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The coalescence of a ferrofluid drop at its bulk surface, with or without a magnetic field, was investigated experimentally by a high-speed camera. Shape deformations of both the pendant ferrofluid drop and the bulk surface in the axial direction were observed during the approaching process even in the absence of a magnetic field. The angle of the upper pendant peak at the first contact decreases with the magnetic flux density, while the lower ferrofluid peak displays an opposite trend. The coalescing width of the ferrofluid drop follows a power-law relationship. The exponent of 0.64 under medium and high magnetic fields as well as the case without magnetic field confirms the inertial regime of drop coalescence. Under the low magnetic field, the significant exponent increasing from 0.59 to 3.02 at about 4 ms is in coincidence with the sudden change to a smooth coalescing section according to the visualized images. A high-speed microparticle image velocimetry (micro-PIV) technique was employed with a transparent model fluid to reveal the flow fields during the drop coalescence instead of opaque ferrofluids.

An effective mass transfer intensification method was proposed by embedding different triangular obstacles to improve the gas–liquid mass transfer efficiency in microchannel. The influences of triangle obstacles configuration, obstacle interval and gas flow rate on the volumetric mass transfer coefficient, pressure drop and energy consumption were investigated experimentally. The enhancement factor was used to quantify the mass transfer enhancement effect of triangle obstacles. It was found that the isosceles or equilateral triangle obstacles are superior to the rectangular obstacles. The maximum enhancement factor of equilateral triangle obstacles was 2.35. Considering comprehensively mass transfer enhancement and energy consumption, the isosceles triangle obstacle showed the best performance, its maximum enhancement factor was 2.1, while the maximum pressure drop increased only 0.41 kPa (22%) compared to the microchannel without obstacles. Furthermore, a micro-particle image velocimetry (micro-PIV) was utilized to observe the flow field distribution and evolution, in order to understand and analyze the enhancement mechanism. The micro-PIV measurement indicated that the obstacle structure could induce the formation of vortex, which promotes convective mass transfer and thins the flow boundary layer, accordingly, the gas–liquid mass transfer efficiency is remarkably improved. This study can provide theoretical guidance and support for the design and optimization of microchannel with triangular obstacles.

This study focuses on the effect of chemical absorption on the formation dynamic characteristics and initial length of Taylor bubbles. The temporal evolutions of neck width and length of gaseous thread and initial length with and without chemical absorption were investigated with the Capillary number and Hatta number between 0.0010–0.0073 and 1.8–5.8 respectively. The squeezing regime with typical three stages, expansion, squeezing and pinch off is observed for both two processes. Compared with the non-absorption process, the increase of formation time in the chemical absorption process arises mainly from the expansion stage, and the decrease of initial length is from the necking stage. In addition, the temporal length evolution satisfies the power-law scale with the same exponent but a smaller pre-exponential factor. The correlations of neck width for stage transition and initial length with Hatta number demonstrate the enhancement effect of chemical absorption on bubble formation dynamics and initial length at relatively high chemical reaction rates and long formation time. This study provides insight into the bubble formation mechanism and helps to regulate the bubble initial size with chemical absorption.

The bubble formation dynamics and size manipulation in the slurry of polystyrene microspheres in the microfluidic T-junction were visually investigated by a high-speed camera. Based on the evolution law of the bubble neck width with time, the formation process of bubbles is divided into three stages: filling, squeezing and pinch-off. The particle concentration has an obvious effect on the squeezing stage, while less impact on the filling and pinch-off stages. In the squeezing stage, the evolution of the dimensionless minimum neck width of bubbles with time could be described by a power-law relationship. The increase of the particle concentration or continuous phase flow rate could lead to the increase of body flow of the continuous phase flow rate and the enhancement of the squeezing force acted on the bubble neck, correspondingly, the power-law index α in the squeezing stage enlarges. Moreover, the bubble size increases with the increase of the gas phase flow rate and the decrease of the particle concentration and continuous phase flow rate. However, the effect of the particle concentration on the bubble size weakens with the increase of the continuous phase flow rate. In addition, a new prediction correlation of the bubble size for the slurry system in a T-shape microchannel was proposed with good prediction accuracy.

Monodispersed microbubbles and microdroplets are widely used as reaction carriers in microfluidics. In this study, the generation processes of bubbles and droplets in a step-emulsification microfluidic device are compared to show the similarities and differences in the emulsification process. By changing the placement of the microdevice, the effects of buoyancy and gravity on the generation of bubbles and droplets are introduced, and the feedback mechanism of the bubble layer and the effect of droplet accumulation on the emulsification process are clarified. Finally, based on the analysis of the difference of the pinch-off of the dispersed phase between the bubble and the droplet in this configuration, the Plateau-Rayleigh instability processes for the formation of bubble and droplet are revealed by using a high-speed camera system, and the reasons for the difference of the operating ranges of the gas flow rate and liquid flow rate in the dripping flow regime are explained.

The coalescence behavior of two droplets with different viscosities in the funnel-typed expansion chamber in T-junction microchannel was investigated experimentally and compared with droplet coalescence of the same viscosity. Four types of coalescence regimes were observed: contact non-coalescence, squeeze non-coalescence, two-droplet coalescence and pinch-off coalescence. For droplet coalescence of different viscosities, the operating range of non-coalescence becomes narrowed compared to the droplet coalescence of same viscosity, and it shrinks with increasing viscosity ratio η of two droplets, indicating that the difference in the viscosity of two droplets is conducive to coalescence, especially when 1＜η＜6. Furthermore, the influences of viscosity ratio and droplet size on the film drainage time (Tdr) and critical capillary number (Cac) were studied systematically. It was found that the film drainage time declined with the increase of average droplet size, which abided by power-law relation with the size difference and viscosity ratio of the two droplets: Tdr ~ (ld)0.25±0.04 and Tdr ~ (η)−0.1±0.02. For droplet coalescence of same viscosity, the relation of critical capillary number with two-phase viscosity ratio and dimensionless droplet size is Cac = 0.48λ0.26l−2.64, while for droplet coalescence of different viscosities, the scaling of critical capillary number with dimensionless average droplet size, dimensionless droplet size difference and viscosity ratio of two droplets is Cac = 0.11η−0.07ls−2.23ld0.16.

This study focuses on the generation of bubbles in a step-emulsification microdevice via a two-angle photography method. It's found that the bubble generation mechanism is controlled by the interfacial tension, below a critical capillary number; while controlled by viscous force, inertial force and the disturbance induced by the bubble swarm, above the critical capillary number. From the two-angle photography method, a model is established for predicting the bubble size, by taking into account of the dynamic contact angle between gas-liquid interface and wall, and the hydrodynamic feedback of the cavity on bubble formation via the quantification of resistance by the volume fraction of gas in the cavity.

Rapid and robust generation of uniformly sized droplets is a research hotspot for emulsification technology. Compared with conventional microfluidic devices such as cross-junction and flow-focusing, the size of the droplets generated in the step emulsifiers based on the spontaneous transformation mechanism is independent of the flow fluctuation, making this type of devices have many advantages and important application value. To introduce the research progress of step emulsification technology in recent years, this review discusses the device configurations, interfacial evolution for droplet formation mechanism, flow pattern classification, the influence of control variables on the emulsification process and the application advantages of such devices, and illustrates the design and selection principles of such devices, the control laws of manipulation variables, and the future application fields and directions.

The densities and viscosities of maltitol in glycylglycine aqueous solutions were measured at T = 293.15, 303.15, 313.15, 323.15, and 333.15 K and at atmospheric pressure. Furthermore, the apparent...

The performance and effectiveness of the conventional and heat-integrated side-stream extractive distillation arrangements are explored with the simultaneous consideration of the steady-state economics, thermodynamics and dynamic controllability aspects of the process. The overall performance of simple stream-to-stream heat-integrated F-S-E configuration (serially preheating column inlet feeds by solvent stream) is superior to others because of the least total annual cost (TAC), CO2 emissions and highest second-law efficiency among these side-stream cases. It can further reduce 8.16% in TAC compared to the conventional extractive distillation configuration. Exergy and entropy generation analyses are employed in these heat-integrated side-stream extractive distillation arrangements to thorough evaluate the energy degeneration in each component. Furthermore, dynamic control analyses for conventional and heat-integrated side-stream extractive distillation processes are also explored. Two novel compromised tactics are proposed to make the economically optimal heat-integrated arrangement more stable at rejecting large feed disturbances according to the physical implemented issues of economizers. Both effective and robust dual-product quality controls are provided for tackling the large (20%) throughput and feed composition perturbations. Furthermore, a new developed control scheme with parallel arrangement works better than that with series exchangers’ arrangement by comprehensively weighing results of steady-state economic design and dynamic controllability.

A cross-focused microchannel with symmetrical chamber is constructed to improve coalescence efficiency of droplets. The flow behavior of droplets and time scale are investigated experimentally. Results show that the flow patterns could be divided into four regimes: long droplet, multiple coalescence, double coalescence and non-coalescence. Moreover, the evolutions of deformation value and rotation angle of droplets are studied to understand the mechanism of droplets coalescence. Asynchronous coalescence could appear in this configuration on account of the existence of stagnancy time. The dimensionless liquid drainage time declines exponentially with the dimensionless velocity of continuous phase. The present configuration allows droplets to coalesce asynchronously, which greatly expands applicable operation region and accordingly facilitates the controllability of droplets coalescence.

The feasibility and performance of the strengthened vapor recompression-assisted side-stream pressure-swing distillation configuration with preheating and/or precooling column(s) feed stream(s) to ...

The mass transfer in CO2 absorption can be intensified in the miniaturized devices. In this paper, the weak CO2 absorption in aqueous glycerol solutions in a serpentine minichannel is studied, with...

Droplet coalescence in a novel funnel-typed expansion chamber in the T-junction microchannel was visually investigated to improve and precisely control droplet coalescence. Three types of coalescen...

Formation of satellite droplets during drop formation or breakup leads to an increment of polydispersity of emulsion and runs contrary to the original intention of microfluidics. Thus, more attention is needed to shed light on the mechanism of satellite droplets formation to alleviate polydispersity. In this work, we studied the satellite droplet formation process in microfluidic flow-focusing junction and T-junction with capillary number Ca = μcuc/σ in a range of 0.039–0.32. The formation of satellite droplets during the thinning of neck filament was recorded by high speed camera. Using self-written programs, the evolution of the minimal width, the volume of and the length of neck filament were analyzed to understand the satellite formation process. It was found that before final pinch-off, liquid neck expriences four different thinning regimes. There is a critical length Lc = μcμd/σρd during the evolution of the neck thinning processes. When the neck width is less than the critical length, the thinning processes always experience the transition from rapid shrinking to slow shrinking facilitating the formation of satellite droplets.

The flow distribution and mass transfer for gas-liquid flow in two parallel microchannels with different tree-shaped distributor are studied. Two parallel microchannels, both consist of four parallel branches, operated in the slug flow and with Reynolds number in the range of 0.2-10. The operating range in the halving-width microchannel (Tree-1) is significantly narrower than the constant-width microchannel (Tree-2). As the ratio of gas-liquid flow rate increases, the flow distribution in the Tree-1 gradually deteriorates. While in the Tree-2, there exists a critical ratio of gas-liquid flow rate to attain ideal flow distribution. Meanwhile, the volumetric mass transfer coefficient in the Tree-1 is lower, while in the Tree-2, the opposite is true. However, the high mass transfer coefficient is always accompanied by the high pressure drop. The pressure drop in the Tree-2 is higher than the Tree-1. Furthermore, the performance ratio is adopted to evaluate the mass transfer performance.

The experimental values of density and viscosity for binary solutions diethylenetriamine (DETA) + H2O, DETA + N,N-diethylethanolamine (DEEA), and ternary solution DETA + DEEA + H2O over the entire concentration range were reported at atmospheric pressure and T = (293.15, 303.15, 313.15, 323.15, 333.15) K. The measured densities and viscosities were used to calculate the thermal expansion coefficient, excess molar volume, partial molar volume, apparent molar volume, partial molar volume at infinite dilution, viscosity deviation, and excess Gibbs free energy of activation for viscous flow. The molecular interactions in mixtures were discussed based on the volumetric and viscometric properties.

The gas-liquid two-phase flow pattern, absorption rate and pressure drop of CO2 absorbed into the aqueous solution of the task-specific ionic liquid (1-aminopropyl-3-methylimidazole tetrafluoroborate [Apmim][BF4] and 1- hydroxyethyl-3-methylimidazole tetrafluoroborate [OHemim][BF4]) and halide-free ionic liquid 1- butyl -3-methylimidazolium methylsulfate [Bmim][CH3SO4] were investigated in a microreactor. The absorption mechanism of the three ionic liquids was analyzed employing the 13C NMR spectroscopy. The [Apmim][BF4] was found to have the best ability of CO2 capture compared to the other two ionic liquids, as chemical absorption occurred between [Apmim][BF4] and CO2, while only physical absorption took place between [OHemim][BF4] / [Bmim][CH3SO4] and CO2. The sequence of CO2 absorption rate in three ionic liquid aqueous solutions is: [Apmim][BF4] > [Bmim][CH3SO4] >[OHemim][BF4]. Furthermore, the effects of gas-liquid flow rate and ionic liquids concentration on CO2 absorption rate and pressure drop were studied, the pressure drop models based on various flow patterns were proposed.

Droplet dynamics at double T-junction microdisperser are usually occurred in multiphase flow and complex fluids within microfluidics and microreactors. However, the mechanism of droplet dynamics in double T-junction microdisperser with the double input of the continuous phase remains unclear. In this article, the deformation and breakup of droplets in double T-junction microdisperser is experimentally studied with the double input of the continuous phase. Various flow patterns are observed for the dynamics of droplets. The droplet deformation is analyzed and discussed, and a prediction formula for the maximum droplet deformation is proposed. In addition, the breakup dynamics of droplets are studied systematically. The breakup of the droplet can be divided into squeezing stage, transition stage and pinch-off stage. The influence of the capillary number and the viscosity ratio of both phases on each stage of the droplet breakup are discussed. Finally, it is found that in the transition zone between the deformation and breakup of droplets, the tail of a droplet is often accompanied by corrugated filaments, and the filament breakage presents either tip streaming or self-similar fractures.

The densities and viscosities of binary mixtures monoethanolamine (MEA) + H2O, 2-(Diethylamino)ethanol (DEEA) + H2O, MEA + DEEA and ternary mixtures MEA + DEEA + H2O were measured over the entire composition at temperatures ranging from (293.15 to 333.15) K and atmospheric pressure. The volumetric and viscometric properties such as excess molar volume, partial molar volume, partial molar volume at infinite dilution, apparent molar volume, viscosity deviation, molar enthalpy, molar entropy and the molar Gibbs energy of activation for viscous flow of mixtures were calculated in terms of experimental data, respectively. Furthermore, excess molar volumes were fitted by the Redlich–Kister equation for binary mixture and by Cibulka equation for ternary solution. The volumetric and viscometric properties were analyzed through the molecular structure and intermolecular interactions.