface condition and also complex external structures that is hardly obtained in typical ways using isotropic materials. This report demonstrates for the first time a high availability of anisotropic materials for generation of multiphase microparticles controlled with internal and external structures.

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.

Janus Droplet Formation via Thermally Induced Phase Separation: A Numerical Model with Diffusion and Convection.

Microscale Janus particles have versatile potential applications in many physical and biomedical fields, such as microsensor, micromotor, and drug delivery. Here, we present a phase-field approach of multicomponent and multiphase to investigate the Janus droplet formation via thermally induced phase separation. The crucial kinetics for the formation of Janus droplets consisting of two polymer species and a solvent component via an interplay of both diffusion and convection is considered in the Cahn–Hilliard–Navier–Stokes equation. The simulation results of the phase-field model show that unequal interfacial tensions between the two polymer species and the solvent result in asymmetric phase separation in the formation process of Janus droplets. This asymmetric phase separation plays a vital role in the establishment of the so-called core–shell structure that has been observed in previous experiments. By varying the droplet size, the surface tension, and the molecular interaction between the polymer species, several novel droplet morphologies are predicted in the development process of Janus droplets. Moreover, we stress that the hydrodynamics should be reckoned as a non-negligible mechanism that not only accelerates the Janus droplet evolution but also has great impacts on the coarsening and coalescence of the Janus droplets.

Responsive emulsions with Janus topology constructed by magneto-surfactants and remote control of emulsion stability

The fluids with well-defined inner structure and intrinsic responsibility are demanding to the maintenance and regulation of normal functions in living systems. Inspired by this, responsive Janus droplets are constructed by small molecules of magneto-surfactants, and the traditional vortex mixing is applied to guarantee large-scale fabrication. Silicone oil (SO) and Vegetable oil (VO) as two immiscible oils are introduced as inner phases of the Janus emulsions. The topology and subsequently the magnetic responsive performance of Janus emulsion is controlled by adjusting the oil composition with various viscosity, the surfactant concentration and the mass ratio of two oils. Moreover, the remote control of the emulsion stability is realized by the external magnetic field, of which the demulsification mechanism is investigated. The anisotropic and responsive droplets with advantages of deformability, encapsulation capability, and biocompatibility would expand the application of emulsions in targeted therapy, drug delivery and magnetic materials science.

Chemical design of self-propelled Janus droplets

Chemical design of self-propelled Janus droplets

Compartmentalized Janus droplets of photoresponsive cholesteric liquid crystals and poly(dimethylsiloxane)-based oligomers

A new kind of Janus droplet containing photoresponsive cholesteric liquid crystals (CLCs) was fabricated for the first time and their formation, compartment structure, mesophase texture and function were thoroughly investigated. In the droplets, the CLC compartments included a typical nematic LC (4'-pentyl-4-biphenylcarbonitrile) doped with an azobenzene-containing chiral dopant, and the other compartments were formed of a poly(dimethylsiloxane)-based oligomer. Janus droplets were fabricated through microphase separation of the incompatible components in chloroform solution dispersed in an aqueous medium, induced by slow evaporation of chloroform. The mesophase structures of the CLC phase in Janus droplets, both suspended in aqueous medium and spreading on substrates, were controlled by the bulk elastic free energy of the CLC phase, surface anchoring and confining geometries. The helix pitch of the cholesteric phase in the droplets was determined by the doping concentration of the chiral dopant. For the suspended Janus droplets with the helix pitch obviously smaller than the droplet sizes, the CLC compartments mainly possessed a bipolar structure instead of the Frank-Pryce structure typically observed on CLC droplets. After the Janus droplets spread on the substrates, the CLC compartments changed to crescent shapes due to the different wettability characteristics of the two compartments, and the formed stable and metastable CLC configurations were distinctively different from those in the suspensions. Interestingly, when the Janus droplets spreading on substrates were irradiated with a laser beam (λ = 488 nm) of low intensity, the directors in the CLC compartments rearranged to form fingerprint structures with minimum total energy.

Focal conic flowers, dislocation rings, and undulation textures in smectic liquid crystal Janus droplets.

Liquid crystalline phases of matter often exhibit visually stunning patterns or textures. Mostly, these liquid crystal (LC) configurations are uniquely determined by bulk LC elasticity, surface anchoring conditions, and confinement geometry. Here, we experimentally explore defect textures of the smectic LC phase in unique confining geometries with variable curvature. We show that a complex range of director configurations can arise from a single system, depending on sample processing procedures. Specifically, we report on LC textures in Janus drops comprised of silicone oil and 8CB in its smectic-A LC phase. The Janus droplets were made in aqueous suspension using solvent-induced phase separation. After drop creation, smectic layers form in the LC compartment, but their self-assembly is frustrated by the need to accommodate both the bowl-shaped cavity geometry and homeotropic (perpendicular) anchoring conditions at boundaries. A variety of stable and metastable smectic textures arise, including focal conic domains, dislocation rings, and undulations. We experimentally characterize their stabilities and follow their spatiotemporal evolution. Overall, a range of fabrication kinetics produce very different intermediate and final states. The observations elucidate assembly mechanisms and suggest new routes for fabrication of complex soft material structures in Janus drops and other confinement geometries.

Magnetic response Janus emulsions stabilized by Mangeto-surfactant

Magnetic response emulsions, with regulated motion of droplets by magnetic fields, provide opportunities in system design. Research in Janus emulsion is blooming attributed to excellent multiple chambers, diverse interfaces, and advanced structures. Responsive Janus emulsions are prepared, using two immiscible oils of dimethyl phthalate (DMP)/vegetable oil (VO) and small molecules of magneto-surfactants ([H-G-C16]FeCl4) as stabilizer, in batch-scale by traditional vortex mixing. The composition of emulsion, the concentration of surfactant, and the emulsification energy are applied to control the topology of the structured fluid, and more importantly achieve the quantitative regulation of magnetic response. Janus droplets with clear inner interfaces are successfully constructed solely by magneto-surfactant, and the strategy is universal and even extended to Cerberus emulsions composed of three separated compartments. The magnetic response is visualized by directed moving against gravity and saturated magnetization. The responsivity is enhanced about 44.5%-54.5% accompany with VO lobe transformation from “waxing crescent” to “full moon with a hole”. The magnetic response is increased as high as 80.0% by decreasing the droplet size. Delicate calculations reveal that the ordered arrangement of [H-G-C16]FeCl4 at the interfaces, mainly contributes to the magnetic response compared with aggregates in the continuous phase. The adsorption on the VO/aq interface dominates the overall response due to the higher affinity of the surfactant to the VO phase. The findings open an avenue in the production of magneto-responsive multiple emulsions, which would push forward future application in smart microreactors, intelligent fluids, and responsive delivery systems.

Inner Rotation of Pickering Janus Emulsions.

Janus droplets were prepared by vortex mixing of three non-mixable liquids, i.e., olive oil, silicone oil and water, in the presence of gold nanoparticles (AuNPs) in the aqueous phase and magnetite nanoparticles (MNPs) in the olive oil. The resulting Pickering emulsions were stabilized by a red-colored AuNP layer at the olive oil/water interface and MNPs at the oil/oil interface. The core–shell droplets can be stimulated by an external magnetic field. Surprisingly, an inner rotation of the silicon droplet is observed when MNPs are fixed at the inner silicon droplet interface. This is the first example of a controlled movement of the inner parts of complex double emulsions by magnetic manipulation via interfacially confined magnetic nanoparticles.

Modeling of three-phase displacement in three-dimensional irregular geometries using a lattice Boltzmann method

Three-phase displacement process in a porous media is significantly influenced by wettability of the solid surfaces. It remains a research challenge to model and simulate three-phase flows with moving contact lines, especially on arbitrary complex three-dimensional surfaces. We propose a color-gradient lattice Boltzmann model to simulate immiscible three-phase flows with contact-line dynamics. Unlike the previous models in dealing with wettability through assigning virtual mass fractions to the solid surfaces, the present model realizes the prescribed contact angles by correcting the orientation of the color-gradient parameter at the solid surfaces. We first validate this model against analytical solutions by simulating several typical cases, including a Janus droplet deposited on a flat surface and on a spherical surface, and the spontaneous imbibition of ternary fluids in a cylindrical tube. We then use it to study the displacement of residual oil during tertiary gas injection process. To mimic this process, gas is injected into a simple pore-throat connecting structure after water flooding. Results show that in displacement process, an oil spreading layer forms, which changes the oil flow state from adhering to the bottom wall to spreading over the entire interface between gas and water. Therefore, residual oil can be easily recovered in the form of gas-to-oil-to-water double displacement chain, leading to a high oil recovery regardless of the wetting properties of the solid surface. In the cases without the presence of oil spreading layer, increasing capillary number favors the oil recovery under certain wetting conditions only.

Actuation of Janus Emulsion Droplets via Optothermally Induced Marangoni Forces.

Microscale Janus emulsions represent a versatile material platform for dynamic refractive, reflective, and light-emitting optical components. Here, we present a mechanism for droplet actuation that exploits thermocapillarity. Using optically induced thermal gradients, an interfacial tension differential is generated across the surfactant-free internal capillary interface of Janus droplets. The interfacial tension differential causes droplet-internal Marangoni flows and a net torque, resulting in a predictable and controllable reorientation of the droplets. The effect can be quantitatively described with a simple model that balances gravitational and thermal torques. Occurring in small thermal gradients, these optothermally induced Marangoni dynamics represent a promising mechanism for controlling droplet-based micro-optical components.

Flexible fabrication of lipophilic-hydrophilic micromotors by off-chip photopolymerization of three-phase immiscible flow induced Janus droplet templates

Self-propelled microparticles are promising for lots of applications ranging from analytical detection to water treatment. Herein, we present an effective approach to fabricate lipophilic-hydrophilic micromotors via the photocuring of three-phase immiscible flow induced droplet templates. In the microfluidic system, two immiscible inner fluids, the lipophilic 1, 6-Hexanediol diacrylate (HDDA), and the hydrophilic poly (ethylene glycol) diacrylate (PEGDA), are simultaneously injected into a theta-shaped cylindrical capillary from two separate inlets, and they are emulsified into Janus drops when encountering the outer immiscible silicone oil. Because of the immiscible feature of droplet templates, off-chip photopolymerization strategy has been used, which can significantly decrease the blocking chance of microdevice. And also, the lipophilic-hydrophilic structure of droplets is convenient for the loading of cargos with different characteristics. More importantly, the size and configuration of droplet templates can be flexibly regulated by changing the flow rates of three different phases. Accordingly, multifunctional micromotors can be fabricated by adding different nanoparticles and materials into the HDDA or PEGDA phase first and then photocuring the droplets. Taking the bubble-propelled micromotors for example, we prepare microswimmers by loading Ag, TiO2 and Fe3O4 nanoparticles into the PEGDA phase. The swimming behaviors of micromotors in H2O2 solution are systematically investigated, finding that the proportion of PEGDA phase and the concentration of H2O2 both positively affect the moving speed. Furthermore, the applicability of motor particles on water treatment is successfully demonstrated by using neutral red solution as the model pollutant. And the micromotors can be recycled using magnets after the catalytic degradation process. Therefore, this micromotor generation technique and this kind of micromotor can be attractive for many applications.

Lattice Boltzmann simulation of three-phase flows with moving contact lines on curved surfaces.

A numerical method for simulating three-phase flows with moving contact lines on arbitrarily complex surfaces is developed in the framework of the lattice Boltzmann method. In this method, the immiscible three-phase flow is modeled through a multiple-relaxation-time color-gradient model, which not only allows for a full range of interfacial tensions but also produces stable outcomes for a wide range of viscosity ratios. A characteristic line model is introduced to implement the wetting boundary condition, which is not only easy to implement but is also able to handle arbitrarily complex boundaries with prescribed contact angles. The developed method is first validated by the simulation of a Janus droplet resting on a flat surface, a perfect Janus droplet deposited on a cylinder, and the capillary intrusion of ternary fluids for various viscosity ratios. It is then used to study a compound droplet subject to a uniform incoming flow passing through a multipillar structure, where three different values of surface wettability are considered. The simulated results show that the surface wettability has significant impact on the droplet dynamic behavior and final fluid distribution.

Outflow boundary condition of multiphase microfluidic flow based on phase ratio equation in lattice Boltzmann method

This article proposes a new outflow boundary condition for the color gradient model in the multiphase lattice Boltzmann method. The boundary condition is based on the phase ratio equation and made use of the Zou–He boundary condition in single-phase flow. The boundary condition is provided in two-dimension-nine-velocity (D2Q9) and three-dimension-twenty-seven-velocity (D3Q27) schemes, for which an extension of the Zou–He boundary condition to D3Q27 is also derived and its correctness verified. Application cases, including two-phase parallel flows, droplet flows, T-junction flows, three-phase Janus droplet flows in two-dimensional (2D), and three-dimensional (3D) spaces, demonstrate the effectiveness of this new boundary condition, and the performance of a test case shows its improved pressure stability and mass conservation characteristics.

Janus Emulsion Biosensors for Anti-SARS-CoV-2 Spike Antibody.

The spread of the COVID-19 pandemic around the world has revealed that it is urgently important to develop rapid and inexpensive assays for antibodies in general and anti-SARS-CoV-2 IgG antibody (anti-SARS-CoV-2 spike glycoprotein S1 antibody) in particular. Herein we report a method to detect the anti-SARS-CoV-2 spike antibody level by using Janus emulsions or Janus particles as biosensors. Janus emulsions are composed of two immiscible hydrocarbon and fluorocarbon oils. The hydrocarbon/water interfaces are functionalized with a secondary antibody of IgG protein and SARS-CoV-2 spike receptor binding domain (RBD), to produce two different Janus emulsions. Mixtures of these Janus droplets enable the detection of the anti-SARS-CoV-2 spike IgG antibody in an agglutination assay caused by the antibody’s binding to both the secondary antibody of IgG antibody and SARS-CoV-2 spike protein RBD. Both qualitative optical images and quantitative fluorescence spectra are able to detect the level of anti-SARS-CoV-2 spike antibody at concentrations as low as 0.2 μg/mL in 2 h. The detection results of clinical human serum samples using this agglutination assay confirm that this method is applicable to clinical samples with good sensitivity and specificity. The reported method is generalizable and can be used to detect other analytes by attaching different biomolecular recognition elements to the surface of the Janus droplets.