Droplets In Microchannels Research Articles

On-Demand Coalescence of Ferromagnetic Droplets in Microchannels Using an Oscillating Magnetic Field.

Droplet-based microfluidics exhibit remarkable potential in achieving high-throughput chemical reactions with minimal reagent consumption. However, a pivotal challenge lies in the selective coalescence of droplets for precise process control, particularly when dealing with droplets of varying amounts and volumes, which are difficult to trap and coalesce due to their tiny dimensions and incessant movement. Hence, we proposed a method for on-demand coalescence of ferromagnetic droplets using an oscillating magnetic field. Experimental results show that the ferromagnetic droplets can be trapped in different positions in the microchannels according to the applied magnetic field intensity. A high-intensity pulsed amplitude of the magnetic field enables the migration of trapped droplets toward the same position, facilitating their mutual contact and interaction. By programmable modulation of the oscillating magnetic field, a controllable reciprocation of droplets in microchannels was successfully realized, which enabled us to dynamically capture, coalesce, and release two or more (≥3) droplets on demand. The integrated ferromagnetic droplet-based microfluidic platform allows contact-free, easily monitored, and on-demand coalescence of ferromagnetic droplets in microchannels, which holds promise for a wide range of applications, such as microfluidic-based drug synthesis, biosensing, reaction kinetics, and paracrine signaling, particularly.

Arrangement and feedback effects of droplet swarms in a parallel microchannel device

Droplet swarms are the dominant form of highly dispersed microdroplets in cavities. During the self-assembly of droplet swarms, the continuous phase is inclined to flow along the path of the minimum resistance, while the arrangement of the droplet swarms seeks to minimize the potential energy, and the two mechanisms compromise in competition. In this paper, the ability of droplet swarms to timely adjust the morphology is measured by the ratio of the average flow rate of the two-phase flow to that of the droplet swarms, thereby clarifying the dominant mechanism of the arrangement of droplet swarms. The arrangement of droplet swarms dominated by different mechanisms and their distribution are introduced, and the prediction method for the arrangement of droplet swarms is proposed. The mechanism underlying the breakup of the microdroplets in the cavity is elucidated, and three modes of the breakup in the cavity are introduced. Furthermore, a resistance model for the microdevice is established to quantify the fluctuations of pressure difference and flow rate resulting from the formation of droplet swarms. The feedback effects of droplet swarms on the uniformity of droplet formation and flow patterns are analyzed, revealing the ideal flow range for the formation of highly dispersed droplets in microchannels. This paper elucidates the arrangement and the feedback effects of droplet swarms, which will guide the application of microdevices in reaction and mass transfer processes.

The Numerical Simulation of Marangoni Problems Employing Multi-phase Parallel SPH Method

When facing microscale problems, the phenomenon of droplet movement caused by temperature gradients is usually attributed to the Marangoni effect. This study first constructed a multiphase smoothed particle fluid dynamics (SPH) system structure, integrating continuous surface force (CSF) model and multiphase model to explore the behavior of droplets under Marangoni effect. To further improve computational efficiency, this study also utilized the Open Multi Processing (OpenMP) parallel computing framework to perform parallel optimization on the SPH algorithm. Subsequently, the effectiveness of the constructed SPH framework in addressing thermal capillary phenomena was verified by simulating the Marangoni migration phenomenon of silicone oil droplets in fluorine solutions. In addition, this study also investigated the polymerization process of two droplets in microchannels under the Marangoni effect, and obtained the complete dynamic changes of droplets from motion to polymerization. During this period, we conducted a detailed analysis of the variation process of the velocity vector field, the spatial distribution of the temperature field, and the evolution characteristics of the surface tension gradient during the droplet movement process, thereby comprehensively revealing how the Marangoni effect drives the phenomenon of droplet movement and aggregation.

Electrochemical coalescence of oil-in-water droplets in microchannels of TiO2-x/Ti anode via polarization eliminating electrostatic repulsion and ·OH oxidation destroying oil-water interface film

Electrochemistry is a sustainable technology for oil-water separation. In the common flat electrode scheme, due to a few centimeters away from the anode, oil droplets have to undergo electromigration to and electrical neutralization at the anodic surface before they coalesce into large oil droplets and rise to water surface, resulting in slow demulsification and easy anode fouling. Herein, a novel strategy is proposed on basis of a TiO2-x/Ti anode with microchannels to overcome these problems. When oil droplets with several microns in diameter flow through channels with tens of microns in diameter, the electromigration distance is shortened by three orders of magnitude, electrical neutralization is replaced by polarization coupling ·OH oxidation. The new strategy was supported by experimental results and theoretical analysis. Taking the suspension containing emulsified oil as targets, COD value dropped from initial 500 mg/L to 117 mg/L after flowing through anodic microchannels in only 58 s of running time, and the COD removal was 21 times higher than that for a plate anode. At similar COD removal, the residence time was 48 times shorter than that of reported flat electrodes. Coalescences of oil droplets in microchannels were observed by a confocal laser scanning microscopy. This new strategy opens a door for using microchannel electrodes to accelerate electrochemical coalescence of oil-in-water droplets.

Polymeric droplet formation and flow pattern evolution in capillary microchannels: Effect of fluid elasticity

Polymeric droplets are widely employed in fields such as chemical, biomedical, and materials engineering. However, the study of polymeric droplet formation is still insufficient due to the complex elasticity. In this work, the effect of fluid elasticity on the flow patterns for polymeric droplet formation in cross-junction microchannels is investigated by means of finite-volume direct numerical simulations. The volume of fluid method with cell-based adaptive mesh refinement technique is used to capture the interface. Additionally, the rheological behavior of polymeric fluids is described using the exponential Phan-Thien–Tanner constitutive model. The simulated flow behaviors are highly consistent with the experimental observations. The results indicate that three typical flow patterns of dripping, jetting, and threading flows are obtained at different fluid elasticities (denoted by the Weissenberg number Wi) and viscosities (denoted by the Capillary number Ca). Meanwhile, the elastic effect is found to be greater in the dripping flow, significantly reducing the axial tensile stress. It is demonstrated that changes in the stretched state of polymer macromolecules with the same Wi at different Ca lead to variations in the strength of elastic action, which, in turn, affects the extension length and the pinch-off time of droplets. Finally, a relationship equation between the extension length and time of the polymer fluid is established. This present study aims to provide important insight into the preparation of polymeric droplets in microchannels.

Flexible Toolbox of High-Precision Microfluidic Modules for Versatile Droplet-Based Applications.

Although the enormous potential of droplet-based microfluidics has been successfully demonstrated in the past two decades for medical, pharmaceutical, and academic applications, its inherent potential has not been fully exploited until now. Nevertheless, the cultivation of biological cells and 3D cell structures like spheroids and organoids, located in serially arranged droplets in micro-channels, has a range of benefits compared to established cultivation techniques based on, e.g., microplates and microchips. To exploit the enormous potential of the droplet-based cell cultivation technique, a number of basic functions have to be fulfilled. In this paper, we describe microfluidic modules to realize the following basic functions with high precision: (i) droplet generation, (ii) mixing of cell suspensions and cell culture media in the droplets, (iii) droplet content detection, and (iv) active fluid injection into serially arranged droplets. The robustness of the functionality of the Two-Fluid Probe is further investigated regarding its droplet generation using different flow rates. Advantages and disadvantages in comparison to chip-based solutions are discussed. New chip-based modules like the gradient, the piezo valve-based conditioning, the analysis, and the microscopy module are characterized in detail and their high-precision functionalities are demonstrated. These microfluidic modules are micro-machined, and as the surfaces of their micro-channels are plasma-treated, we are able to perform cell cultivation experiments using any kind of cell culture media, but without needing to use surfactants. This is even more considerable when droplets are used to investigate cell cultures like stem cells or cancer cells as cell suspensions, as 3D cell structures, or as tissue fragments over days or even weeks for versatile applications.

Measuring arrangement and size distributions of flowing droplets in microchannels through deep learning using DropTrack

In microfluidic systems, droplets undergo intricate deformations as they traverse flow-focusing junctions, posing a challenging task for accurate measurement, especially during short transit times. This study investigates the physical behavior of droplets within dense emulsions in diverse microchannel geometries, specifically focusing on the impact of varying opening angles within the primary channel and injection rates of fluid components. Employing a sophisticated droplet tracking tool based on deep-learning techniques, we analyze multiple frames from flow-focusing experiments to quantitatively characterize droplet deformation in terms of ratio between maximum width and height and propensity to form liquid with hexagonal spatial arrangement. Our findings reveal the existence of an optimal opening angle where shape deformations are minimal and hexagonal arrangement is maximal. Variations of fluid injection rates are also found to affect size and packing fraction of the emulsion in the exit channel. This paper offers insight into deformations, size, and structure of fluid emulsions relative to microchannel geometry and other flow-related parameters captured through machine learning, with potential implications for the design of microchips utilized in cellular transport and tissue engineering applications.

Droplet formation of yield stress fluids in asymmetric parallelized microchannels

The microemulsion method is limited due to low productivity. Using Carbopol gel (0.05 %, 0.13 %, 0.2 %) and mineral oil as dispersed phase and continuous phase respectively (3 ≤ Qd ≤ 8, 2 ≤ Qc ≤ 350), the numbering-up of YSFs droplets in asymmetric parallel microchannels are experimentally studied. Ca*(10-3-10) and Cac (10-4-10-2) are introduced to divide the two-dual droplet flow pattern which is conducive to production. It is found that yield stress mainly acts on the necking stage of droplet formation, so the continuous phase flow can be increased to shorten the necking time and improve the size uniformity. The influence of rheology on droplet morphology makes the existing multiphase flow pressure drop model no longer applicable. Therefore, a correction pressure drop model of Carbopol slugs and prediction models of droplet size and generation frequency are proposed. This study provides a reference for the high-throughput production of YSFs droplets in microchannels.

Flow and Heat Transfer in Two-Phase Flow Immiscible Droplets in Microchannels

ABSTRACT The enhancement of heat transfer in microchannels without phase change is a significant area of study, primarily driven by the internal fluid recirculation in two-phase flows. This investigation focuses on a circular microchannel, 100 μm in diameter, where mineral oil droplets are introduced into a water flow. The study utilizes the conservative level set method for precise interface tracking and liquid film thickness measurement. This research introduces a modified Nusselt number, specifically tailored to describe the heat transfer characteristics of multiphase flows. The study delves into the effects of varying droplet sizes, from small spheres to a slug. The findings indicate that the most significant heat transfer enhancement occurs with droplets whose volume closely matches that of a sphere fitting within the channel. Moreover, the investigation explores the impact of parameters like inlet velocity, primary-phase slug length, and contact angle. Notably, higher inlet velocities lead to improved heat transfer, resulting in a substantial increase in the Nusselt number compared to single-phase flows. The study underscores the delicate balance between recirculation intensity and droplet heat capacity concerning slug length, as excessive variations can harm thermal performance. It also highlights the pivotal role of surface wettability, showing improved thermal performance on hydrophobic surfaces.

Liquid–Liquid Two-Phase Flow and Size Prediction of Slug Droplets in Microchannels

The liquid–liquid two-phase flow and size prediction of slug droplets in flow-focusing microchannels with different downstream orifice sizes were investigated experimentally. Aqueous solution of 50%-glycerol and mineral oil with 4 wt.% surfactant sorbitanlauric acid ester (Span 20) were used as the dispersed and continuous phases, respectively. Three characteristic flow patterns were identified: slug flow, dripping flow, and jetting flow. The slug flow region decreased but the jetting flow region increased with the decrease in the size of the channel orifice. Afterwards, the universal flow pattern maps of the liquid–liquid two-phase in three microchannels were obtained based on dimensionless analysis. Furthermore, two slug droplet formation regions were found: when φ−1Cac < 0.01, the droplet formation was mainly driven by the squeezing force Fp, while when φ−1Cac > 0.01, both the squeezing force Fp and shear force Fτ contributed to droplet formation. Additionally, the prediction correlations of the dimensionless sizes of the slug droplets in both regions were established based on the flow rate ratio of the two-phase, the dimensionless orifice size, and the Capillary number of the continuous phase. The predicted results are in good agreement with the experimental values.

Thermocapillary flow‐induced core release from double‐emulsion droplets in microchannels

AbstractThe use of double emulsions (DEs), which represent colloidal structures composed of droplets nested within droplets, can provide for unparallel droplet manipulation in droplet‐based microfluidic technology due to their unique core–shell structures. The controlled release of cores in DEs is of particular interest. However, this process remains poorly explored. In this work, the thermocapillary flow induced by a temperature gradient is used as a driving force to control the core release and the impacts of different linear temperature gradients, core diameters, shell diameter, and core/shell diameter ratios on the thermocapillary flow and core release characteristics of DE droplets consisting of a water‐in‐n‐hexadecane‐in‐water system within a cylindrical microchannel are investigated. Most of the core and shell diameter conditions considered result in a double‐core release process, where the inner droplet volume is partially ejected before the remaining core is rewrapped by the outer droplet, and the remaining inner droplet volume is ejected later during a second core release event. However, relatively small core diameters of 50 and 75 μm produce conditions where the full inner droplet volume is ejected during a single‐core release process. In addition, we provide empirical relationships for accurately determining the time at which core release initially occurs under given DE parameters as well as for precisely determining whether the applied conditions will lead to single‐ or double‐core release processes. Therefore, the results of this study provide insights enabling the development of accurate inner droplet release technologies under thermocapillary migration.

A novel approach to determining the hydrodynamic resistance of droplets in microchannels using active control and grey-box system identification

Inaccurate prediction of droplet hydrodynamic resistance has a profound impact on droplet chip performance and lengthens the iterative design process. Previous studies measuring droplet resistance use various approaches such as interface comparison to quantify flow rate, and pressure taps; all these methods are classified as passive. Although each study supports well their own findings, the wide variety of conditions such as channel geometry and use of surfactant in combination with the difficulty in quantifying the droplet resistance leads to poor consensus across the different studies. Overall guidelines would be broadly beneficial to the community, but are currently fairly crude, with a rule of thumb of 2 to 5 times resistance increase. The active droplet control platform previously developed enables a novel approach that is herein confirmed as promising. This proof-of-concept study focuses on verifying this approach that employs a system identification method to determine the hydrodynamic resistance of a channel containing a single droplet, from which the droplet resistance is retrieved. This method has the potential to be further applied to a large variety of conditions, and most importantly, to non-Newtonian fluids once key limitations are overcome to improve measurement resolution. The current results qualitatively agree with the literature and demonstrate the promising future for this novel active approach to quantifying droplet resistance.

One‐step fabrication of moon‐shaped microrobots through in situ solidification of magnetic Janus droplets in microchannels

AbstractSpecial‐shaped microscale structures have shed light on new possibilities in key fields of chemistry, medicine, and energy. Asymmetrical microrobots with sensitive magnetic responses can be useful tools in controlled chemical reactions, drug delivery, and functional material synthesis. Microfluidic‐based emulsion generation technology is adopted as a powerful platform for the fabrication of steerable microrobots with refined control. Specifically, Janus droplets are generated in microfluidic chips featuring a flow‐focusing configuration. Asymmetrical morphologies of the Janus droplets are achieved by balancing the interfacial tensions, where the portion containing magnetic nanoparticles is solidified through the UV‐initiated polymerization process right after the formation while the Janus structure is left intact. We succeed in controlling the morphology of the Janus droplet along with the moon‐shaped robots hydrodynamically and applying them in flow control at the microscale under external magnetic fields, which are characterized and quantified by three‐dimensional profile measurement and high‐speed microparticle velocimetry measurement. Our proposed on‐chip fabrication method using a microfluidic platform not only provides a method for fabricating magnetic robots but also enables tuning the complex morphologies and functionalities at the microscale, which could shed light on new possibilities in key fields of controlled chemistry reaction, medicine synthesis, and energy generation.

Dynamics of Nonmagnetic and Magnetic Emulsions in Microchannels of Various Materials

The formation of droplets in microchannels (droplet microfluidics) has a large number of applications, such as in micro-dosing and gas meters. This paper considers the dynamics of direct and inverse emulsions based on water, polydimethylsiloxane, and synthetic and mineral oil in microfluidic chips based on two technologies: glass–parafilm–glass sandwich structures and removable scaffold in a silicone compound. It is shown that wettability, roughness and chip wall material; channel thickness; magnetic fluid flow rate; and magnetic field strength affect the size of emulsion droplets formed in a microfluidic chip. The addition of another mechanism for regulating the hydrodynamics of emulsions using a magnetic field opens up new possibilities for the development of promising devices.

Lattice Boltzmann Study on the Motion of Dual Droplets in Microchannels With Contact Angle Hysteresis

The contact angle hysteresis is defined as the difference between the advancing and receding contact angles, which is an important phenomenon in the two-phase flow on the wetting surface. In this paper, an improved pseudo-potential lattice Boltzmann (LB) Multiphase flow model, combined with geometric wetting boundary conditions, is employed to study the motion behavior of two droplets in micro-channel with different contact angle hysteresis. We have mainly studied the effects of capillary number, wettability, the width of contact angle hysteresis window, the initial distance between two droplets and the relative size of droplets on the dynamic behavior of two droplets in the micro-channel. The research results show that the increase of capillary number is conducive to the movement of droplets, but not conducive to the discharge of droplets from the pipeline, and the influence of the capillary number on the upstream droplet is greater than that on the downstream droplet. On the other hand, the larger the contact angle hysteresis window, the slower the droplet motion and deformation, but the more obvious about the deformation, and the two droplets merge earlier but discharge the pipeline later; In addition, with the increase of the initial distance between the two droplets, the merging-fracture mode of the two droplets changes from first merging and then fracture to in-process fracture and first fracture and then merging. Correspondingly, when the initial distance between the two droplets is small, the time for the two droplets to completely discharge the channel decreases slowly with the increase of the initial distance, while when the initial distance between the two droplets is large, the discharge time of the two droplets first decreases rapidly with the increase of the distance, and then remains basically unchanged; Finally, the results also show that the larger the relative size difference between upstream and downstream droplets, the more unfavorable it is for droplets to discharge from the pipeline.

Inner circulation flow characteristics of coalescence droplets in microchannel

Controllable coalescence of multiple droplets is an important part of droplet manipulation, which is used to add samples to existing droplets, complete multi-step biological and chemical reactions inside droplet. It is of great importance to study the internal circulation characteristics for enhancing heat and mass transfer inside droplets. In this paper, microchannel experiments and volume of fluid method are conducted, and the evolution process of droplet coalescence is analyzed. The velocity distribution inside droplet is used as the main index to study the internal circulation characteristics under different surface velocities conditions. A characterization method of internal circulation strength based on central radial velocity and axial velocity inside the droplet is proposed, which can characterize the internal circulation region inside droplet caused by shearing action of microchannel wall of continuous phase. Results show that with the decrease of droplet length, the velocity distribution inside the droplet changes from being dominated by shearing action of microchannel wall to shearing action of continuous phase. In the process of droplet coalescence, there is a convective trend in the contact area between two droplets, and the maximum radial and axial velocities reach 3.5 times and 7.5 times of the droplet surface velocity respectively. This work can provide a reference for improving droplet control accurancy and mixing efficiency.

A new method for measuring the dynamic interfacial tension for flowing droplets of three-phase emulsion in the channel

• We invent a method to measure dynamic interfacial tension of three-phase emulsion. • The real-time acquisition of dynamic interfacial tension in the channel is obtained. • The γ-t relationship shows the surfactant adsorption exist throughout the channel. • Ca - t images of various surfactant concentrations show different transfer mechanisms. In this work, we developed a method for measuring the dynamic interfacial tension of three-phase emulsion based on microfluidic devices. The interfacial tension of three-phase emulsion in the flow process changes rapidly and is difficult to measure, which has never been reported in the previous literature. Through the geometric analysis of the structure of three-phase emulsion, the MATLAB program is compiled to simulate the image. By combining the simulated images with the high-speed photographic images of the flowing droplet, the dynamic interfacial tension during the structural transformation of emulsion is derived. This method can obtain the interfacial tension of flowing droplets in microchannels in real time, especially for three-phase emulsion, and can calculate the instantaneous interfacial tension at an interval of 4 ms. The error range of the measured dynamic interfacial tension value is not more than 0.1 mN / m , which is relatively accurate. We also use the measured dynamic interfacial tension to draw the Ca - t image, which shows the obvious differences at high and low surfactant concentrations. This is due to the different mass transfer resistance of surfactants at high and low concentrations. In a short, this study helps us to further understand how surfactants diffuse during the flow process of three-phase emulsions, which further helps us to better regulate the emulsions.

Three-Dimensional Open Water Microchannel Transpiration Mimetics.

The key problem that hinders the water transportation performance and application of microchannels is the annoying gaslock. Realizing liquid transport without the gaslock requires a specially designed pump and a channel system, as well as the reduction of gas concentration in liquids. In nature, to eat viscous nectar with high efficiency, hummingbirds use their open geometric tongue for nectar-sucking. Inspired by hummingbirds' tongue, we report a bionic open microchannel that discharges unwanted gas inside the microchannel from the opening without influencing its fluidic performance. The opening can also be used for extrusion of oil droplets in microchannels, indicating great potential applications in oil-water separation and chemical slow release, especially for bubble discharge in microchannels. Most significantly, a mimicked "leaf" with our bionic open microchannnels exhibits marvelous "transpiration" performance when irradiated by a laser. Our work provides a new strategy for the fabrication of open microchannels and sheds light on potential applications of multiphase phenomena in microchannels including oil-water separation, phase change heat and mass transfer, solar vapor generation, and precisely controllable drug delivery.

Coalescence dynamics of two droplets in T-junction microchannel with a lantern-shaped expansion chamber

BackgroundTo improve the coalescence efficiency of droplets in microchannel, a lantern-shaped expansion chamber is proposed to facilitate droplets coalescence. MethodsA high-speed digital camera was used to study the coalescence process and the mechanism of droplet coalescence was analyzed based on the liquid film draining model. FindingsThe existence of shrinkage mouth could prolong the contact time of droplets and accordingly promote droplet coalescence. The drainage time of continuous phase liquid film decreases with the increase of superficial flow velocity and droplet size, but increases with increasing dispersed phase viscosity. The film drainage time for contact coalescence, squeeze coalescence and tail coalescence increases successively. There exists a minimum critical capillary number Camin and a maximum critical capillary number Camax, and coalescence can occur only when Camin < Ca < Camax. The correlations for predicting the minimum critical capillary number and the maximum critical capillary number are proposed.

Numerical analysis of deformation and breakup of a compound droplet in microchannels

Compound droplets with different sizes are increasingly used in industrial production and academic research. This study aims to improve understanding of the dynamical behaviors of the compound droplet moving in microchannels. The compound droplet consisting of one inner core is initially concentric and placed at the entrance of a circular channel with a cone at the downstream region. Following this primary channel is a secondary channel that is circular and straight. Manipulation of the droplet is assisted by a flow introduced via the gap between two channels. The numerical results show that when passing through these confined channels, the droplet experiences a finite deformation or breakup mode. In each mode, the deformation or breakup of the compound droplet is also different when changing the values of the parameters including the capillary number, the ratio of the droplet radii, the ratio of the interfacial tension, the size of the secondary channel, the gap size and the cone angle. The finitely deformed droplet can be either nearly ellipsoidal with a deformation index D of around unity (i.e., first deformation mode) or highly elongated with a high D (i.e., second deformation mode). Breakup of the compound droplet happens to the outer droplet, and the breakup point is located either in front of the inner droplet (i.e., first breakup mode) or behind it (i.e., second breakup mode). The simple droplets yielded in the first breakup mode has a higher total volume than those in the second breakup mode. It is found that the droplet experiences the first deformation mode to the first breakup mode, then the second deformation mode and finally the second breakup mode when Ca increases. The regime diagrams of these modes, based on these parameters, are also presented.