Double Emulsion Droplets Research Articles

Effect of Eccentricity Difference on the Mechanical Response of Microfluidics-Derived Hollow Silica Microspheres during Nanoindentation.

Hollow microspheres as the filler material of syntactic foams have been adopted in extensive practical applications, where the physical parameters and their homogeneity have been proven to be critical factors during the design process, especially for high-specification scenarios. Based on double-emulsion droplet templates, hollow microspheres derived from microfluidics-enabled soft manufacturing have been validated to possess well-controlled morphology and composition with a much narrower size distribution and fewer defects compared to traditional production methods. However, for more stringent requirements, the innate density difference between the core-shell solution of the double-emulsion droplet template shall result in the wall thickness heterogeneity of the hollow microsphere, which will lead to unfavorable mechanical performance deviations. To clarify the specific mechanical response of microfluidics-derived hollow silica microspheres with varying eccentricities, a hybrid method combining experimental nanoindentation and a finite element method (FEM) simulation was proposed. The difference in eccentricity can determine the specific mechanical response of hollow microspheres during nanoindentation, including crack initiation and the evolution process, detailed fracture modes, load-bearing capacity, and energy dissipation capability, which should shed light on the necessity of optimizing the concentricity of double-emulsion droplets to improve the wall thickness homogeneity of hollow microspheres for better mechanical performance.

Design automation of microfluidic single and double emulsion droplets with machine learning

Droplet microfluidics enables kHz screening of picoliter samples at a fraction of the cost of other high-throughput approaches. However, generating stable droplets with desired characteristics typically requires labor-intensive empirical optimization of device designs and flow conditions that limit adoption to specialist labs. Here, we compile a comprehensive droplet dataset and use it to train machine learning models capable of accurately predicting device geometries and flow conditions required to generate stable aqueous-in-oil and oil-in-aqueous single and double emulsions from 15 to 250 μm at rates up to 12000 Hz for different fluids commonly used in life sciences. Blind predictions by our models for as-yet-unseen fluids, geometries, and device materials yield accurate results, establishing their generalizability. Finally, we generate an easy-to-use design automation tool that yield droplets within 3 μm (<8%) of the desired diameter, facilitating tailored droplet-based platforms and accelerating their utility in life sciences.

Microfluidic fabrication of monodisperse microcapsules with gas cores.

A facile strategy for efficient and continuous fabrication of monodisperse gas-core microcapsules with controllable sizes and excellent ultrasound-induced burst performances is developed based on droplet microfluidics and interfacial polymerization. Monodisperse gas-in-oil-in-water (G/O/W) double emulsion droplets with a gas core and monomer-contained oil layer are fabricated in the upstream of a microfluidic device as templates, and then water-soluble monomers are added into the aqueous continuous phase in the downstream to initiate rapid interfacial polymerization at the O/W interfaces to prepare monodisperse gas-in-oil-in-solid (G/O/S) microcapsules with gas cores. The sizes of both microbubbles and G/O/W droplet templates can be precisely controlled by adjusting the gas supply pressure and the fluid flow rates. Due to the very thin shells of G/O/S microcapsules fabricated via interfacial polymerization, the sizes of the resultant G/O/S microcapsules are almost the same as those of the G/O/W droplet templates, and the microcapsules exhibit excellent deformable properties and ultrasound-induced burst performances. The proposed strategy provides a facile and efficient route for controllably and continuously fabricating monodisperse microcapsules with gas cores, which are highly desired for biomedical applications.

Effective colloidal emulsion droplet regulation in flow-focusing glass capillary microfluidic device via collection tube variation.

Colloidal emulsion droplets, created using glass capillary microfluidic devices, have been found in a myriad of applications, serving as subtle microcarriers, delicate templates, etc. To meet the objective requirements under varying circumstances, it is crucial to efficiently control the morphology and dimensions of the droplets on demand. The glass capillary collection tube is a crucial component of the flow-focusing microfluidic system due to its close association with the geometrical confinement of the multiphasic flow. However, there are currently no guidelines for the design of the morphology and dimensions of the glass capillary collection tube, which shall result in a delay in assessing serviceability until after the microfluidic device is prepared, thereby causing a loss of time and effort. Herein, an experimental study was conducted to investigate the effect of the geometrical characteristics of glass capillary collection tubes on the production of colloidal emulsion droplets. After characterizing the generated colloidal emulsion droplets, it was found that the geometrical variations of the glass capillary collection tube resulted in numerical disparities of droplets due to different degrees of flow-focusing effects. The stronger flow-focusing effect produced smaller droplets at a higher frequency, and the dimensional variation of colloidal emulsion droplets was more responsive to varying flow rates. Furthermore, the transformation from colloidal single-core double-emulsion droplets to multi-core double-emulsion droplets also changed with the flow rate due to the glass capillary collection tube morphology-determined varying flow-focusing effect. These experimental findings can offer qualitative guidance for the design of glass capillary microfluidic devices in the preliminary stage, thus facilitating the smooth production of desired colloidal emulsion droplets.

A portable microfluidic device for thermally controlled granular sample manipulation.

Effective granular sample manipulation with a portable and visualizable microfluidic device is significant for lots of applications, such as point-of-care testing and cargo delivery. Herein, we report a portable microfluidic device for controlled particle focusing, migration and double-emulsion droplet release via thermal fields. The device mainly contains a microfluidic chip, a microcontroller with a DC voltage control unit, a built-in microscope with a video transmission unit and a smartphone. Five microheaters located at the bottom of the microfluidic chip are used to unevenly heat fluids and then induce thermal buoyancy flow and a thermocapillary effect, and the experiments can be conveniently visualized through a smartphone, which provides convenient sample detection in outdoor environments. To demonstrate the feasibility and multifunctionality of this device, the focusing manipulation of multiple particles is first analyzed by using silica particles and yeast cells as experimental samples. We can directly observe the particle focusing states on the screen of a smartphone, and the particle focusing efficiency can be flexibly tuned by changing the control voltage of the microheater. Then the study focus is transferred to single-particle migration. By changing the voltage combinations applied on four strip microheaters, the single particle can migrate at predetermined trajectory and speed, showing attractiveness for those applications needing sample transportation. Finally, we manipulate the special three-phase flow system of double-emulsion drops in thermal fields. Under the combined effect of the thermocapillary effect and increased instability, the shell of double-emulsion droplets gradually thins and finally breaks, resulting in the release of samples in inner cores. The core release speed can also be flexibly adjusted by changing the control voltage of the microheater. These three experiments successfully demonstrate the effectiveness and multifunctionality of this thermally actuated microfluidic device on granular manipulation. Therefore, this portable microfluidic device will be promising for lots of applications, such as analytical detection, microrobot actuation and cargo release.

Hydrogel-Encased Photonic Microspheres with Enhanced Color Saturation and High Suspension Stability.

Regular arrays of colloidal particles can produce striking structural colors without the need for any chemical pigments. Regular arrays of colloidal particles can be processed into microparticles via emulsion templates for use as structural colorants. Photonic microparticles, however, suffer from intense incoherent scattering and lack of suspension stability. We propose a microfluidic technique to generate hydrogel-shelled photonic microspheres that display enhanced color saturation and suspension stability. We created these microspheres using oil-in-water-in-oil (O/W/O) double-emulsion droplets with well-defined dimensions with a capillary microfluidic device. The inner oil droplet contains silica particles in a photocurable monomer, while the middle water droplet carries the hydrogel precursor. Within the inner oil droplet, silica particles arrange into crystalline arrays due to solvation-layer-induced interparticle repulsion. UV irradiation solidifies the inner photonic core and the outer hydrogel shell. The hydrogel shell reduces white scattering and enhances the suspension stability in water. Notably, the hydrogel precursor in the water droplet aids in maintaining the solvation layer, resulting in enhanced crystallinity and richer colors compared with microspheres from O/W single-emulsion droplets. These hydrogel-encased photonic microspheres show promise as structural colorants in water-based inks and polymer composites.

Interface coupling and droplet size under various flow-focusing geometry dimensions in double emulsion formation

The application performance of droplets, including the size and core-shell volumetric ratio of a double emulsion droplet, is investigated under various geometric sizes and flow rates in a flow-focusing capillary device. This study uses three-dimensional (3D) printing to create a novel substrate to assemble capillaries and make the inner dimension of the device tunable. Droplet generation is separated into two regions (uniform or non-uniform) based on the interface coupling shapes, where different generation modes are classified based on the rupture form of the multilayer interfaces. A map of the generation modes is established based on geometric size and flow conditions. In the dripping mode, interface coupling and its effect on generation are analyzed by the interface necking process for the two generation sub-modes (with/without a ball). The local capillary number of the double emulsion droplet is established for these sub-modes to analyze variations in the inner droplet volume, which helps propose the volume predictive model of inner and outer droplets.

In situ quantification of osmotic pressure within living embryonic tissues

Mechanics is known to play a fundamental role in many cellular and developmental processes. Beyond active forces and material properties, osmotic pressure is believed to control essential cell and tissue characteristics. However, it remains very challenging to perform in situ and in vivo measurements of osmotic pressure. Here we introduce double emulsion droplet sensors that enable local measurements of osmotic pressure intra- and extra-cellularly within 3D multicellular systems, including living tissues. After generating and calibrating the sensors, we measure the osmotic pressure in blastomeres of early zebrafish embryos as well as in the interstitial fluid between the cells of the blastula by monitoring the size of droplets previously inserted in the embryo. Our results show a balance between intracellular and interstitial osmotic pressures, with values of approximately 0.7 MPa, but a large pressure imbalance between the inside and outside of the embryo. The ability to measure osmotic pressure in 3D multicellular systems, including developing embryos and organoids, will help improve our understanding of its role in fundamental biological processes.

Osmosis-induced hydrodynamic centering of W/O/W double emulsion droplets for quasi-concentric microcapsule/microsphere fabrication

Water-in-oil-in-water (W/O/W) double emulsion droplets are a typical colloidal system applied in various practical fields. However, certain high-specification applications necessitate exact geometric homogeneity in terms of both shell diameter and thickness. Despite achieving uniformity in shell diameter, on-chip microfluidic emulsification always leads to an eccentric phenomenon due to innate density differences of the droplet solution. At the same time, the current regulation methods have multiple limitations, including their applicability, regulatory scope, and objective restrictions. Herein, we propose an off-chip osmosis-induced hydrodynamic method to accurately regulate the eccentricity of W/O/W double emulsion droplets. The rational selection of hypertonic osmosis can energize the droplets and induce inward transmembrane water flux. This flow helps to reposition the droplet core to align concentrically with the shell, regardless of the initial dimension and eccentricity of the droplets. As the osmotic induction reduces due to the dilution of the inner solute molar concentration, the optimal droplet configuration can be achieved when osmosis-induced hydrodynamic regulation sufficiently counteracts the adverse effects of droplet solution density difference. The microcapsules can thus be produced by instant UV polymerization of the regulated double emulsion droplets, and the silica microspheres with fine geometric homogeneity can be prepared through further heat treatment, which is expected to meet the stringent morphological requirements in practical applications.

Production and Reconfiguration of Double Emulsions by Temperature Control.

Double emulsions are of great importance for both science and engineering. However, the production of multicore double-emulsion droplets is challenging and normally requires sophisticated microfluidic devices, which limits their availability to broader communities. Here, we propose a simple, precise, and scalable batch method for producing double emulsions with monodispersed multicores at milliliter per minute rates, using the most common means in laboratory, temperature. By rapidly cooling liquid crystal emulsions, the introduced temperature gradient around the emulsion droplets leads to the injection of monodispersed guest droplets to form double-emulsion droplets. The number of injected water droplets can be precisely controlled by adjusting the thermally induced mechanical force through the temperature difference and the cooling rate. In contrast to conventional microfluidic fabrication, this method processes all emulsion droplets simultaneously in a noncontact and in situ manner. Therefore, it has great flexibility, allows multiple processing of double emulsions of arbitrary shape, has good capacity for mass production, and offers excellent compatibility with technologies such as microfluidics. Finally, we demonstrate that temperature changes can also be used to release the inner droplets from the double emulsion. The proposed method offers a reversible tool for processing double emulsions with minimal cost and expertise and is applicable to droplet-based microsystems in materials science, photonics, sensors, pharmaceuticals, and biotechnology.

Facile and Programmable Capillary-Induced Assembly of Prototissues via Hanging Drop Arrays.

An important goal for bottom-up synthetic biology is to construct tissue-like structures from artificial cells. The key is the ability to control the assembly of the individual artificial cells. Unlike most methods resorting to external fields or sophisticated devices, inspired by the hanging drop method used for culturing spheroids of biological cells, we employ a capillary-driven approach to assemble giant unilamellar vesicles (GUVs)-based protocells into colonized prototissue arrays by means of a coverslip with patterned wettability. By spatially confining and controllably merging a mixed population of lipid-coated double-emulsion droplets that hang on a water/oil interface, an array of synthetic tissue-like constructs can be obtained. Each prototissue module in the array comprises multiple tightly packed droplet compartments where interfacial lipid bilayers are self-assembled at the interfaces both between two neighboring droplets and between the droplet and the external aqueous environment. The number, shape, and composition of the interconnected droplet compartments can be precisely controlled. Each prototissue module functions as a processer, in which fast signal transports of molecules via cell-cell and cell-environment communications have been demonstrated by molecular diffusions and cascade enzyme reactions, exhibiting the ability to be used as biochemical sensing and microreactor arrays. Our work provides a simple yet scalable and programmable method to form arrays of prototissues for synthetic biology, tissue engineering, and high-throughput assays.

Tunable Structural Coloration in Eccentric Water-in-Oil-in-Water Droplets.

Structural colors show diverse advantages such as fade resistance, eco-friendliness, iridescence, and high saturation in comparison with chemical pigments. In this paper, we show tunable structural coloration in colorless water-in-oil-in-water double emulsion droplets via total internal reflection and interference at the microscale concave interfaces. Through experimental work and simulations, we demonstrate that the shell thickness and the eccentricity of the core-shell structures are key to the successful formation of iridescent structural colors. Only eccentric thin-shell water-in-oil-in-water droplets show structural colors. Importantly, structural colors based on water-oil interfaces are readily responsive to a variety of environmental stimuli, such as osmotic pressure, temperature, magnetic fields, and light composition. This work highlights an alternative structural coloration that expands the applications of droplet-based structural colors to aqueous systems.

Continuous gradient temperature control of microfluidic chip based on thermoelectric cooler

High-efficiency and accurate temperature control is the key to the application of microfluidic technology in biology, chemistry and medicine fields. At present, gradient temperature control of microfluidic chip in a wide range below room temperature still faces challenges. Based on a microfluidic platform of double emulsion droplets, this study builds a temperature control system and attempts to achieve continuous gradient temperature control on time scale of microfluidics based on thermoelectric cooler. Temperature control unit is designed and relevant control strategies are discussed to achieve accurate temperature control. Meanwhile, the strategy to achieve continuous gradient temperature control of microfluidic chip is mainly explored. Finally, combining with the working characteristics of thermoelectric cooler, continuous gradient temperature control of microfluidic chip is experimentally realized based on adaptive fuzzy PID control, while PID control shows remarkable overshoot, deviation and oscillation. Moreover, small gradient temperature control from 10 °C down to −15 °C, −20 °C and then −25 °C is precisely realized by using adaptive fuzzy PID control, as well as large gradient temperature control from 20 °C down to 0 °C and then −20 °C. Specifically, the maximum deviation of temperature is 0.192 °C in small gradient temperature control, and the steady state error is within ±0.07 °C. In large gradient temperature control, the maximum deviation of temperature is 0.309 °C, and the steady state error was within ±0.1 °C. This study provides diversified temperature control in a wide range for microfluidic analysis.

Smart microsubmarine shells operated by controlled buoyancy force through metal-ion adsorption/desorption and their biosensor applications

Small and uniform-sized poly(polyethylene glycol (PEG)–co-acrylic acid (AA)) shells engulfing a mineral oil droplet (oil@poly(PEG-co-AA) shells, ∼180 μm) are prepared using the in situ polymerization of the O(mineral oil)–W(PEGDA/AA, PEGDA = polyethylene glycol diacrylate)–O(mineral oil) double emulsion droplets produced in a microfluidic chip. The lightweight oil core and heavy-weight poly(PEG-co-AA) shells exhibit positive buoyancy and negative gravitational forces in water, respectively. Densities of oil@poly(PEG-co-AA) shells were finely tuned in water to be sensitive to a small weight change by controlling microfluidic parameters. The poly(PEG-co-AA) shells are pH-responsive in terms of charged state and shell size. The pH-controlled metal adsorption/desorption at poly(PEG-co-AA) shells breaks the density balance, leading to the sinking or floating of oil@poly(PEG-co-AA) shells in the water. Herein, this levitation and size change of oil@poly(PEG-co-AA) shells are applied to glucose and urea biosensors by immobilizing their enzymes. The divalent (or higher) metal adsorption is distinguishable from monovalent metal adsorption by the diminished size of the oil@poly(PEG-co-AA) shells and their different levitation behavior in water. Thus, these oil@poly(PEG-co-AA) shells facilitate smart motile sensor applications with an unexplored concept, which can be further expanded to other applications such as smart valves in microfluidics and smart windows.

Co-encapsulation of collagen peptide and astaxanthin in WG/OG/W double emulsions-filled alginate hydrogel beads: Fabrication, characterization and digestion behaviors

The double emulsions-filled hydrogel beads delivery systems with controlled lipolysis and sustained-release property of co-encapsulated bioactive substances will be highly desired. Herein, the water-in-oil-in-water emulsion with gelled inner water phase and oil phase (WG/OG/W) filled hydrogel beads as a novel co-delivery system were developed with varied concentrations of rice bran wax and W/O emulsions to achieve effectively controlled release of lipolysis and nutraceuticals. Interestingly, the gelation of oil phase triggered by rice bran wax could enhance the storage stability of WG/OG/W emulsions due to the enhanced viscoelastic property. Increasing the mass fractions of W/O emulsions improved the stability of double emulsions due to increased viscosity and decreased particle size. Cryo-SEM observation showed that the double emulsion droplets were scattered in the three-dimensional network of alginate gel beads. Increased the addition of rice bran wax or W/O emulsions, the encapsulation efficiency of collagen peptide and astaxanthin was significantly improved. The in vitro digestion results indicated that increasing the concentrations of rice bran wax and W/O emulsion fractions in WG/OG/W emulsion-filled gel beads could effectively delay the release extent of free fatty acids and encapsulated nutraceuticals. The presence of rice bran wax contributed to increase the bioaccessibility of collagen peptide and astaxanthin.

Structural Color Mixing in Microcapsules through Exclusive Crystallization of Binary and Ternary Colloids.

Colloidal crystals are designed as photonic microparticles for various applications. However, conventional microparticles generally have only one stopband from a single lattice constant, which restricts the range of colors and optical codes available. Here, photonic microcapsules are created that contain two or three distinct crystalline grains, resulting in dual or triple stopbands that offer a wider range of colors through structural color mixing. To produce distinct colloidal crystallites from binary or ternary colloidal mixtures, the interparticle interaction is manipulated using depletion forces in double-emulsion droplets. Aqueous dispersions of binary or ternary colloidal mixtures in the innermost droplet are gently concentrated in the presence of a depletant and salt by imposing hypertonic conditions. Different-sized particles crystallize into their own crystals rather than forming random glassy alloys to minimize free energy. The average size of the crystalline grains can be adjusted with osmotic pressure, and the relative ratio of distinct grains can be controlled with the mixing ratio of particles. The resulting microcapsules with small grains and high surface coverage are almost optically isotropic and exhibit highly-saturated mixed structural colors and multiple reflectance peaks. The mixed color and reflectance spectrum are controllable with the selection of particle sizes and mixing ratios.

Dynamic mechanism of double emulsion droplets flowing through a microfluidic T-junction

A microfluidic chip is adopted to study the droplet dynamic behaviors when flowing through the T-junction. Large ranges of initial length (Lo/w = 0.8–2.8), capillary number (Cao = 0.030–0.165), and viscosity ratio (λo = 0.16–5.90) are considered to identify the deformation characteristics and the breakup results. Three flow patterns are categorized in the T-junction, and critical conditions are expressed as power law relations between the normalized length and the capillary number. The coupling competition exists between outer neck thinning and inner droplet shifting, which influences the deformation process. A state diagram indicating the number of breakups is built through two key features of the shifting behavior, namely, the shifting distance and the shifting velocity. With increasing viscosity ratio, thresholds of both breakups decrease due to the reduced deformation resistance caused by vortex flow and the weakened coupling effect, resulting from the suppressed shifting behavior. The shell thickness via twice-breakup pattern depends solely on the viscosity ratio.

Evaluating the Accuracy of Impedance Flow Cytometry with Cell-Sized Liposomes.

Electrical properties of single cells are important label-free biomarkers of disease and immunity. At present, impedance flow cytometry (IFC) provides important means for high throughput characterization of single-cell electrical properties. However, the accuracy of the spherical single-shell electrical model widely used in IFC has not been well evaluated due to the lack of reliable and reproducible single-shell model particles with true-value electrical parameters as benchmarks. Herein, a method is proposed to evaluate the accuracy of the single-cell electrical model with cell-sized unilamellar liposomes synthesized through double emulsion droplet microfluidics. The influence of three key dimension parameters (i.e., the measurement channel width w, height h, and electrode gap g) in the single-cell electrical model were evaluated through experiment. It was found that the relative error of the electrical intrinsic parameters measured by IFC is less than 10% when the size of the sensing zone is close to the measured particles. It further reveals that h has the greatest influence on the measurement accuracy, and the maximum relative error can reach ∼30%. Error caused by g is slightly larger than w. This provides a solid guideline for the design of IFC measurement system. It is envisioned that this method can advance further improvement of IFC and accurate electrical characterization of single cells.

Numerical characterization of transient electrohydrodynamic deformation and coalescence of single-core double emulsion droplets by AC field dielectrophoresis

Considering its good biocompatibility, the water-in-oil-in-water type compound double emulsion (DE) droplet serves as an ideal microreaction vessel upon their on-demand electro-coalescence. By coupling phase field description to fluid dielectrophoresis, transient numerical simulation is conducted to characterize the dynamic feature of surface charge wave and electric stress induced at various phase interfaces of single-core DE drops subjected to external AC electric fields. A novel prolate/prolate electro-deformation mode is discovered by numerical analysis for A/B/A type two-phase droplet when both the conductivity and permittivity of oil shell are much smaller than that of water phase. Based upon this, AC electrostatic attraction between two opposing single-core DE droplets is then investigated in a broad parametric space. Collision of two adjacent oil shells is always followed by the dielectrophoretic integration of corresponding water cores wrapped within them, leading to three different possible modes of inter-droplet electro-coalescence under distinct core/shell radius ratio. Our numerical analysis herein unveils for the first time the hitherto unknown spatial-temporal electrohydrodynamic behavior of single-core DE droplets driven by Maxwell-Wagner structural polarization, during their deformation and coalescence as a consequence of an active interplay among those induced droplet dipoles.

Double emulsions as a high-throughput enrichment and isolation platform for slower-growing microbes

Our understanding of in situ microbial physiology is primarily based on physiological characterization of fast-growing and readily-isolatable microbes. Microbial enrichments to obtain novel isolates with slower growth rates or physiologies adapted to low nutrient environments are plagued by intrinsic biases for fastest-growing species when using standard laboratory isolation protocols. New cultivation tools to minimize these biases and enrich for less well-studied taxa are needed. In this study, we developed a high-throughput bacterial enrichment platform based on single cell encapsulation and growth within double emulsions (GrowMiDE). We showed that GrowMiDE can cultivate many different microorganisms and enrich for underrepresented taxa that are never observed in traditional batch enrichments. For example, preventing dominance of the enrichment by fast-growing microbes due to nutrient privatization within the double emulsion droplets allowed cultivation of slower-growing Negativicutes and Methanobacteria from stool samples in rich media enrichment cultures. In competition experiments between growth rate and growth yield specialist strains, GrowMiDE enrichments prevented competition for shared nutrient pools and enriched for slower-growing but more efficient strains. Finally, we demonstrated the compatibility of GrowMiDE with commercial fluorescence-activated cell sorting (FACS) to obtain isolates from GrowMiDE enrichments. Together, GrowMiDE + DE-FACS is a promising new high-throughput enrichment platform that can be easily applied to diverse microbial enrichments or screens.