Immiscible Continuous Phase Research Articles

OsciDrop: A Versatile Deterministic Droplet Generator.

This paper describes OsciDrop, a versatile chip-free droplet generator used to produce size-tunable droplets on demand. Droplet generation is fundamental to miniaturized analysis. We designed OsciDrop to segment the fluid flowing out of the orifice of a disposable pipette tip into droplets by oscillating its distal end underneath an immiscible continuous phase. We described the theoretical model and investigated the effect of flow rate, oscillating amplitude, frequency, and waveform on droplet generation. Our study revealed a previously underexplored Weber number-dominated regime that leverages inertial force instead of viscous force to generate droplets. The same pipette tip allowed robust and deterministic generation of monodisperse droplets with programmable sizes ranging from 200 pL to 2 μL by asymmetrical oscillation. We validated this platform with two droplet-based nucleic acid amplification tests: a digital loop-mediated isothermal amplification assay for absolute quantification of African swine fever virus and a multi-volume digital polymerase chain reaction assay for the high dynamic range measurement of human genomic DNA. The OsciDrop method opens a facile avenue to miniaturization, integration, and automation, exhibiting full accessibility for digital molecular diagnostics.

Ultrasonic Intensification of Mass Transfer in Organic Acid Extraction

The mechanism of ultrasonic extraction was discovered and analyzed in detail for the liquid membrane technique from the consideration of the specific features of the radial vibrations of a droplet of the dispersed phase placed into an immiscible continuous phase subjected to ultrasonic irradiation. Analytical formulas were derived for the rate of mass transfer as a function of the amplitude of acoustic pressure oscillations and the time of ultrasonic treatment of an extraction system. Conditions for achieving the maximum efficiency of the extraction of a substance under the stimulating effect of ultrasound were analyzed. A nonlinear equation was derived for the radial vibrations of a spherical droplet of the dispersed phase in an immiscible continuous phase under forcing in the form of acoustic pressure periodically changing with time. Experimental study of the dependence of sulfosalicylic acid distribution on time in an aqueous two-phase system with ultrasound shows good agreement of experimental results with the calculations performed.

Drop Generation in Cross-Flow of Liquid Rotating in Rigid Body Motion

Abstract Various methods have been developed to generate monodisperse drops of a dispersed phase (DP) liquid in an immiscible continuous phase (CP) liquid, which include the membrane emulsification method and the microfluidic drop generation. This study proposes an easy-to-adopt drop generation method using cross-flow: a DP liquid is injected through a stationary vertical syringe needle into a CP liquid rotating in rigid body motion. The developed method was tested and characterized using de-ionized water as the DP liquid and mineral oil as the CP liquid. Drops were generated mainly either in the dripping mode or the jetting mode, and the former resulted in higher monodispersity. Smaller drops were generated when a thinner syringe needle was used, the average flow speed of the DP liquid through the needle was decreased, or the linear flow speed of the CP liquid at the needle location was increased. Especially, the power–law relationship was observed between the drop diameter and the flow speeds, and the dripping-to-jetting transition (DJT) was observed when the Weber number of the DP liquid was about 5.

On-chip integration of normal phase high-performance liquid chromatography and droplet microfluidics introducing ethylene glycol as polar continuous phase for the compartmentalization of n-heptane eluents

We introduce an integrated chip-approach for a postcolumn segmentation of normal phase liquid chromatography. This is achieved by the seamless integration of a chiral NP-chip-HPLC column, a flow-focusing droplet generator, and a segmented flow channel in a single microfluidic glass chip. This allows a continuous segmentation of the eluent into droplets which are picked up and transported via an immiscible continuous phase. The combination of NP-chip-HPLC and droplet microfluidics enables to fractionate and conserve chromatographic runs for further downstream processes at picoliter scale. An essential aspect is the proper choice of the continuous phase concerning polarity, wetting properties and viscosity. For this purpose, ethylene glycol is introduced which facilitates this first combination of normal phase chromatography and droplet microfluidics. By adjusting the flow rates and varying the generator geometry, the size and frequency of the droplets could be precisely controlled.

All-Aqueous-Phase Microfluidics for Cell Encapsulation.

Cell-laden hydrogel microcarriers are widely used in diverse biomedical applications like three-dimensional (3D) cell culture, cellular therapy, and tissue engineering, where microcarriers were generally produced by oil, which is the common but not optimal choice, as oil may cause cytotoxicity or protein denaturation. Here, an all-aqueous-phase microfluidics is presented to achieve oil-free emulsification of cell-laden microcapsules and 3D cell culture. Aqueous solutions with different concentration gradients are used as an immiscible continuous phase and a dispersed phase, and oscillation from a solenoid valve facilitates the formation of microcapsules at the water-water interface. By adjusting aqueous-phase flow rates and oscillating frequencies, core-shell microcapsules with controllable structures can be stably and continuously generated. In further 3D cell culture, encapsulated cells maintained good viabilities and aggregated together. These features show that the oil-free microfluidic method may have broad prospects in many biomedical applications.

Surfactant-free fabrication of pNIPAAm microgels in microfluidic devices

Conventional pNIPAAm microgel synthesis utilizes surfactants to suspend pre-gel droplets in the immiscible continuous phase due to the slow polymerization required for synthesizing pNIPAAm in aqueous solvent. To improve the fabrication process and to eliminate the effects of surfactant on microgel quality, a surfactant-free and water-free method was developed. Rapid polymerization of high-quality microgels was achieved in a single-channel microfluidic device to help maintain the integrity of gel particles without the addition of surfactants. The droplet generation mechanism and the effect of flow rate of the two in-going immiscible fluid on the geometry of the produced microgels were studied. The produced microgels have low polydispersity with a dispersity index of 6.4%. The pNIPAAm hydrogels fabricated in the DMSO solvent has smaller pore size and more uniform microstructure compared to that synthesized in water. The fabricated pNIPAAm microgels show a sharp volume phase transition at ∼32 °C and high deswelling/swelling rate.

Phase inversion of slug flow on step surface to form high viscosity droplets in microchannel

Slug flow is a typical two phase flow in which droplets are lubricated by an immiscible continuous phase in a microchannel. The step surface is found to break the lubrication film and induce a phase inversion of the slug flow. The reduction in the film thickness by the step is quantitatively calculated and measured, and the critical droplet length for the breakup of the film is also revealed. This step-induced phase inversion is demonstrated to form high-in-low-viscosity emulsion droplets from a low-in-high-viscosity slug flow.

Membrane emulsification: Formation of water in oil emulsions using a hydrophilic membrane

It is shown that formation of water based droplets in an immiscible (i.e. oil) continuous phase can be achieved using a hydrophilic porous metal membrane without prior hydrophobic treatment of the membrane surface. This avoids the need for “health and safety approval” of typical hydrophobic treatments for the membrane, which often use chemicals incompatible with pharma or food applications. To investigate this, wetting experiments were carried out: sessile droplets were used to determine static contact angles and a rotating drum system was used to determine contact angles under dynamic conditions. In the latter case the three-phase contact line was observed between the rotating drum, water and the continuous phase used in the emulsification process; a surfactant was present in the continuous phase which, in this process, has a double function: to assist the wetting of the membrane by the continuous phase, and not the disperse phase, and to stabilize the droplets formed at the surface of the porous membrane during membrane emulsification.

Microfluidic synthesis of monodisperse pectin hydrogel microspheres based on in situ gelation and settling collection

AbstractBACKGROUNDGeneration of monodisperse hydrogel microspheres is needed to make exquisite microenvironments, provide effective delivery system, and obtain reliable results. In this work, we present a simple microfluidic approach for the preparation of monodisperse pectin hydrogel microspheres because of efficient collection and shape of hydrogel.RESULTSBased on the mechanism of in situ gelation and efficient collection, aqueous droplets of pectin polysaccharides are continuously generated in an immiscible continuous phase dissolving divalent metal ions, such as calcium. Under in situ gelation conditions, calcium ions are diffused into the interface between the continuous phase and the aqueous droplets, which triggers gelation of the pectin polysaccharides. The settling collection method, which involves dropping consecutive hydrogels from the outlet hole, is able to maintain the shape of the soft pectin hydrogels. Thus, pectin microspheres produced show high monodispersity (a coefficient of variation of 3.5%). In addition, the stiffness of the pectin hydrogels produced can be manipulated by a simple change of the concentration of pectin in the aqueous phase. The control of the mechanical properties can also be confirmed by measurement of the deformation of the pectin hydrogels using the micropipette aspiration method. Furthermore, the versatility of this approach enables the preparation of monodisperse pectin hydrogels with the capability to encapsulate or release nanoparticles on demand under mild conditions. The pectin microspheres are freely manipulated by the control of magnetic fields.CONCLUSIONWe believe that the in situ microfluidic synthesis method combined with settling collection provides an efficient approach for the preparation of soft, monodisperse hydrogel microspheres. © 2016 Society of Chemical Industry

Reduction in aggregation and energy transfer of quantum dots incorporated in polystyrene beads by kinetic entrapment due to cross-linking during polymerization.

We report the development of barcoded polystyrene microbeads, approximately 50 μm in diameter, which are encoded by incorporating multicolored semiconductor fluorescent nanocrystals (quantum dots or QDs) within the microbeads and using the emission spectrum of the embedded QDs as a spectral label. The polymer/nanocrystal bead composites are formed by polymerizing emulsified liquid droplets of styrene monomer and QDs suspended in an immiscible continuous phase (suspension polymerization). We focus specifically on the effect of divinylbenzene (DVB) added to cross-link the linearly growing styrene polymer chains and the effect of this cross-linking on the state of aggregation of the nanocrystals in the composite. Aggregated states of multicolor QDs give rise to nonradiative resonance energy transfer (RET) which distorts the emission label from a spectrum recorded in a reference solvent in which the nanocrystals are well dispersed and unaggregated. A simple barcode is chosen of a mixture of QDs emitting at 560 (yellow) and 620 nm (red). We find that for linear chain growth (no DVB), the QDs aggregate as is evident from the emission spectrum and the QD distribution as seen from confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM) images. Increasing the extent of cross-linking by the addition of DVB is shown to significantly decrease the aggregation and provide a clear label. We suggest that in the absence of cross-linking, linearly growing polymer chains, through enthalpic and entropic effects, drive the nanocrystals into inclusions, while cross-linking kinetically entraps the particle and prevents their aggregation.

Ultrasonic Studies of Emulsion Stability in the Presence of Magnetic Nanoparticles

Pickering emulsions are made of solid particle-stabilized droplets suspended in an immiscible continuous liquid phase. A magnetic emulsion can be obtained using magnetic particles. Solid magnetic nanoparticles are adsorbed strongly at the oil-water interface and are able to stabilize emulsions of oil and water. In this work emulsions stabilized by magnetite nanoparticles were obtained using high-energy ultrasound waves and a cavitation mechanism and, next, their stability in time was tested by means of acoustic waves with a low energy, without affecting the structure. An acoustic study showed high stability in time of magnetic emulsions stabilized by magnetite particles. The study also showed a strong influence of an external magnetic field, which can lead to changes of the emulsion properties. It is possible to control Pickering emulsion stability with the help of an external stimulus—a magnetic field.

Coexistence of different droplet generating instabilities: new breakup regimes of a liquid filament.

The coexistence of multiple droplet breakup instabilities in a Step-emulsification geometry is studied. A liquid filament, which is confined in one dimension by channel walls and surrounded by a co-flowing immiscible continuous phase, decays into droplets when subject to a sudden release of confinement. Depending on the filament aspect ratio and liquid flow rates, an unexpectedly rich variety of droplet breakup regimes is found. All of these breakup regimes are composed of two basic instabilities, i.e. a step- and a jet-instability, that coexist in various combinations on the same filament. Surprisingly, even an asymmetric breakup regime is found, producing droplet families of significantly different diameters, while the filament is subject to a fully symmetric flow field. We suggest key physical principles explaining the spontaneous symmetry breaking and the transitions between individual droplet breakup regimes. The particular ability to produce distinct droplet families from a single filament is demonstrated to allow for simultaneous concentration and encapsulation of particles into one droplet family while excess bulk liquid is released into another family of droplets.

Effects of Emulsion Droplet Size on the Structure of Electrospun Ultrafine Biocomposite Fibers with Cellulose Nanocrystals

Electrospinning of cellulose nanocrystals (CNC)/poly(lactic acid) (PLA) emulsions has been demonstrated to be an effective dispersion and alignment method to control assembly of CNC into continuous composite ultrafine fibers. CNC-PLA nanocomposite random-fiber mats and aligned-fiber yarns were prepared by emulsion electrospinning. A dispersed phase of CNC aqueous suspension and an immiscible continuous phase of PLA solution comprised the CNC-PLA water-in-oil (W/O) emulsion system. Under a set of specific conditions, the as-spun composite ultrafine fibers assumed core-shell or hollow structures. In these structures, CNCs were aligned along the core in the core-shell case, or on the wall of the hollow cylinder in the hollow fiber case. CNCs act as nucleating agents influencing PLA crystallinity, and improve the strength and stiffness of electrospun composite fibers. The effects of emulsion droplet size on fiber structural formation and CNC distribution within the electrospun fibers have been carefully examined.

Preparation of uniformly sized alginate microspheres using the novel combined methods of microchannel emulsification and external gelation

The purpose of this study was to prepare alginate (ALG) microspheres with narrow size distribution using a combination of microchannel (MC) emulsification technique and external gelation method. ALG solution was dispersed as water-in-oil (W/O) emulsion droplets in iso-octane containing 5 wt% Span 85 as the immiscible continuous phase via MC emulsification technique using hydrophobic MC array. The MC array used in this experiment is a grooved-type MC consisting of 1070 channels fabricated on a 25 mm × 28 mm silicon microchip. The monodisperse W/O emulsion droplets generated from the MCs were in the mean particle diameter ( d av) range of 18–22 μm and coefficient of variation (CV) of 5–26% at the ALG concentrations of 0.5–3.0 wt% and flow rates of 0.05–0.4 mL/h. The d av of the emulsion droplets hardly changed below a dispersed phase threshold flow rate of 0.2 mL/h but gradually became smaller when the dispersed phase concentration was increased. The resulting emulsion droplets were then congealed to form rigid ALG gel particles by reacting them with calcium chloride (CaCl 2) solution. Gelation of the ALG droplets by calcium ion (Ca 2+) resulted in shrinkage of its d av, forming uniformly sized ALG microspheres with an average diameter of 6.2 μm and a CV of below 10% at the ALG concentration of 3 wt%.

Generation of Monodisperse Inorganic–Organic Janus Microspheres in a Microfluidic Device

AbstractThis study presents a simple synthetic approach for the in situ preparation of monodisperse hybrid Janus microspheres (HJM) having organic and inorganic parts in a PDMS‐based microfluidic device. Based on the mechanism of shear‐force‐driven break‐off, merged droplets of two photocurable oligomer solutions having distinctive properties are generated into an immiscible continuous phase. Functionalized perfluoropolyether (PFPE) as the organic phase and hydrolytic allylhydridopolycarbosilane (AHPCS) as the inorganic phase are used for the generation in aqueous medium of HJM with well‐defined morphology and high monodispersity (average diameter of 162 µm and a 3.5% coefficient of variation). The size and shape of the HJM is controlled by varying the flow rate of the disperse and continuous phases. The HJM have two distinctive regions: a hydrophobic hemisphere (PFPE) having a smooth surface and a relatively hydrophilic region (AHPCS) with a rough, porous surface. In addition, pyrolysis and subsequent oxidation of these HJM convert them into SiC‐based ceramic hemispheres through the removal of the organic portion and etching off the silica shell. The selective incorporation of magnetic nanoparticles into the inorganic part shows the feasibility of the forced assembly of HJM in an applied magnetic field.

Nucleation and solidification in static arrays of monodisperse drops.

The precise measurement of nucleation and non-equilibrium solidification are vital to fields as diverse as atmospheric science, food processing, cryopreservation and metallurgy. The emulsion technique, where the phase under study is partitioned into many droplets suspended within an immiscible continuous phase, is a powerful method for uncovering rates of nucleation and dynamics of phase changes as it isolates nucleation events to single droplets. However, averaging the behavior of many drops in a bulk emulsion leads to the loss of any drop-specific information, and drop polydispersity clouds the analysis. Here we adapt a microfluidic technique for trapping monodisperse drops in planar arrays to characterize solidification of highly supercooled aqueous solutions of glycerol. This system measured rates of nucleation between 10(-5) and 10(-2) pL(-1) s(-1), yielded an ice-water interfacial energy of 33.4 mJ m(-2) between -38 and -35 degrees C, and enabled the specific dynamics of solidification to be observed for over a hundred drops in parallel without any loss of specificity. In addition to the physical insights gained, the ability to observe the time and temperature of nucleation and subsequent growth of the solid phase in static arrays of uniform drops provides a powerful tool to discover thermodynamic protocols that generate desirable crystal structures.

Effect of dispersed phase rheology on the drag of single and of ensembles of fluid spheres at moderate Reynolds numbers

In this work, the effect of dispersed phase rheology on the drag phenomena of single and of ensembles of fluid spheres translating in an immiscible power-law continuous phase has been studied numerically at moderate Reynolds numbers. The results presented herein encompass the following ranges of conditions: 1 ≤ Reo ≤ 200, 0.1 ≤ k ≤ 50, 0.2 ≤ Φ ≤ 0.6, 0.6 ≤ ni ≤ 1.6 and 0.6 ≤ no ≤ 1.6, thereby enabling the effects of the Reynolds number (Reo), of the internal to external fluid characteristic viscosity ratio (k), of the volume fraction of the dispersed phase (Φ) and of the two power-law indices (ni, no) on drag coefficient to be delineated. This information facilitates the estimation of the rate of sedimentation of single fluid spheres and their ensembles in quiescent continuous phase. Within the range of conditions studied herein, the effect of the dispersed phase rheology is found to be rather small.

Novel one-pot route to monodisperse thermosensitive hollow microcapsules in a microfluidic system

We present a simple one-pot synthetic approach for the preparation of monodisperse thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) microcapsules in a microfluidic system. Based on the mechanism of shear force-driven break-off, aqueous droplets of monomer solution are continuously generated in an immiscible continuous phase containing photoinitiators. Under UV irradiation, activated initiators are diffused into the interface between the continuous phase and the aqueous droplets, which trigger polymerization of NIPAM monomers. The PNIPAM microcapsules produced are hollow microcapsules with a thin shell membrane, high monodispersity, and fast response to environmental temperature. In addition, the size of microcapsules produced can be manipulated by the flow rate of the continuous phase or aqueous phase and different concentrations of surfactant to control interfacial tension between continuous phase and aqueous phase. Furthermore, the versatility of this approach enables the preparation of monodisperse microcapsules having the capability to encapsulate various materials such as proteins and nanoparticles under mild conditions. The in situ microfluidic synthetic method provides a novel approach for the preparation of monodisperse hollow microcapsules via a one-pot route.

Microdroplets: A sea of applications?

The exploitation of microdroplets produced within microfluidic environments has recently emerged as a new and exciting technological platform for applications within the chemical and biological sciences. Interest in microfluidic systems has been stimulated by a range of fundamental features that accompany system miniaturization. Such features include the ability to process and handle small volumes of fluid, improved analytical performance when compared to macroscale analogues, reduced instrumental footprints, low unit cost, facile integration of functional components and the exploitation of atypical fluid dynamics to control molecules in both time and space. Moreover, microfluidic systems that generate and utilize a stream of sub-nanolitre droplets dispersed within an immiscible continuous phase have the added advantage of allowing ultra-high throughput experimentation and being able to mimic conditions similar to that of a single cell (in terms of volume, pH, and salt concentration) thereby compartmentalizing biological and chemical reactions. This review provides an overview of methods for generating, controlling and manipulating droplets. Furthermore, we discuss key fields of use in which such systems may make a significant impact, with particular emphasis on novel applications in the biological and physical sciences.

Predicting of Temperature Distribution in Direct Contact Heat Transfer

An experimental and theoretical investigation of three phase direct contact heat transfer by evaporation of refrigerant drops in an immiscible liquid has been carried out. Refrigerant Rl2 and R134a were used for the dispersed phase, while water and brine were the immiscible continuous phase. A numerical analysis is presented to predict the temperature distribution throughout the circular test column radially and axially is achieved. Experimental measurements of the temperature distribution have been compared with the numerical results and are discussed .A comparison between the experimental and theoretical results showed acceptable agreement and applicability of the derived equations. Comparison with other related work showed similar behavior.