Droplet Breakup Process Research Articles

Numerical simulations of secondary atomization characteristics of shear-thinning kerosene gels

Owing to their unique non-Newtonian characteristics, gel propellants have become a new type of fuel with promising applications in the aerospace industry. However, the complex rheological properties and high viscosity of gels pose significant challenges for flow control in gel engines, particularly in terms of fuel atomization and combustion efficiency. Focusing primarily on kerosene gel, this study investigates the secondary atomization characteristics of shear-thinning droplets, where the volume of fluid method is employed to capture interface dynamics on adaptive grids, and the viscosity of the gel is described by a power-law model. The accuracy of the numerical model is validated by comparing it with the experimental observations of the breakup process of 1% silica kerosene gel droplets. Numerical simulations are conducted to analyze the droplet breakup processes and formation mechanisms under bag, multimode, and shear breakup. Compared to kerosene, the apparent viscosity distribution inside the kerosene gel droplets is non-uniform, and the inhibition of viscous force delays the breakup time and reduces the droplet deformation, thereby affecting the transition of the breakup mode. By varying the Weber number, We, the rheological parameters, and the density ratio, the quantitative effects of these factors on droplet deformation, centroid velocity, average viscosity, the breakup time, etc. are revealed. The results indicate that the breakup becomes more severe with increasing We number, resulting in larger deformation and a greater number of smaller droplets; the rheological parameters significantly affect the droplet breakup by altering the fluid viscosity; moreover, under the same flow condition, the higher the density ratio, the more difficult it is for the droplet to break up; however, on dimensionless scales, the effect of density ratio appears weak. Additionally, a breakup regime diagram is constructed in the We–Oh (Ohnesorge) number space for low Weber numbers. This study validates the numerical method for simulating the secondary atomization of kerosene gel propellants, providing a thorough analysis of three breakup mechanisms and the impact of key parameters on kerosene gel droplets. These insights offer a theoretical foundation for the optimization of gel propellants, aiding in the development of efficient and stable propulsion systems.

Numerical study and experimental validation: A portioned calculation method for a large atomization field

Atomization and sprays are widely used in industry and agriculture. An appropriate atomization simulation method is essential in analyzing the liquid film-breaking process and atomization performance, especially in large-scale atomization field calculations. This study innovatively proposes a portioned method that combines existing fundamental atomization calculation models to balance computational accuracy and speed, finally achieving a full-scale numerical study of large atomization fields. This study employs the volume of fluid (VOF) model to measure the two-phase flow in the inner flow field and applies the discrete particle model (DPM) to analyze droplet behavior in the far-atomization field. In the near-atomization field, the VOF-to-DPM method connects the nozzle with the jet space, providing an effective numerical simulation of the liquid film formation and droplet breakup processes. Additionally, experiments on atomization using a pressure-swirl nozzle at different flow rates were conducted. Experimental data, such as atomization cone angle, flow distribution, and droplet particle size distribution, were obtained, and numerical calculations were performed using the large atomization field partitioned calculation model. The simulation results are utilized to explain the mechanisms of liquid film disintegration, while the experimental results are employed to validate the accuracy of the numerical model. The comparison revealed that the calculated results of the partitioned simulation approach align well with the experimental data. The maximum error in flow characteristics is 9.53%, in atomization cone angle is 6.16%, and in flow distribution is 3.67%, and there is a good agreement in particle size distribution with a maximum error of 17.58% in Sauter mean diameter, validating the accuracy of the portioned calculation method for large atomization fields.

Influence of triangular obstacles on droplet breakup dynamics in microfluidic systems

Microfluidic devices with complex geometries and obstacles have attracted considerable interest in biomedical engineering and chemical analysis. Understanding droplet breakup behavior within these systems is crucial for optimizing their design and performance. This study investigates the influence of triangular obstacles on droplet breakup processes in microchannels. Two distinct types of triangular obstructions, positioned at the bifurcation (case I) and aligned with the flow (case II), are analyzed to evaluate their impact on droplet behavior. The investigation considers various parameters, including the Capillary number (Ca), non-dimensional droplet length (L*), non-dimensional height (A*), and non-dimensional base length (B*) of the triangle. Utilizing numerical simulations with COMSOL software, the study reveals that the presence of triangular obstacles significantly alters droplet breakup dynamics. Importantly, the shape and location of the obstacle emerge as key factors governing breakup characteristics. Results indicate faster breakup of the initial droplet when the obstacle is positioned in the center of the microchannel for case I. For case II, the study aims to identify conditions under which droplets either break up into unequal-sized entities or remain intact, depending on various flow conditions. The findings identify five distinct regimes: no breakup, breakup without a tunnel, breakup with a tunnel, droplet fragmentation into unequal-sized parts, and sorting. These regimes depend on the presence or absence of triangular obstacles and the specific flow conditions. This investigation enhances our understanding of droplet behavior within intricate microfluidic systems and provides valuable insights for optimizing the design and functionality of droplet manipulation and separation devices. Notably, the results emphasize the significant role played by triangular obstacles in droplet breakup dynamics, with the obstacle’s shape and position being critical determinants of breakup characteristics.

A dynamic droplet breakup model for Eulerian-Lagrangian simulation of liquid-fueled detonation

This study proposes a dynamic model to reflect the physical image of the droplet breakup process in two-phase detonation flows. This breakup model is implemented in a two-phase detonation solver developed based on an open-source computational fluid dynamic platform, OpenFOAM, and compared with three prevalent models (TAB, PilchErdman, and ReitzKH-RT model) under different droplet diameters (30–70 μm) in one- and two-dimensional detonation problems. The simulating results show that the present breakup model well predicts experimentally determined detonation parameters such as detonation velocities and post-wave temperature. In addition, the present model has the advantage of being free of the KH breakup time parameter, which the ReitzKH-RT model needs to fit the experimental data. The one-dimensional detonation simulations indicate that different breakup models slightly impact the detonation wave velocity because the droplet breakup process does not significantly affect the total heat release as long as it is sufficiently fast to sustain the detonation. However, the two-dimensional detonation simulations show that the breakup model and the droplet initial diameter significantly affect the detonation cell size due to the different droplet distributions predicted by various models. The breakup length, which is the distance from the shock wave to the location at which sufficiently small child droplets appear, affects the chemical reaction zone thickness and then the detonation cell size. A longer breakup length will result in a larger detonation cell size.

Aerodynamic breakup of gel suspension droplets loaded with aluminum particles

The persistent and frequent space explorations are affected by the performance of propellants, among which the closely related gel propellants have been widely used due to their unique characteristics. Currently, research efforts primarily concentrate on the metallization and atomized combustion of gel propellants, emphasizing the combustion performance of metallized gel propellants. However, little attention has been given to the secondary breakup process that occurs just before combustion. By utilizing high-speed flow visualization technology, this study examines the key aerodynamic breakup process of gel suspension droplets loaded with micro aluminum particles, experimentally and theoretically investigating the droplet dynamics induced by particle volume fraction. The experimental confirmation of the three breakup behaviors exhibited by gel suspension droplets, namely oscillation mode, bag-stamen breakup, and hemline breakup, is presented along with the construction of a phase diagram. The impact of heterogeneous effects resulting from an increase in particle volume fraction on the critical breakup Weber number is revealed, with a concentration of 9 % as the turning point. Within the range of moderate Weber numbers, the tail flick deformation of suspension droplets in airflow is observed and reported for the first time. At different particle concentrations, there keeps a linear correlation between the dimensionless tail length and Weber number. Spherical suspension droplets will deform into thin disks in the airflow, with a nearly constant deformation rate regardless of changes in particle concentration. Additionally, the relationships between characteristic times and droplet parameters are discussed through statistical analysis of the results. As Ohnesorge number and particle volume fraction increase, the characteristic times both show a monotonically increasing trend. Moreover, a model of unstable growth rate associated with the particle volume fraction is established through linear instability analysis, accurately predicting the variation in critical Weber number for gel suspension droplet breakup. The congruence between our theoretical framework and experimental findings not only enhances comprehension of the impact of micro aluminum particles on the secondary breakup of gel suspension droplets but also provides valuable guidance for the aerospace applications of metallized gel propellants.

Oscillatory flow driven microdroplet breakup dynamics and microparticle formation in LMPA-water two phase flow system

This study presents a mathematical modeling and numerical investigation of oscillatory flow induced microdroplet breakup in low melting point alloy (LMPA)-in-water two-phase flow system in a microchannel. The effects of perturbation flow Weber number, and oscillating frequencies on the droplet breakup dynamics have been elucidated. Lowering the wall temperature will accelerate the solidification process of LMPA droplets, giving rise to the rapid formation of ultra-small particles with size down to 1 micron in less than 3 ms. The versatile approach combines oscillatory flow dominated droplet breakup process and phase change process, leading to the formation of LMPA microparticles with monodispersed size distribution and well-tailored properties by using a straight microchannel, which obviates the need for sophisticated design and complex fabrication process of microdevices. This work will provide a strategy to manipulating LMPA droplet size and structure for the massive production of ultra-small LMPA microparticles via a facile control over the flow conditions in the two-phase flow in a microfluidic system. It will open up for a diversity of applications of LMPA microparticles in various fields such as energy storage and thermal management.

A Simple Non-Embedded Single Capillary Device for On-Demand Complex Emulsion Formation.

This study includes an examination of the design, fabrication, and experimentation of a rudimentary droplet generator. The device has potential applications in on-demand double and higher-order emulsions as well as tailored emulsions with numerous cores. The phenomenon of a pendant double droplet creation is observed when an inner phase is transported through a capillary, while a middle phase envelops the external surface of the capillary. This leads to the occurrence of a pinching-off process at the tip of the pulled capillary. Following this, the double droplet is introduced into a container that is filled with the outer phase. The present study examines the force equilibrium throughout the droplet break-up process and aims to forecast the final morphology of the droplets within the container by considering the impact of interfacial tension ratios. The shell thickness in a core-shell formation can be calculated based on the inner and middle phase flow rates as well as the middle droplet formation period. The present platform, which enables the simple production of double and higher emulsions, exhibits promising prospects for the controlled manufacturing of complex emulsions. This technology holds potential for various applications, including the experimental exploration of collision behavior or electro-hydrodynamics in emulsions as well as millimeter-size engineered microparticle fabrication.

Experimental investigation and theoretical prediction of droplet breakup under a combined electric field and shear flow field

The electrohydrodynamic breakup of a droplet under a combined electric field (EF) and shear flow field (SFF) is experimentally investigated. The interface morphology characteristics of the droplet undergoing different breakup patterns are analyzed, and the interactions among electric forces, viscous forces, and interfacial tension during the droplet breakup process are also revealed. In addition, a diagram of droplet breakup patterns with different hydrodynamic and electric capillary numbers (Ca and CaE, respectively) is summarized. In particular, a prediction model for the criteria of droplet breakup under combined EF and SFF is proposed. The results indicate that three typical patterns of droplet breakup occur in turn with increasing CaE, i.e., satellite droplet detachment from a migrating droplet, a lobe detachment pattern, and a lobe disintegration pattern. As the Ca increases, the breakup pattern of satellite droplets detaching from the migrating droplet gradually disappears, and the critical CaE for droplet breakup decreases because the enhanced viscous force facilitates the detachment of the lobes from the mother droplet. Additionally, the breakup pattern of charged lobe disintegration occurs under a large CaE when the charge of the droplet reaches the Rayleigh limit. The critical CaE of the lobe disintegration breakup pattern decreases first and then increases with increasing Ca because the changes in droplet shape, interfacial tension, electrical potential energy, and fluid potential energy lead to a variation in the Rayleigh limit. Moreover, the breakup speeds of the three patterns are also altered by the strengths of the SFF and EF. Particularly, a reliable prediction model for the criterion of droplet breakup is proposed and compared with experimental data. This model provides a feasible approach for determining the criterion of droplet breakup through a very small amount of data for droplet-based applications.

Breakup of particle-laden droplets in airflow

The atomisation of a suspension containing liquid and dispersed particles is prevalent in many applications. Previous studies of droplet breakup mainly focused on homogeneous fluids, and the heterogeneous effect of particles on the breakup progress is unclear. In this study, the breakup of particle-laden droplets in airflow is investigated experimentally. Combining synchronised high-speed images from the side view and the 45 $^\circ$ view, we compare the morphology of particle-laden droplets with that of homogeneous fluids in different breakup modes. The results show that the higher effective viscosity of particle-laden droplets affects the initial deformation, and the heterogeneous effect of particles appears in the later breakup stage. To evaluate the heterogeneous effect of particles quantitatively, we eliminate the effect of the higher effective viscosity of particle-laden droplets by comparing cases corresponding to the same inviscid Weber number. The quantitative comparison reveals that the heterogeneous effect of particles accelerates the fragmentation of the liquid film and promotes localised rapid piercing. A correlation length that depends on the particle diameter and the volume fraction is proposed to characterise the length scale of the concentration fluctuation under the combined effect of the initial flattening and later stretching during the droplet breakup process. Based on this correlation length, the fragment size distributions are analysed, and the scaling results agree well with the experimental data.

Droplet size distribution in annular flow affected by the breakup and coalescence processes

The present work studies the influence of droplet breakup and coalescence processes on droplet size distribution in annular flow. The influence of the breakup and coalescence processes on droplet number is reflected in four factors, i.e., breakup-induced droplet birth, breakup-induced droplet death, coalescence-induced droplet birth and coalescence-induced droplet death. The four effects jointly shape the probability density distribution of the droplet size after reaching a dynamic equilibrium. A numerical procedure is compiled using the microscopic mechanisms of the droplet breakup and coalescence process to calculate the droplet size distribution and characteristic droplet sizes. The numerical predictions are fully validated by comparison with the experimental data available in the literature. In addition, the birth and death of droplets in the breakup and coalescence processes are discussed in detail.

Impact of hybrid surfaces on the droplet breakup dynamics in microgravity slug flow: A dynamic contact angle analysis

Microfluidic devices, which enable precise control and manipulation of fluids at the microscale, have revolutionized various fields, including chemical synthesis and space technology. A comprehensive understanding of fluid behavior under diverse conditions, particularly in microgravity, is essential for optimizing the design and performance of these devices. This paper aims to investigate the effects of discontinuous wettability on droplet breakup structures under microgravity conditions using a microchannel wall. The approach we adopt is underpinned by the volume-of-fluid methodology, an efficient technique renowned for its accurate resolution of the fluid interface in a two-phase flow. Furthermore, a modified dynamic contact angle model is employed to precisely predict the shape of the droplet interface at and near the wall. Our comprehensive model considers influential parameters such as slug length and droplet generation frequency, thereby providing crucial insights into their impact on the two-phase interface velocity. Validated against existing literature data, our model explores the impact of various configurations of discontinuous wettability on breakup morphology. Our findings highlight the significance of employing a dynamic contact angle methodology for making accurate predictions of droplet shape, which is influenced by the wall contact angle. Emphasis is placed particularly on the effects of slug length and droplet generation frequency. Notably, we demonstrate that the use of a hybrid surface at the junction section allows for precise control over the shape and size of the daughter droplets, contrasting with the symmetrical division observed on uniformly hydrophilic or superhydrophobic surfaces. This study contributes valuable insights into the complex dynamics of the droplet breakup process, which has profound implications for the design and optimization of microfluidic devices operating under microgravity conditions. Such insights are further poised to augment applications in space exploration, microreactors, and more.

Time-varying evolutionary mechanism analysis of the multiphase flow during high-velocity oxygen-fuel (HVOF) thermal spraying WC-12Co particle

HVOF thermal spraying with the fuel of aviation kerosene has unique advantages for spraying WC-12Co ceramic particles. The time-varying evolution mechanism of the multiphase flow formed by the particles wrapping with the combustion flame flow is quantitatively revealed, which is essential for preparing of high-quality coatings. In this study, the breakup and gasification process of kerosene droplet was considered. The fuel droplet, flame flow and spraying particles were directly coupled. Based on the computational fluid dynamics (CFD) method, a transient evolution model of HVOF thermal spraying multiphase flow with JP5000 spray gun was established. The Euler-Lagrange method of bidirectional coupling was used to track the continuous and discrete phases. The spraying combustion flame characteristics were expressed by the realizable k-ε turbulence model and eddy dissipation model (EDM). The results show that the multiphase coupling transient evolution model quantitatively captures the nonlinear changes of the breaking and gasification for the kerosene droplet, combustion reaction, particle flight behavior and others during spraying. The kerosene droplets in the combustion chamber aggregate with each other to form large droplets, which are rapidly broken and gasified in the peripheral areas. Due to the violent combustion, a gas-phase circulation was formed by the heat and cold air. As the combustion progresses, the flame flow becomes stable and orderly. When particles with a size of 20 to 30 μm impact the substrate, the particle temperature is close to the solid phase temperature, and the velocity is fast. The particle temperature and velocity decrease with the increase of the particle size.

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.

Numerical Investigation of the Effect of Supersonic Air Temperature on the Mixing Characteristics of Liquid Fuel

The effect of supersonic air temperature on the mixing characteristics of liquid hydrocarbon fuel injected into three different supersonic airflows elevated in three steps from 373 K to 673 K was investigated numerically. Compressible Reynolds-averaged Navier–Stokes (RANS) equations were solved together with species conservation equation using ANSYS Fluent for two-phase flow simulations including fuel droplet breakup and evaporation. The turbulence model needed to close the RANS equations used the Shear Stress Transport (SST) k-ω model. The Eulerian–Lagrangian model was employed to track fuel droplets in mainstream air, and the Kelvin–Helmholtz and Rayleigh–Taylor (KH-RT) models were used to simulate the droplet breakup process. Numerical solutions were validated using experimental data. The higher the air temperature, the stronger the streamwise vortices downstream of the pylon. When the air temperature was 373 K, the liquid fuel hardly evaporated, but as the air temperature increased, and the mass fraction of the vaporized fuel and the mixing efficiency increased linearly downstream of the pylon. At air temperatures of 523 K and 673 K, the mixing efficiencies were 10% and 51% at the combustor outlet, respectively. The total pressure loss decreased slightly due to droplet evaporation as the temperature increased from 373 K to 673 K.

Numerical simulation of secondary breakup of shear-thinning droplets

The breakup of non-Newtonian droplets is ubiquitous in numerous applications. Although the non-Newtonian property can significantly change the droplet breakup process, most previous studies consider Newtonian droplets, and the effects of non-Newtonian properties on the breakup process are still unclear. This study focuses on the secondary breakup of shear-thinning droplets by numerical simulation. The volume of fluid method is used to capture interface dynamics on adaptive grids. To compare shear-thinning droplets and Newtonian droplets, a new definition of the Ohnesorge number is proposed by considering the characteristic shear rate in the droplet induced by the airflow. The results show that compared with the Newtonian fluid, the shear-thinning properties can change the apparent viscosity distribution inside the droplet, alter the local deformation, change the droplet morphology, and affect the transition in the droplet breakup regime.

Wave Structure of Heterogeneous Detonations at High Operating Pressures

ABSTRACT An analytical model has been constructed to evaluate the behavior of a single droplet in a heterogeneous detonation flowfield. In this study, mass shed off the droplet is estimated based on viscous boundary layer instabilities at the free surface of the gas/liquid interface at high operating pressures. The trajectory of the droplet is computed by assuming that it travels through a gas medium that matches a Zel’dovich-Von Neumann-Döring (ZND) profile, i.e., gas/liquid momentum exchange is ignored. With application to rocket combustors, three different mixtures are considered: liquid oxygen droplets with both methane and hydrogen gas, as well as kerosene droplets in gaseous oxygen. It is identified that the initial droplet diameter and the dynamic pressure behind the leading shock front are the two most important factors influencing the droplet breakup process. It is found that smaller droplet diameters, higher base pressures, and reactants with lower sound speeds are desired for close coupling of the leading shock front to the heat-release zone. With the droplet atomization model employed, it is concluded that increasing the operating pressure may increase the minimum droplet size that will permit coupling of the heat release with the detonation front. This is due to the increased dynamic pressure behind the wavefront when the operating pressure is increased. Finally, it is noted that the sound speed in the compressed reactant mixture is also of prime importance as induction zone lengths scale with this parameter. As a result the liquid kerosene/gaseous oxygen system provided the smallest induction zone length of the three systems studied.

Role of water micro-explosion on fuel droplet size distribution, engine performance, and emissions in a water-diesel emulsified engine: A comprehensive numerical investigation

The global warming crisis and strict regulations have pushed the automotive industry into technological change, but it will take time, so finding quick solutions can be very helpful. Using water in diesel emulsion is one of the low-cost and fast solutions to reduce pollution and improve the performance of diesel engines. In this study, the effects of adding water to diesel fuel and the phenomena that occur when water enters the combustion chamber are investigated. For this purpose, a simulation of a four-cylinder diesel engine has been used with different content of water in diesel (0% or pure diesel, 5%, 10%, 15%, and 20%). The simulations are performed in Converge CFD using dynamic mesh and adaptive mesh refinement. The results showed that the use of water in diesel leads to an improvement of about 10% in engine indicated power and a simultaneous reduction in soot and NOx by 10 and 30%, respectively. The simulation results showed that water droplet micro-explosions accelerated the fuel droplet breakup process and reduced the combustion duration by 10%–30%.

Secondary droplet size distribution upon breakup of a sub-milimeter droplet in high speed cross flow

The droplet breakup behaviors under the action of shock wave are frequently observed in liquid-fueled detonation-based engines. In this work, the secondary droplet size distribution after the breakup of a sub-milimeter droplet under high velocity cross flow conditions (the gas flow velocity is 105.64 m/s) was experimentally determined on a horizontal shock tube with the developed digital in-line holography technique. During the droplet breakup process, the spatial (two-dimensional, three-dimensional) and droplet size distributions of two characteristic stages (early and late stages) were majorly concerned based on the reconstructed droplet cloud field. At the early stage, the liquid-mist field appeared as the barrel shape, and the probability distribution of the droplet number in the cross-stream direction presented as “M shape”. While the secondary droplet cloud field appeared as the conical shape, and the probability distribution presented as the parabolic shape at the late stage. Both the probability distribution of the droplet number along the flow direction increased firstly then decreased at the early and late stages with the increasing of distance. In addition, the peak position of the droplet number probability distribution also changed with time. The secondary droplets mainly distributed in the range of 0∼20 μm for the early and late stages, and its number accounted for more than 93%. Meanwhile, the corresponding volume probabilities changed from 12% to 72%, due to the consistent stripping process. In addition, the droplet size distribution could be well described by the log-normal and root-normal distribution curves at both stages. The study of the spatial and droplet size distributions is believed to be of importance for corresponding droplet breakup modeling.

Novel glass capillary microfluidic devices for the flexible and simple production of multi-cored double emulsions

HypothesisDouble emulsions with many monodispersed internal droplets are required for the fabrication of multicompartment microcapsules and tissue-like synthetic materials. These double emulsions can also help to optically resolve different coalescence mechanisms contributing to double emulsion destabilization. Up to date microfluidic double emulsions are limited to either core–shell droplets or droplets with eight or less inner droplets. By applying a two-step jet break-up within one setup, double emulsion droplets filled with up to several hundred monodispersed inner droplets can be achieved. ExperimentsModular interconnected CNC-milled Lego®-inspired blocks were used to create two separated droplet break-up points within coaxial glass capillaries. Inner droplets were formed by countercurrent flow focusing within a small inner capillary, while outer droplets were formed by co-flow in an outer capillary. The size of inner and outer droplets was independently controlled since the two droplet break-up processes were decoupled. FindingsWith the developed setup W/O/W and O/W/O double emulsions were produced with different surfactants, oils, and viscosity modifiers to encapsulate 25–400 inner droplets in each outer drop with a volume percentage of inner phase between 7% and 50%. From these emulsions monodispersed multicompartment microcapsules were obtained. The report offers insights on the relationship between the coalescence of internal droplets and their release.

Ferrofluid droplet breakup process and neck evolution under steady and pulse-width modulated magnetic fields

Numerous applications in engineering and biotechnology have attracted the attention of many researchers to the analysis of underlying physical phenomena during the droplet pinch-off. In this study, the neck evolution during the formation of a ferrofluid droplet from a capillary is investigated under two types of magnetic field for a drop-on-demand system. The two types are steady and Pulse-Width Modulated (PWM) magnetic fields. First, under steady magnetic field, the necking process is studied for different values of magnetic Bond number and various angles between magnetic coil centerline and gravity. Subsequently, self-similar behavior in the vicinity of the detachment moment is observed. In the last stage of droplet pinch-off, a scaling law is fitted to the variation of minimum neck diameter versus remaining time to the breakup. The scaling exponent is obtained to be 0.67 which is in good agreement with that for the ferrofluid droplet formation using a syringe pump reported in previous studies. Second, under PWM magnetic field, the necking process during the detachment of a ferrofluid droplet is explored and compared with that in the presence of the steady magnetic field. A new pattern of minimum neck diameter variation with remaining time to the breakup is observed for the first time. The trend of variation can be divided into two regions which are called “slow-going” and “fast-going” necking. Two mechanisms are introduced which explain the transition from the slow-going to fast-going necking region. The effect of magnetic Bond number and dimensionless applied frequency on the neck evolution is studied. In the proximity of the detachment moment, a universal behavior for pinch-off process is observed and the exponent of the fitted scaling law is equal to 0.84. Finally, the variation of relative neck position under the PWM magnetic field is compared with that in the presence of steady magnetic field.