During the last few decades, microfluidic liquid–liquid extractors have been developed to address the need for separating solutes in analytical chemistry and efficiently recovering products in microfluidic reactors. This review classifies the various microfluidic liquid–liquid extractors into three major groups based on their flow arrangement: stop-flow microfluidic extractors (MEs), cocurrent MEs, and countercurrent MEs. Each group is further classified into several subcategories based on flow pattern and/or working principle. The review focuses on how to establish these three groups of microfluidic liquid–liquid extractors, including the difficulties and corresponding solutions for establishing these MEs, as well as their advantages and disadvantages. The review ends with conclusions and the outlook of the field.

•A 3-D microextractor is designed using the vortex effect.•Dripping, jetting, and swirl flows with and without droplets are formed.•The swirl flow enhances shear force and facilitates droplet production.•A high extraction efficiency can be achieved even at low flux ratios.•Co- and counter-current arrangement without any moving parts are established.A single-stage cocurrent arrangement was described based on a three-dimensional (3-D) vortex microextractor using the vortex effect, and the 3-D microextractor was further modified to establish a countercurrent arrangement without any interstage pumps or check valves. The 3-D vortex microextractor mainly composed of an inlet channel, two circular chambers, a slender cylindrical connection channel, and an outlet channel. In both cocurrent and countercurrent operations, the flow pattern and extraction performance were investigated and analyzed. In the cocurrent arrangement, the hydraulics experiment results demonstrated that the 3-D vortex microextractor could passively and effectively produce droplets even at high fluxes of the dispersed phase. The extraction experiment results confirmed that the vortex effect of the 3-D microextractor could enhance the shear force and facilitate the production of droplets, consequently enhancing the extraction efficiency. Furthermore, it was experimentally proved that the countercurrent arrangement could be realized relying on the modified 3-D microextractor and alternatively pulsed feeding of both phases.Download high-res image (145KB)Download full-size image

•A 2-D high-efficiency micromixer was designed using the Coanda effect.•Transverse flow perturbation was generated through the feedback channel.•Time-periodic chaotic mixing behavior was investigated.•Lagrangian particle tracking was employed to characterize chaotic advection.An oscillating feedback micromixer comprising an inlet channel, two Coanda steps, a divergent chamber, a splitter, two feedback channels, and an outlet channel was designed considering the Coanda effect. Two-dimensional unsteady simulations were employed to study the impact of the Reynolds number on the oscillation frequency, pressure drop, and chaotic mixing. The switching mechanism of the fluidic oscillation based on the Coanda effect was examined in detail. Three Lagrangian particle tracking indicators, the Poincaré maps, particle dispersion distribution, and stretching of fluid filaments, were employed to observe and quantify the mixing induced by chaotic advection. The Lagrangian simulation results showed that the average stretching index increased from 4.91 to 5.57 with Reynolds number (Re) increased from 33.3 to 100. In addition, the mixing efficiency was quantified using a mixing index based on the standard deviation of the scalar species distribution. The results indicated that the mixing efficiency increased with the increase of Reynolds number, and the mixing efficiency of 75.3% could be achieved at Re = 100. The mixing of colorless and blue deionized water was tested experimentally to verify the simulated concentration fields, and the experimental results were in good agreement with the simulation results.Download high-res image (76KB)Download full-size image

•A high-throughput microextractor is designed using the fluidic oscillation.•A concave obstacle is used to produce the fluidic oscillation.•Stratified, single oscillating and full oscillating flow patterns are formed.•The oscillating flow enhances mass transfer performance.•A high extraction efficiency can be achieved for two immiscible liquids.A highly efficient microextractor with large fluxes was designed based on oscillation to improve its extraction performance. The flow patterns and mass transfer performance of the microextractor were experimentally investigated. The influence of Reynolds number on these metrics of a liquid-liquid system was discussed. From the flow visualization experiment, three different flow patterns were observed: stratified flow, single oscillating flow, and full oscillating flow. It was found that the mass transfer performance was closely related to the conversion of the flow patterns. In the stratified flow, the extraction efficiency decreased with an increase in Reynolds number, while in the single oscillating flow and fully oscillating flow regimes, the extraction efficiency increased with increasing Reynolds number. When the flux of the aqueous phase was 45 mL/min with a corresponding residence time of 0.027 s, an extraction efficiency of 98.9% was achieved. Furthermore, the overall volumetric mass transfer coefficient of the oscillating extractor was about two to three orders of magnitude higher than that of traditional liquid-liquid macro-scale extractors and one magnitude higher than that of other liquid-liquid micro-scale extractors. The high efficiency, high throughput, and short residence time gives the oscillating extractor a competitive advantage over other liquid-liquid extractors.Download high-res image (164KB)Download full-size image

In this study, a mini hydrodynamic focusing device with two feedback channels was designed to produce droplets of two immiscible liquids with a high throughput. Compared to conventional hydrodynamic focusing devices, the design can prevent the two liquids from forming parallel flows that suppress the dispersed phase from breaking up into droplets with high throughputs. A red aqueous solution and fully hydrogenated kerosene with 0.3 wt % Span 80 surfactant were used as the dispersed phase and continuous phase, respectively. Experiments confirmed that the feedback channels can enhance the interface instability and consequently facilitate the production of droplets. Moreover, an arrangement where the dispersed phase is on both sides of the continuous phase was found to remarkably improve the production of droplets because of the increased interface. The mini hydrodynamic focusing device is suitable for solvent extraction and other chemical processes that require a high surface/volume ratio.

The effects of different geometrical factors such as the jet nozzle exit width, feedback channel width, oscillator depth, and the oscillating chamber shape on the feedback fluidic oscillator performance were investigated experimentally. A characteristic curve relating to only the oscillation frequency f and Reynolds number Re was found to be insensitive to the oscillator configurations and is expressed as f (Hz) = 6.05 exp(−α/3.30) + 14.5 exp(−α/0.859) + 0.669, where α = ln(Re)/f. The characteristic curve can be used to determine the Reynolds number and thereby the flowrate through the oscillators according to the detected oscillation frequency.

A compact pneumatic pulse-jet pump with a Venturi-like reverse flow diverter, which consists of a nozzle and diffuser, is designed for lifting and transporting a hazardous fluid through a narrow mounting hole. The pumping performance for a liquid mixture or a liquid-solid mixture is examined in terms of the effects of liquid viscosity, particle mass concentration, lifting height, and compression pressure. Results reveal that the pumping performance of the compact pneumatic pulse-jet pump is controlled by jet inertia and the flow resistance of the riser tube positioned after the diffuser. The capacity of the compact pneumatic pulse-jet pump increases with compression pressure and decreases with liquid viscosity. However, even for a liquid mixture with a high viscosity of 7.38 mPa·s, a pumping capacity of 170.7 L·h−1 was observed. For a liquid mixture, two dimensionless indices of performance were found to be the ratio of Euler numbers Euout/EuDV and the suction factor q. As the liquid-solid mixture was lifted to elevation of 6.74 m by the compact pump, the particle size distributions of the liquid-solid mixture in the tank and from the riser tube outlet were determined by a particle size analyzer and found to coincide well.

Feedback fluidic flowmeters with curved attachment walls instead of conventional straight attachment walls were designed and machined. We experimentally investigated the effects of the dimensions of the jet nozzle and feedback channel on the oscillatory frequency, using water as the working fluid. The results reveal that in most cases, feedback fluidic flowmeters with curved attachment walls have a greater oscillatory frequency than those with straight attachment walls. A performance characteristic curve related to only the oscillatory frequency f and Reynolds number Re was found to be insensitive to the jet nozzle width and feedback channel width, even for asymmetric feedback channels [f(Hz)=7.67·exp(–α/2.56)+31.2·exp(–α/0.554)+0.757, where α=ln(Re)/f]. The characteristic curve can be used to measure the flow rates of liquids through a feedback fluidic flowmeter with curved attachment walls without the need for frequent calibrations.Graphical abstractDownload full-size imageHighlights► Curved attachment walls improve the performance of the feedback fluidic flowmeters. ► A characteristic curve was found to be insensitive to the geometrical factors. ► Frequent calibrations are consequently unnecessary during applications. ► The flowrate can be determined using the curve regardless of dimensional changes.