Droplet-based microextraction in the aqueous two-phase system

Engineering advancements in microfluidic systems for enhanced mixing at low Reynolds numbers

Mixing within micro- and millichannels is a pivotal element across various applications, ranging from chemical synthesis to biomedical diagnostics and environmental monitoring. The inherent low Reynolds number flow in these channels often results in a parabolic velocity profile, leading to a broad residence time distribution. Achieving efficient mixing at such small scales presents unique challenges and opportunities. This review encompasses various techniques and strategies to evaluate and enhance mixing efficiency in these confined environments. It explores the significance of mixing in micro- and millichannels, highlighting its relevance for enhanced reaction kinetics, homogeneity in mixed fluids, and analytical accuracy. We discuss various mixing methodologies that have been employed to get a narrower residence time distribution. The role of channel geometry, flow conditions, and mixing mechanisms in influencing the mixing performance are also discussed. Various emerging technologies and advancements in microfluidic devices and tools specifically designed to enhance mixing efficiency are highlighted. We emphasize the potential applications of micro- and millichannels in fields of nanoparticle synthesis, which can be utilized for biological applications. Additionally, the prospects of machine learning and artificial intelligence are offered toward incorporating better mixing to achieve precise control over nanoparticle synthesis, ultimately enhancing the potential for applications in these miniature fluidic systems.

Encapsulation of Caenorhabditis elegans in Water-in-Water Microdroplets to Study the Worm Viability: Alternative Avenue to Manipulate Microdroplet Environment

Due to the great biocompatibility of the aqueous two phase system (ATPS), biological cells have been widely encapsulated in ATPS microdroplets (diameter < 50 μm). However, the immobilization of relatively large multicellular organisms such as Caenorhabditis elegans in ATPS droplets remains challenging as the spontaneous generation of droplets greater than 200 μm is difficult without external perturbations. In this study, we utilize a microneedle-assisted coflow microfludic channel to passively form ATPS microdroplets larger than 200 μm and successfully entrap C. elegans in the microdroplets. We monitor the worm viability and its temporal stroke frequency up to 6 h. We study the effects of dextran (DEX)-to-polyethylene glycol (PEG) flow ratios and worm concentration on the droplet diameter, worm encapsulation efficiency, and the number of droplets containing individual worms. Larger ATPS microdroplets (>200 μm) form in the ranges of capillary number (Ca) between 0.020 to 0.20 and Weber number (We) between 10-5 and 10-3. An ATPS with the encapsulation ability and biocompatibility can offer an alternative immobilization tool for multicellular organisms to existing platforms such as water/oil droplets.

Water-in-water droplet microfluidics: A design manual

Droplet microfluidics is utilized in a wide range of applications in biomedicine and biology. Applications include rapid biochemical analysis, materials generation, biochemical assays, and point-of-care medicine. The integration of aqueous two-phase systems (ATPSs) into droplet microfluidic platforms has potential utility in oil-free biological and biomedical applications, namely, reducing cytotoxicity and preserving the native form and function of costly biomolecular reagents. In this review, we present a design manual for the chemist, biologist, and engineer to design experiments in the context of their biological applications using all-in-water droplet microfluidic systems. We describe the studies achievable using these systems and the corresponding fabrication and stabilization methods. With this information, readers may apply the fundamental principles and recent advancements in ATPS droplet microfluidics to their research. Finally, we propose a development roadmap of opportunities to utilize ATPS droplet microfluidics in applications that remain underexplored.

Recent progress in the synthesis of all-aqueous two-phase droplets using microfluidic approaches

An aqueous two-phase system (ATPS) is a system with liquid-liquid phase separation and shows great potential for the extraction, separation, purification, and enrichment of proteins, membranes, viruses, enzymes, nucleic acids, and other biomolecules because of its simplicity, biocompatibility, and wide applicability [1-4]. The clear aqueous-aqueous interface of ATPSs is highly advantageous for their implementation, therefore making ATPSs a green alternative approach to replace conventional emulsion systems, such as water-in-oil droplets. All aqueous emulsions (water-in-water, w-in-w) hold great promise in the biomedical field as glucose sensors [5] and promising carriers for the encapsulation and release of various biomolecules and nonbiomolecules [6-10]. However, the ultralow interfacial tension between the two phases is a hurdle in generating w-in-w emulsion droplets. In the past, bulk emulsification and electrospray techniques were employed for the generation of w-in-w emulsion droplets and the fabrication of microparticles and microcapsules in the later stage. Bulk emulsification is a simple and low-cost technique; however, it generates polydisperse w-in-w emulsion droplets. Another technique, electrospray, involves easy experimental setups that can generate monodisperse but nonspherical w-in-w emulsion droplets. In comparison, microfluidic platforms provide monodisperse w-in-w emulsion droplets with spherical shapes, deal with the small volumes of solutions and short reaction times and achieve portability and versatility in their design through rapid prototyping. Owing to several advantages, microfluidic approaches have recently been introduced. To date, several different strategies have been explored to generate w-in-w emulsions and multiple w-in-w emulsions and to fabricate microparticles and microcapsules using conventional microfluidic devices. Although a few review articles on ATPSs emulsions have been published in the past, to date, few reviews have exclusively focused on the evolution of microfluidic-based ATPS droplets. The present review begins with a brief discussion of the history of ATPSs and their fundamentals, which is followed by an account chronicling the integration of microfluidic devices with ATPSs to generate w-in-w emulsion droplets. Furthermore, the stabilization strategies of w-in-w emulsion droplets and microfluidic fabrication of microparticles and microcapsules for modern applications, such as biomolecule encapsulation and spheroid construction, are discussed in detail in this review. We believe that the present review will provide useful information to not only new entrants in the microfluidic community wanting to appreciate the findings of the field but also existing researchers wanting to keep themselves updated on progress in the field.

Progress in all-aqueous droplets generation with microfluidics: Mechanisms of formation and stability improvements

All-aqueous systems have attracted intensive attention as a promising platform for applications in cell separation, protein partitioning, and DNA extraction, due to their selective separation capability, rapid mass transfer, and good biocompatibility. Reliable generation of all-aqueous droplets with accurate control over their size and size distribution is vital to meet the increasingly growing demands in emulsion-based applications. However, the ultra-low interfacial tension and large effective interfacial thickness of the water-water interface pose challenges for the generation and stabilization of uniform all-aqueous droplets, respectively. Microfluidics technology has emerged as a versatile platform for the precision generation of all-aqueous droplets with improved stability. This review aims to systematize the controllable generation of all-aqueous droplets and summarize various strategies to improve their stability with microfluidics. We first provide a comprehensive review on the recent progress of all-aqueous droplets generation with microfluidics by detailing the properties of all-aqueous systems, mechanisms of droplet formation, active and passive methods for droplet generation, and the property of droplets. We then review the various strategies used to improve the stability of all-aqueous droplets and discuss the fabrication of biomaterials using all-aqueous droplets as liquid templates. We envision that this review will benefit the future development of all-aqueous droplet generation and its applications in developing biomaterials, which will be useful for researchers working in the field of all-aqueous systems and those who are new and interested in the field.

Microfluidic generation of ATPS droplets by transient double emulsion technique

An aqueous two-phase system (ATPS) is considered as a promising candidate for biological applications due to its excellent permeability, selective separation, rapid mass transfer and all-biocompatible nature. However, it remains difficult to generate ATPS emulsions with controllable and prolonged stability due to ultra-low interfacial tension. Here, we present a transient double emulsion (TDE) technique to address this challenge. The method involves two steps: (i) water-in-oil-in-water (W1/O/W2) transient double emulsion droplets are first produced with droplet microfluidics by introducing an additional middle oil phase, and (ii) the W1/O/W2 droplets dewet into oil-in-water and ATPS droplets in a controllable manner. Using the TDE method, both dextran-in-polyethylene glycol (PEG) and PEG-in-dextran ATPS droplets can be generated with high uniformity (coefficient of variation (CV) ranging from 0.66% to 2.55%), a wide range of droplet sizes (∼100 to 250 μm in radius), and tunability in the generation frequency (∼4 to 170 Hz). Tuning the oil viscosity controls the destabilization time for on-demand dewetting and releasing of aqueous core droplets. The stability of ATPS droplets is significantly improved by storing aqueous droplets in double emulsion vessels, which would benefit various applications, such as small molecule encapsulation.

Uncommon biphasic behaviour induced by very high metal ion concentrations in HCl/H2O/[P44414]Cl and HCl/H2O/PEG-600 systems

In this work, the important role played by metal ions such as Fe(ii/iii), Cr(iii) and Ni(ii) in the formation and binodal behaviour of aqueous biphasic systems (ABS) composed of HCl and the ionic liquid, [P44414]Cl, or the polymer, PEG-600, is investigated. The concentration of metal ions used in this work exceeds several g L-1 for an industrial foreseen application. Experiments have also been carried out by varying the concentration of metal ions at different temperatures. Fe exhibits a totally different behaviour compared to Ni and Cr. In particular, the binodal curves in the presence of the ionic liquid are far from the classical curves found in the literature, displaying an onion-shape form, while for Ni and Cr, the curves follow the classical trend. When any of the three metal ions is mixed with the polymer and HCl medium, only Fe(iii) induces a biphasic system. Insights into the chemical driving forces at work are discussed.

Emerging aqueous two-phase systems: from fundamentals of interfaces to biomedical applications

This review summarizes recent advances of aqueous two-phase systems (ATPSs), particularly their interfaces, with a focus on biomedical applications.

Polymer-Salt Aqueous Two-Phase System (ATPS) Micro-Droplets for Cell Encapsulation

Biosample encapsulation is a critical step in a wide range of biomedical and bioengineering applications. Aqueous two-phase system (ATPS) droplets have been recently introduced and showed a great promise to the biological separation and encapsulation due to their excellent biocompatibility. This study shows for the first time the passive generation of salt-based ATPS microdroplets and their biocompatibility test. We used two ATPS including polymer/polymer (polyethylene glycol (PEG)/dextran (DEX)) and polymer/salt (PEG/Magnesium sulfate) for droplet generation in a flow-focusing geometry. Droplet morphologies and monodispersity in both systems are studied. The PEG/salt system showed an excellent capability of uniform droplet formation with a wide range of sizes (20-60 μm) which makes it a suitable candidate for encapsulation of biological samples. Therefore, we examined the potential application of the PEG/salt system for encapsulating human umbilical vein endothelial cells (HUVECs). A cell viability test was conducted on MgSO4 solutions at various concentrations and our results showed an adequate cell survival. The findings of this research suggest that the polymer/salt ATPS could be a biocompatible all-aqueous platform for cell encapsulation.

Interfacial Complexation Induced Controllable Fabrication of Stable Polyelectrolyte Microcapsules Using All-Aqueous Droplet Microfluidics for Enzyme Release

Water-in-water (w/w) emulsions are particularly advantageous for biomedical-related applications, such as cell encapsulation, bioreactors, biocompatible storage, and processing of biomacromolecules. However, due to ultralow interfacial tension, generation and stabilization of uniform w/w droplets are challenging. In this work, we report a strategy of creating stable and size-controllable w/w droplets that can quickly form polyelectrolyte microcapsules (PEMCs) in a microfluidic device. A three-phase (inner, middle, outer) aqueous system was applied to create a stream of inner phase, which could be broken into droplets via a mechanical perturbation frequency, with size determined by the stream diameter and vibration frequency. The interfacial complexation is formed via electrostatic interaction of polycations of poly(diallyldimethylammoniumchloride) with polyanions of polystyrene sodium sulfate in the inner and outer phases. With addition of negatively charged silica nanoparticles, the stability, permeability, and mechanical strength of the PEMC shell could be well manipulated. Prepared PEMCs were verified by encapsulating fluorescein isothiocyanate-labeled dextran molecules and stimuli-triggered release by varying the pH value or osmotic pressure. A model enzyme, trypsin, was successfully encapsulated into PEMCs and released without impairing their catalytic activity. These results highlight its potential applications for efficient encapsulation, storage, delivery, and release of chemical, biological, pharmaceutical, and therapeutic agents.

High-Throughput Aqueous Two-Phase System Droplet Generation by Oil-Free Passive Microfluidics

Aqueous two-phase system (ATPS) droplet generation has significant potential in biological and medical applications because of its excellent biocompatibility. However, the ultralow interfacial tension of ATPS makes droplet generation extremely challenging when compared with the conventional water-in-oil (W/O) system. In this paper, we passively produced ATPS droplets with a wide range of droplet size and high production rate without the involvement of an oil phase and external forces. For the first time, we reported important information of the flow rate and capillary (Ca) number for passive, oil-free ATPS droplet generation. It was found that the range of Ca numbers of the continuous phase under the jetting flow regime is 0.3-1.7, as compared to less than 0.1 in the W/O system, indicating the ultralow interfacial tension in ATPS. In addition, we successfully generated ATPS droplets with a radius as small as 7 μm at the maximum frequency up to 300 Hz, which has not been achieved in previous studies. The size and generation frequency of ATPS droplets can be controlled independently by adjusting the inlet pressures and corresponding flow rates. We found that the droplet size is correlated with the pressure and flow rate ratios with the power-law exponents of 0.8 and 0.2, respectively.

Emerging Biotechnology Applications of Aqueous Two-Phase Systems

Liquid-liquid phase separation between aqueous solutions containing two incompatible polymers, a polymer and a salt, or a polymer and a surfactant, has been exploited for a wide variety of biotechnology applications throughout the years. While many applications for aqueous two-phase systems fall within the realm of separation science, the ability to partition many different materials within these systems, coupled with recent advances in materials science and liquid handling, has allowed bioengineers to imagine new applications. This progress report provides an overview of the history and key properties of aqueous two-phase systems to lend context to how these materials have progressed to modern applications such as cellular micropatterning and bioprinting, high-throughput 3D tissue assembly, microscale biomolecular assay development, facilitation of cell separation and microcapsule production using microfluidic devices, and synthetic biology. Future directions and present limitations and design considerations of this adaptable and promising toolkit for biomolecule and cellular manipulation are further evaluated.

Droplet Formation by Rupture of Vibration-Induced Interfacial Fingers

By imposing vibration to a core-annular flow of an aqueous two-phase system (ATPS) with ultralow interfacial tension, we observe a liquid finger protruding from the interface of an expanding jet. We find that the protruded finger breaks up only when its length-to-width ratio exceeds a threshold value. The breakup follows a constant wavelength-to-width ratio that is consistent with that of breakup under Rayleigh-Plateau instability. The mechanism is applicable to aqueous two-phase systems with a large range of viscosity ratios. The protruded finger can break up into small droplets that are monodisperse in size, controllable in generation frequency under a wide range of flow rates. This work suggests a way to generate small water-water droplets with high monodispersity and production rate from a single nozzle.

Microfluidic generation of aqueous two-phase-system (ATPS) droplets by oil-droplet choppers

Existing approaches for droplet generation with an ultra-low interfacial tension using aqueous two-phase systems, ATPS, are either constricted by a narrow range of flow conditions using passive methods or subjected to complex chip fabrication with the integration of external components using active actuation. To address these issues, we present a simple approach to produce uniform ATPS droplets facilitated by oil-droplet choppers in microfluidics. Our solution counts on the synchronized formation of high-interfacial-tension oil-in-water and low-interfacial-tension water-in-water droplets, where the ATPS interface is distorted by oil droplets and decays into water-in-water droplets. In the synchronization regime, the size and generation frequency of ATPS droplets can be controlled independently by tuning the flow rates of the dispersed aqueous and oil phases, respectively. Our method demonstrates high uniformity of droplets (coefficient of variation between 0.75% and 2.45%), a wide range of available droplet size (droplet radius from 5 μm to 180 μm), and a maximum generation frequency of about 2.1 kHz that is nearly two orders of magnitude faster than that in existing methods. We develop theoretical models to precisely predict the minimum and maximum frequencies of droplet generation and the droplet size. The produced ATPS droplets and oil choppers are separated in the channel using density difference. Our method would boost emulsion-based biological applications such as cell encapsulation, biomolecule delivery, bioreactors, and biomaterials synthesis with ATPS droplets.

High-Efficiency and High-Throughput On-Chip Exchange of the Continuous Phase in Droplet Microfluidic Systems

This article describes an integrated platform for the on-chip exchange of the continuous phase in droplet microfluidic systems. The drops used in this work are stabilized by amphiphilic nanoparticles. For some characterizations and applications of these nanoparticle-stabilized drops, including the measurement of adsorption dynamics of nanoparticles to the droplet surface, it is necessary to change the composition of the continuous phase from that used during the droplet generation process. Thus far, no work has reported the exchange of the continuous phase for a large number (>1 million) of drops in a microfluidic system. This article describes the design and characterization of a high-efficiency and high-throughput on-chip exchanger of the continuous phase in a continuous-flow droplet microfluidic system. The efficiency of exchange was higher than 97%. The throughput was greater than 1 million drops/min, and this can be increased further by increasing the number of parallel exchangers used. Because drops are injected into the exchanger in a continuous-flow manner, the method is directly compatible with automation to further increase its reliability and potential scale-up.

Liquid–liquid microflow reaction engineering

Engineering characteristics of liquid–liquid microflow and its advantages in chemical reactions.

Water-in-Water Droplets by Passive Microfluidic Flow Focusing

We present a simple microfluidic system that generates water-in-water, aqueous two phase system (ATPS) droplets, by passive flow focusing. ATPS droplet formation is achieved by applying weak hydrostatic pressures, with liquid-filled pipette tips as fluid columns at the inlets, to introduce low speed flows to the flow focusing junction. To control the size of the droplets, we systematically vary the interfacial tension and viscosity of the ATPS fluids and adjust the fluid column height at the fluid inlets. The size of the droplets scales with a power law of the ratio of viscous stresses in the two ATPS phases. Overall, we find a drop size coefficient of variation (CV; i.e., polydispersity) of about 10%. We also find that when drops form very close to the flow focusing junction, the drops have a CV of less than 1%. Our droplet generation method is easily scalable: we demonstrate a parallel system that generates droplets simultaneously and improves the droplet production rate by up to one order of magnitude. Finally, we show the potential application of our system for encapsulating cells in water-in-water emulsions by encapsulating microparticles and cells. To the best of our knowledge, our microfluidic technique is the first that forms low interfacial tension ATPS droplets without applying external perturbations. We anticipate that this simple approach will find utility in drug and cell delivery applications because of the all-biocompatible nature of the water-in-water ATPS environment.

Droplet formation and shrinking in aqueous two-phase systems using a membrane emulsification method

Using a membrane emulsification method based on porous hollow-fiber membranes in combination with an aqueous two-phase system (ATPS), we are able to produce "water-in-water" droplets with narrow-dispersed size distributions. The equilibrium phases of the aqueous two-phase system polyethylene glycol-dipotassium hydrogen phosphate are used for this purpose. The droplet diameter of a given fluid system is determined by the flow rates of the continuous and disperse phase as well as the hollow fiber dimensions. When diluting the disperse phase and thus moving the ATPS system out of equilibrium, the droplet size can be further reduced in comparison to the equilibrium case. Generally, droplets formed with this method have diameters 20%-60% larger than the inner hollow fiber diameter. The new strategy of diluting the disperse phase allows the production of droplet diameter below the inner diameter of the membrane.

Microfluidic generation of aqueous two-phase system (ATPS) droplets by controlled pulsating inlet pressures

We present a technique that generates droplets using ultralow interfacial tension aqueous two-phase systems (ATPS). Our method combines a classical microfluidic flow focusing geometry with precisely controlled pulsating inlet pressure, to form monodisperse ATPS droplets. The dextran (DEX) disperse phase enters through the central inlet with variable on-off pressure cycles controlled by a pneumatic solenoid valve. The continuous phase polyethylene glycol (PEG) solution enters the flow focusing junction through the cross channels at a fixed flow rate. The on-off cycles of the applied pressure, combined with the fixed flow rate cross flow, make it possible for the ATPS jet to break up into droplets. We observe different droplet formation regimes with changes in the applied pressure magnitude and timing, and the continuous phase flow rate. We also develop a scaling model to predict the size of the generated droplets, and the experimental results show a good quantitative agreement with our scaling model. Additionally, we demonstrate the potential for scaling-up of the droplet production rate, with a simultaneous two-droplet generating geometry. We anticipate that this simple and precise approach to making ATPS droplets will find utility in biological applications where the all-biocompatibility of ATPS is desirable.

Ultra-thin liquid film extraction based on a gas–liquid–liquid double emulsion in a microchannel device

A gas–liquid–liquid double emulsion with ultra-thin liquid film is proposed for the mass transfer enhancement of an extreme phase ratio system.

Dripping and jetting in microfluidic multiphase flows applied to particle and fiber synthesis

Dripping and jetting regimes in microfluidic multiphase flows have been investigated extensively, and this review summarizes the main observations and physical understandings in this field to date for three common device geometries: coaxial, flow-focusing and T-junction. The format of the presentation allows for simple and direct comparison of the different conditions for drop and jet formation, as well as the relative ease and utility of forming either drops or jets among the three geometries. The emphasis is on the use of drops and jets as templates for microparticle and microfiber syntheses, and a description is given of the more common methods of solidification and strategies for achieving complex multicomponent microparticles and microfibers.

Slow growth of the Rayleigh-Plateau instability in aqueous two phase systems

This paper studies the Rayleigh-Plateau instability for co-flowing immiscible aqueous polymer solutions in a microfluidic channel. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. Experiments for the well-known aqueous two phase system (ATPS, or aqueous biphasic systems) of dextran and polyethylene glycol solutions exhibited a growth rate of 1 s(-1), which was more than an order of magnitude slower than an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were ruled out as explanations for the observed behavior. The results are remarkable because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case. Microfluidic uses of aqueous two phase systems include separation of labile biomolecules but have hitherto be limited because of the difficulty in making droplets. The results of this work teach how to design devices for biological microfluidic ATPS platforms.

Microfluidic fabrication of water-in-water (w/w) jets and emulsions

We demonstrate the generation of water-in-water (w/w) jets and emulsions by combining droplet microfluidics and aqueous two-phase systems (ATPS). The application of ATPS in microfluidics has been hampered by the low interfacial tension between typical aqueous phases. The low tension makes it difficult to form w/w droplets with conventional droplet microfluidic approaches. We show that by mechanically perturbing a stable w/w jet, w/w emulsions can be prepared in a controlled and reproducible fashion. We also characterize the encapsulation ability of w/w emulsions and demonstrate that their encapsulation efficiency can be significantly enhanced by inducing formation of precipitates and gels at the w/w interfaces. Our work suggests a biologically and environmentally friendly platform for droplet microfluidics and establishes the potential of w/w droplet microfluidics for encapsulation-related applications.

Microfluidics with aqueous two-phase systems

An overview is given about research activities in which aqueous two phase systems (ATPSs) are utilized in microfluidic setups. ATPSs consist of two immiscible aqueous phases and have traditionally been used for the separation and purification of biological material such as proteins or cells. Microfluidic implementations of such schemes are usually based on a number of co-flowing streams of immiscible phases in a microchannel, thereby replacing the standard batch by flow-through processes. Some aspects of the stability of such flow patterns and the recovery of the phases at the channel exit are reviewed. Furthermore, the diffusive mass transfer and sample partitioning between the phases are discussed, and corresponding applications are highlighted. When diffusion is superposed by an applied electric field normal to the liquid/liquid interface, the transport processes are accelerated, and under specific conditions the interface acts as a size-selective filter for molecules. Finally, the activities involving droplet microflows of ATPSs are reviewed. By either forming ATPS droplets in an organic phase or a droplet of one aqueous phase inside the other, a range of applications has been demonstrated, extending from separation/purification schemes to the patterning of surfaces covered with cells.

Rounded multi-level microchannels with orifices made in one exposure enable aqueous two-phase system droplet microfluidics

Exposure of a negative photoresist-coated glass slide with diffused light from the backside through a mask with disconnected features provides multi-level rounded channels with narrow orifices in one exposure. Using these structures, we construct microfluidic systems capable of creating aqueous two-phase system droplets where one aqueous phase forms droplets and the other aqueous phase forms the surrounding matrix. Unlike water-in-oil droplet systems, aqueous two-phase systems can have very low interfacial tensions that prevent spontaneous droplet formation. The multi-level channels fabricated by backside lithography satisfy two conflicting needs: (i) the requirement to have narrowed channels for efficient valve closure by channel deformation and (ii) the need to have wide channels to reduce the flow velocity, thus reducing the capillary number and enhancing droplet formation.

Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase systems

This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 μm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment.