Flow Rate Of Continuous Phase Research Articles

Non-Newtonian droplet breakup in a T-junction microdevice containing constriction induced asymmetric parallel branches

Here, we describe the breakup and post-breakup dynamics of a non-Newtonian droplet of xanthan gum aqueous solution in asymmetric parallel branch microdevices. Our experimental results reveal that the droplet breakup regimes, namely, obstruction, tunnel, combined, non-breakup, and parallel, are the functions of xanthan gum concentration and the continuous phase flow rate. We examined the influence of fluid properties on droplet breakup stages by varying the xanthan gum concentration in an aqueous solution that exhibited increasing shear-thinning and elastic properties with its concentration. Four sequential stages (squeezing, transition, pinch-off, and filament thinning) are identified during the droplet breakup process. We found that upstream pressure controlled the squeezing stage, and fluid properties mainly steered the filament rupture stage. A complex interaction between elastic, capillary, and inertial forces further divided the final stage into the stretching and fluid-drainage stages. The Hencky strain characterized the formation of a persistent cylindrical filament in the stretching stage that decayed exponentially in the fluid-drainage stage. Eventually, this study highlights the significance of parallel branches with asymmetric geometric confinements on droplet splitting. Enhanced asymmetry is observed for the elongated filament, emphasizing the dominance of feedback from the downstream confinement.

Droplet separation and sieving mechanism by grooved microchannel

A novel droplet sieving method utilizing grooved microchannel is proposed to achieve the monodispersity of droplets. The sieving performance and mechanism are investigated experimentally. Three different sieving flow regimes are observed in the experiment: incomplete sieving, complete sieving and main droplet rupture. The effects of different operating conditions on sieving efficiency are studied systematically. The results showed that when the total flow rate of the continuous phase in the main channel is lower, the small droplets would accumulate between the main droplets and could not be separated. Complete sieving could be achieved when the main body of the small droplet enters the sieving layer before arriving at the sieving channel. The larger continuous phase flow rate in sieving channel could create thicker sieving layer, which is conducive for small droplets to entering the sieving layer to achieve complete sieving. On the contrary, the higher continuous phase flow rate in the main channel would lead to thinner sieving layer, and the required flow rate in sieving channel would be higher to achieve complete sieving. Under the same sieving channel flow rate, the longer droplet is more apt to rupture. The sieving mechanism of droplets is proposed according to the force analysis in the channel. Whether the main droplet ruptures at the sieving junction depends on the combined action of the suction of the sieving channel, the interfacial tension, and the supporting force of the sieving groove.

Insights into the dynamics of non-Newtonian droplet formation in a T-junction microchannel

The non-Newtonian shear-thinning droplet formation mechanism in a T-junction microchannel is experimentally investigated using the aqueous solutions of xanthan gum as the dispersed phase and mineral oil as the continuous phase. Influences of both phase flow rates and polymer concentration on flow regime transition are explored. It is observed that the initial vertical expansion stage is present only for the Newtonian and lower shear-thinning systems. The droplet evolution rate shows the influence of continuous phase flow rate and shear-thinning properties on the dynamics of necking stages, viz., squeezing, transition, pinch-off, and filament thinning. Analysis of Ohnesorge number (Oh) reveals that inertial force dominates in the squeezing stage, whereas viscous and interfacial force control in the filament thinning stage. Longer and stable filament generation is detected as a discerning feature for non-Newtonian systems that appears more prominent with increasing dispersed phase shear-thinning properties. The results also indicate an inverse relation of droplet length with the continuous phase flow rate and xanthan gum concentration, while the droplet formation frequency and its polydispersity vary directly with those parameters.

미세액적 제조를 위한 다층 액적 분할 미세유체 시스템 개발

In this study, we present the multilayered symmetrical droplet splitting microfluidic system for preparation of microspheres. The microfluidic device was fabricated by conventional photolithography and PDMS casting. Multiple layers of microfluidic channels for symmetrical droplet splitting were stacked and integrated into a device. Each layer was designed to obtain 16 microdroplets from one droplet by droplet splitting. The droplet size was controlled with flow rate of dispersed phase (DI-water) and continuous phase (Mineral Oil with 3 wt.% SPAN80) by using a syringe pump. The droplet splitting behavior and production rate were analyzed by high-speed camera and inverted microscope in one layer of the microfluidic device. Additionally, the droplet size and size distribution were observed in each layer of the microfluidic device. The droplet size could be controlled by flow control of two phase flows with high uniformity of droplet size less than 5% coefficient of variation.

Formation dynamics and size prediction of bubbles for slurry system in T-shape microchannel

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 of the bubble neck 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 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.

Dynamic interfacial tension and adsorption kinetics of nonionic surfactants during microfluidic droplet formation process

Surfactant adsorption plays an important role in the microfluidic droplet formation process, which manifested as dynamic interfacial tension phenomenon. Due to rapid droplet expansion and convection flow around the droplet, traditional methods of interfacial tension measurement are difficult to investigate the surfactant adsorption kinetics in microfluidic process. We thus developed a differential pressure based microfluidic platform and proposed an in-situ method to determine time-evolving interfacial tension during microfluidic droplet formation according to Young-Laplace equation in this research. Four classical nonionic surfactants: Triton X-100, Tween 20, Brij 56, and Brij 58 were tested with classical n-octant/water as the working system. The concentrations of these surfactants at interface were further determined from the measured interfacial tension based on Frumkin adsorption model. The adsorption rates of surfactant showed that increasing surfactant concentration in the bulk solution and flow rate of continuous phase both promoted surfactant adsorption in the cases of Tween 20, Brij 56, and Brij 58, indicating the mass transfer-controlled mechanism of adsorption kinetics. The average thickness of mass transfer boundary layer ranged from 0.1 to 5 μm was determined from the mass transfer rate and a correlation equation was proposed to predict the thickness. However, the adsorption mechanism of Triton X-100 turned to be kinetics-controlled for its high critical micelle concentration, which helped to determine the adsorption and desorption rate constants.

Single Microdroplet Breakup-Assisted Viscosity Measurement.

Recently, with the development of biomedical fields, the viscosity of prepolymer fluids, such as hydrogels, has played an important role in determining the mechanical properties of the extracellular matrix (ECM) or being closely related to cell viability in ECM. The technology for measuring viscosity is also developing. Here, we describe a method that can measure the viscosity of a fluid with trace amounts of prepolymers based on a simple flow-focused microdroplet generator. We also propose an equation that could predict the viscosity of a fluid. The viscosity of the prepolymer was predicted by measuring and calculating various lengths of the disperse phase at the cross junction of two continuous-phase channels and one disperse-phase channel. Bioprepolymer alginates and gelatin methacryloyl (GelMA) were used to measure the viscosity at different concentrations in a microdroplet generator. The break-up length of the dispersed phase at the cross junction of the channel gradually increased with increasing flow rate and viscosity. Additional viscosity analysis was performed to validate the standard viscosity calculation formula depending on the measured length. The viscosity formula derived based on the length of the alginate prepolymer was applied to GelMA. At a continuous phase flow rate of 400 uL/h, the empirical formula of alginate showed an error within about 2%, which was shown to predict the viscosity very well in the viscometer. Results of this study are expected to be very useful for hydrogel tuning in biomedical and tissue regeneration fields by providing a technology that can measure the dynamic viscosity of various prepolymers in a microchannel with small amounts of sample.

Interfaces coupling deformation mechanisms of liquid–liquid–liquid three-phase flow in a confined microchannel

Interfaces coupling deformation mechanisms of liquid–liquid–liquid three-phase flow in a confined microchannel are ascertained by both experimental investigation and numerical simulation. The deformation behaviors of inner and outer interfaces of compound droplets mainly depend on the flow rate of continuous phase (Q) and the diameter ratio of core droplet to shell droplet (D*). Compared with that of single droplets, the outer interface deformation degree of compound droplets is always limited at large Q and D* because of the hindering effect of inner interface. However, the hindering effect can be weakened by decreasing the D*, and thus the outer interface deformations for both single and compound droplets remain almost consistent. Additionally, the interfaces coupling deformation mechanisms are also clarified, and the results show that due to the existence of outer interface, the differential pressure force and shearing force acting on the inner interface decrease, resulting in a low deformation coefficient of core droplet. The influence of inner interface on outer interface deformation is only reflected by the disturbance to the flow field structures. To achieve precise control over the outer interface deformation, a prediction correlation for aspect ratio with several dimensionless numbers is established with good precision. The results in this study provide valuable guidance for practical applications of liquid–liquid–liquid three-phase flow microsystems.

Continuous rotary membrane emulsification for the production of sustainable Pickering emulsions

A continuous rotary membrane emulsification (cRME) system, allowing the decoupling of droplet generation from continuous phase (CP) flowrate, is presented here for the first time. The decoupling results in higher productivity and greater control compared to traditional crossflow and rotational membrane emulsification processes. A design of experiment (DoE) investigated the influence on droplet formation of CP flow, membrane rotational speed and emulsion composition, using a Pickering emulsion consisting of 1 wt% keratin solution and varying concentrations of oxidised cellulose nanofibrils. Experiments showed that CP flowrate had a negligible effect on droplet diameters in a wide range (between 78 and 241 µm), with uniformity index as low as 0.14 for optimal membrane rotational speeds and different oxidised cellulose nanofibrils (OCNF) concentration.cRME has the potential to overcome low emulsion concentration limitations of continuous membrane emulsification systems, paving the way to significantly increase the productivity and application in personal care, food and drugs industries.

Modeling and optimization of the thermal-hydraulic performance of direct contact heat exchanger using quasi-opposite Jaya algorithm

In this study, the thermal-hydraulic performance of a direct contact heat exchanger (DCHE) was explored and optimized by applying the multi-objective quasi-opposite Jaya (MOQO-Jaya) algorithm. Inlet temperature of the continuous phase, continuous phase flow rate, dispersed phase flow rate, evaporator height, nozzle diameter, and number of jets were considered as the design variables for the optimization. The optimization functions were the volumetric heat transfer coefficient (VHTC), entransy dissipation, and entropy production. Study results indicated that the entransy dissipation and entropy production were negatively correlated with the heat transfer capacity in a DCHE, with a Pearson correlation coefficient above 0.96; that is, the smaller the entransy dissipation and entropy production, the better the heat transfer performance in the DCHE. The sensitivity of design parameters demonstrated that temperature played an important role. The optimization results and performance of the MOQO-Jaya algorithm were compared with those of the non-dominated sorting genetic algorithm (NSGA-II). The convergence of MOQO-Jaya was 2.24 times quicker than that of NSGA-II in DCHE optimization. The correctness of numerical model for optimizing the DCHE was verified through experiments. The errors of the experimental and algorithm optimizations both were within ±10%. Results demonstrated that after multi-objective optimization, the VHTC increased by 33.89% and entransy dissipation reduced by 44%, compared with those of traditional experiments.

Effect of Inlet Contactors of Splitting Distributors for Parallel Microchannels

The upstream effect of inlet contactors (T, Y, Cross-T, and Cross-Y) with splitting distributors for parallel microchannels on gas–liquid flow uniformity were studied. Splitting distributors were designed to have a width reduction by a factor of √2 between successive blocks to ensure a flow uniformity in terms of relative lengths of bubbles/slugs (LBubble/Slug/WChannel). Hydrodynamic parameters such as formation dynamics, splitting dynamics, relative lengths, and dimensionless volumes of bubbles/slugs were investigated for different inlet contactors. For Cross-T and Cross-Y inlet contactors, symmetrical splitting of bubbles/slugs and flow uniformity were observed for a wider range of continuous phase flow rates (QWater+SDS) at a fixed dispersed phase flow rate (QAir) than for T and Y inlet contactors channels. Nonsplitting of bubbles/slugs was observed for CaCritical > 0.0035 and 0.0038 for T and Y inlet contactors, respectively. However, the dimensionless volumes of bubbles/slugs in all blocks of Cross-T and Cross-Y channels were found to be constant for QWater+SDS > 5.32 mL/min. The slug flow regime was observed in all inlet contactors, whereas a bubbly flow regime was observed only in the Cross-Y contactor channel. On the basis of the experimental results, a unique correlation was proposed to predict the relative lengths of bubbles/slugs in parallel microchannels with a mean relative deviation of 14.65%.

Investigation of viscous effects on droplet generation in a co-flowing step emulsification device

In the previous articles, we proposed a co-flowing step emulsification method to generate microdroplets. Various influencing factors, including interfacial tension, continuous and dispersed phase flow rates, and critical device dimensions, were analysed, however, without considering the effects of viscosities. Here, the volume-of-fluid (VOF) based CFD simulation method is used to investigate the effects of viscosities on the co-flowing step emulsification in this paper. The continuous and dispersed phase viscosities researched vary from 0.05 Pa s to 2.0 Pa s and 0.001 Pa s to 30 Pa s, respectively. The resulting droplet diameter ranges from 35.6 µm to 522.6 µm. It is found that the continuous phase viscosity has a significant effect on the droplet diameter, frequency and generation regime. As the continuous phase viscosity increases, the droplet diameter decreases, and the droplet generation regime changes from dripping to tip-streaming gradually. Whereas the dispersed phase viscosity increase nearly does not cause droplet diameter change, besides satellite droplets generation. The results are corresponding to the droplet diameter prediction model we proposed. This work effectively provides fundamental insights into microfluidic droplet generation characteristics in co-flowing step emulsification.

Experimental investigation of dynamic mass transfer during droplet formation using micro-LIF in a coaxial microchannel

The mass transfer of Rhodamine 6G from the droplet to the continuous phase in a coaxial micro-channel is studied using micro-LIF (Laser-Induced Fluorescence). The mass distribution inside droplet is measured and visualized. The experimental results affirm that there exists the internal circulation inside the droplet and it could enhance the convective mass transfer. The stagnant center of vortices is also observed. The extraction fraction could reach 40%−80%. In order to establish the mass transfer model, different flow rates of the dispersed and continuous phase are adopted. The high continuous phase flow rate and low dispersed phase flow rate are both beneficial to enhance mass transfer by expediting the internal circulation. A modified mass transfer model is found to calculate the extraction fraction. A good agreement between the model and experiment in various conditions demonstrates that the mass transfer model in this work is reliable and feasible.

Microfluidic preparation of double emulsions using a high aspect ratio double co-flow device

An innovative microfluidic device for one-step production of W/O/W and O/W/O double emulsions is introduced. The proposed geometry inspired by a combination of single and two-step methods consists of two same-level co-flow configurations with a high aspect ratio of height to the channel's width. The designed geometry offers a mechanism to generate double-emulsion drops without requiring regular partial surface modification methods. It offers a simple, robust, and permanent method to fabricate compound droplets in a profoundly controlled manner. Due to the high aspect ratio of the designed geometry, the inner phase jet stream remains enclosed to the middle phase jet, which prevents it from encountering microchannel walls. Hence, the wettability of the channels' surface does not influence the core droplet, implying the double emulsion formation can be achieved without requiring partial wettability treatment. The effects of continuous phase flow rate on the double emulsion size and formation frequency are experimentally investigated. The proposed microfluidic device is able to fabricate relatively high monodispersed and high throughput double emulsions successfully, i.e., the generation frequency can be reached up to 350 Hz while the monodispersity remains below 6%.

Determination of mass transfer coefficient in an L-shaped pulsed column with sieve-plate structure: Application of best-fit technique, drop size distribution, and forward mixing model

The mass transfer coefficient in extraction columns is extremely affected by the size of the droplets. Among the available models to describe the extraction columns, only the forward mixing model (FMM) has considered the effect of droplets size on the column performance. In this research, the performance of an L-shaped pulsed sieve-plate column (LPSPC) was studied using the FMM. A real-code using the best-fit technique and PSO algorithm was developed for finding the optimal values of the axial dispersion and overall mass transfer coefficients. The effect of the operating parameters including pulsing velocity, continuous and dispersed phase flow rate on the volumetric overall mass transfer coefficient, the axial dispersion coefficients, and the mass transfer driving force was studied for two standard chemical systems. It was found that the LPSPC has an optimal operation at the pulsing velocity of 0.95 cm/s in each of the horizontal and vertical sections. Moreover, the performance of the column is strongly dependent on the ratio of the inlet velocity of the phases. It is worth noting that, the separation efficiency in the vertical section is slightly (on average 5%) higher than in the horizontal section of the column.

Dynamics of droplet formation and mechanisms of satellite droplet formation in T-junction microchannel

The dynamics of droplet formation in T-junction microchannel was visually investigated. Three different droplet formation flow patterns were observed: single satellite droplet, no satellite droplet, multiple satellite droplets. The formation process of droplets could be divided into four sequential stages: squeezing stage, transition stage, pinch-off stage, and filament thinning stage. In the squeezing stage and pinch-off stage, the minimum neck width of the droplet varies linearly with time, and the neck thinning rate is independent of continuous phase capillary number. In the transition stage, the minimum neck width of the droplet follows power-law relation with time, and the thinning rate of the droplet neck increases with continuous phase capillary number. In the filament rupture stage, the rupture mode of the filament varies with the increase of continuous phase flow rate. The different satellite droplet formation flow patterns and mechanisms were studied and elucidated.

Numerical CFD investigation of liquid-liquid two-phase flow separation in a microseparator

ABSTRACT Two-phase flow liquid-liquid separation in a microseparator has promising use in microfluidic field such as downstream biodiesel purification. The present study aims to investigate the effect of daughter channel (DC) geometry on liquid-liquid separation in a new design of microseparator. Olive oil and water were selected as continuous and dispersed phase, respectively. Different DC intervals (500–1000 µm) and angles (60°-120°) were designed to generate nine single-sided DC microseparator configurations. Further, these were arranged in mirrored configurations to generate nine double-sided DC microseparator configurations. At continuous phase flow rate (QCP) of 0.5 ml/h, interval of 500 µm and angle of 90° achieved best performance with highest oil purity of 100% in secondary channel outlet (SC_Outlet) and lowest oil purity of 26% in main channel outlet (MC_Outlet). The results revealed that the design of DC with lower pressure drop led to better separation performance. Furthermore, the occurrence of Jamin effect in the DC could cause hindrance in the two-phase flow liquid-liquid separation. The use of double-sided DC microseparator configuration further improves the separation performance. Overall, an excellent separation of oil was successfully achieved.

Experimental investigation on dispersed phase holdup in an L-shape pulsed packed extraction column

In this study, the dispersed phase hold-up has been investigated in a pilot plant of a new type of extraction column entitled “L-shaped pulsed packed extraction column” by using two liquid systems of toluene/acetone/water and n-butyl/acetone/water in the presence of mass transfer in two directions. The effects of operational variables, physical properties and mass transfer direction have been considered on the dispersed phase hold-up. It is found that the dispersed phase hold-up is significantly affected by the pulsation intensity, dispersed phase flow rate and interfacial tension of liquid system. However, the continuous phase flow rate and mass transfer direction have a weaker impact. Furthermore, it is observed that the behavior of hold-up in the horizontal and vertical sections of the used column is distinctive in some case. Such that any factor which reduces the dispersed drop size, leads to an increase in hold-up in the vertical section of the column and a decrease in hold-up in the horizontal section of the column. Finally, new correlation is proposed to accurately predict the dispersed phase hold-up. Good agreement between predictions and experiments was found for all operating conditions, chemical systems and mass transfer direction that were investigated.

Mapping Bubble Formation and Coalescence in a Tubular Cross-Flow Membrane Foaming System.

Membrane foaming is a promising alternative to conventional foaming methods to produce uniform bubbles. In this study, we provide a fundamental study of a cross-flow membrane foaming (CFMF) system to understand and control bubble formation for various process conditions and fluid properties. Observations with high spatial and temporal resolution allowed us to study bubble formation and bubble coalescence processes simultaneously. Bubble formation time and the snap-off bubble size () were primarily controlled by the continuous phase flow rate (); they decreased as increased, from 1.64 to 0.13 ms and from 125 to 49 µm. Coalescence resulted in an increase in bubble size (), which can be strongly reduced by increasing either continuous phase viscosity or protein concentration—factors that only slightly influence . Particularly, in a 2.5 wt % whey protein system, coalescence could be suppressed with a coefficient of variation below 20%. The stabilizing effect is ascribed to the convective transport of proteins and the intersection of timescales (i.e., μs to ms) of bubble formation and protein adsorption. Our study provides insights into the membrane foaming process at relevant (micro-) length and time scales and paves the way for its further development and application.

A comprehensive study of droplet formation in a capillary embedded step T-junction: From squeezing to jetting

The understanding of microdroplet formation characteristics in unconfined T-junction microchannels is relatively weak. Accordingly, an unconfined step T-junction microdevice was specially designed and fabricated to systematically investigate the droplet formation mechanism. A new flow named pseudo-squeezing flow is defined. A liquid–liquid flow pattern map including the pseudo-squeezing, dripping, and jetting flow in the unconfined T-junction is plotted, which shows that a much higher capillary number is needed to realize the flow regime transformation when compared with the confined T-junction. The unconfined T-junction could provide a much wider dripping flow regime, and generate monodispersed smaller droplets. Besides, the droplet diameter decreases with the continuous phase flow rate in the pseudo-squeezing and dripping flow but suddenly increases when the flow regime transfers from dripping to jetting. Importantly, the very short liquid plug can be easily formed in this unconfined T-junction. Finally, because of the same formation mechanism, the droplet size in pseudo-squeezing and dripping flow can be perfectly scaled with a same dimensionless model. This work could be the first comprehensive study of the droplet formation in three flow regimes in the unconfined T-junction.