Outer Droplet Research Articles

Numerical simulation of the deformation and breakup of a two-core compound droplet in an axisymmetric T-junction channel

The rheological behaviors of a compound droplet in a confined geometry are of importance in many industrial and natural processes. However, a detailed numerical simulation of the finite deformation and its transition to the breakup of the multi-core compound droplet in an axisymmetric T-junction channel is still lacking. The present study is to fill this gap through the numerical simulations of a two-core compound droplet that deforms and breaks up in this channel configuration. The numerical results are obtained by the axisymmetric front-tracking method. Our new finding is that the compound droplet in the channel can experience the finite deformation or the breakup depending on the flow condition or the configuration of the channel. In the finite deformation mode (i.e. non-breakup mode), the droplet rapidly reaches the maximum deformation before approaching the perpendicular rigid wall. The most deformation occurs with the outer droplet, and the inner droplet closer to the wall is less deformed than the other inner core droplet. In the breakup mode, three breakup patterns are recognized: (i) breakup type I occurring when breaking up only the outer droplet; (ii) breakup type 2 occurring when breaking up only the inner droplets; (iii) breakup type 3 occurring when breaking up both inner and outer droplets. The transition from the non-breakup mode to the breakup mode is available when increasing the Reynolds number Re (from 0.16 to 40.0), the capillary number Ca (from 0.04 to 4.0), the size Ro of the outer droplet and the middle-to-outer fluid viscosity ratio μ21, or decreasing the size Ri of the inner droplets, the radial size C2 of the channel (normalized by the channel axial size C1) and the interfacial tension ratio of the inner to the outer droplets. The transition diagrams based on some of these parameters are also proposed to provide a more complete picture of the two-core compound droplet behaving in the axisymmetric T-junction channel..

One-step microdevices for synthesizing morphology-controlled ultraviolet-curable polysiloxane shell particles

Double emulsions have great potential application in many fields. Although many companies and research institutions have developed various strategies for forming double emulsions, production and stability are not straightforward. Meanwhile, none of the well-organized simple device and scalable technique was available to form highly monodispersed double emulsions. Herein, we present a one-step strategy to produce highly monodisperse double emulsion via assembled micro-cross device. This device has a lot of advantages, such as easy assembly/disassembly and high repeatability. High-throughput is also easily achievable for its high-pressure capacity. Moreover, we systematically studied the effects of the three flow rates on the size and structure of the double emulsions. A mathematical model has been developed to predict the inner and outer droplets size of the double emulsion. We prepared the flame-retardant microcapsules with a dimethyl methylphosphonate (DMMP) inner core and an ultraviolet-curable polysiloxane shell. We hope that this assembled micro-cross device can be employed extensively in the area of microfluidics.

Numerical study of rheological behaviors of a compound droplet in a conical nozzle

The dynamics of compound droplets is more and more attractive because of their applications in a wide range of industrial and natural processes. This study aims to improve the understanding of dynamical rheological behaviors of a compound droplet moving in a nozzle with a conical shape in the downstream region via front-tracking-based simulations. The numerical results show that the compound droplet experiences three stages of deformation: the entrance stage (in front of the conical region), the transit stage (within the conical region), and the exit stage (in the exit of the nozzle). The droplet receives the maximum deformation in the axial direction during the transit stage, and the radially maximum deformation occurs during the exit stage. Because of the acceleration induced by the conical region, the inner droplet of the compound droplet can break up into smaller droplets during the exit stage. To reveal the transition between the finite deformation and the breakup, many parameters including the Capillary number Ca (varied in the range of 0.0125–1.6), the droplet size relative to the nozzle size R1/R0 (varied in the range of 0.2–0.9), the droplet radius ratio R21 (varied in the range of 0.3–0.8), the viscosity ratios μ21 and μ31 (varied in the range of 0.05–3.2), the interfacial tension ratio σ21 (varied in the range of 0.125–8.0), the conical angle α (varied in the range of 4°–34°) and the initial location of the inner droplet (i.e. the droplet eccentricity) are considered. From the finite deformation mode, the transition to the breakup mode of the inner droplet occurs when increasing any of Ca, R1/R0, R21 and α, or decreasing any of μ21 and σ21. The breakup mode is also enhanced when the inner droplet is initially located closer to the leading side of the outer droplet. However, varying μ31 induces no transition between these modes. The regime diagrams of these modes, based on these parameters, are also proposed.

Controlled preparation of compound droplets in a double rectangular co-flowing microfluidic device

This paper presents a 3D numerical simulation on a microfluidic device for creating double emulsions, controlling their sizes, and also manipulating the encapsulation process using a two-sequential co-flowing microfluidic configuration. In the proposed geometry, double emulsions can be generated as a result of the liquid’s interaction using dripping instability in a highly controllable manner. Dimensionless numbers, including Weber and capillary of the inner phase (Wein and Cain), Weber and capillary of the middle phase (Wemid and Camid), and capillary number of the outer phase (Caout) are considered to peruse effects of the phase flow rates on compound droplets characteristics. The mentioned dimensionless parameters can individually affect the formation process of the compound droplet, size of the inner and the outer droplets, and formation frequency. A three-dimensional numerical model based on the Volume of Fluid-Continuum Surface Force (VOF-CSF) with adaptive mesh refinement was utilized to perform parametric analysis on the double emulsion process. The model predicted the process of emulsification successfully in both dripping and jetting regimes and was used to predict the effects of phase flow rates on the size, morphology, and frequency of compound droplets. As the inner phase flow rate is increased, both inner and outer droplet sizes increase, while increasing the outer phase flow rate results in a reduction in compound droplet size. However, the middle phase flow rate increment eventuates to an increase and decrease in detached droplet size of the outer and inner phases, respectively. Also, it was observed that the number of dripping instabilities increases by increasing both middle and outer phases flow rates, which also lead to the higher compound jet filament length. The comparison of results with previous works reveals that the present geometry has high throughput in a broad range of flow rates while it has the capability of generating relatively spherical droplets even at high production rates. This is while the fabrication of this device is straightforward and precise in comparison with capillary-based devices.

Collision Modes of Two Eccentric Compound Droplets

A compound droplet with its single inner droplet appears in a broad range of applications and has received much attention in recent years. However, the role of the inner droplet location on the dynamical behaviors of the compound droplet is still not completely understood. Accordingly, the present study numerically deals with the eccentricity of the compound droplet affecting its colliding behaviors with the other droplet in a simple shear flow. The solving method is a front-tracking technique that treats the droplet interface as connected elements moving on a rectangular fixed grid. Initially, two compound droplets assumed circular are placed at a distance symmetrically to the domain center and they come into contact, because of the shear flow, when time progresses. During the collision process, the inner droplet that is initially located at a distance to its outer droplet center circulates around this center. It is found that this rotation also contributes to the formation of the collision modes including the reversing, passing-over and merging ones. Starting from a passing-over mode, a transition to a reversing mode or a merging mode can appear when the inner droplets, in terms of their centroids, are closer than their outer droplets. However, the location of the inner droplet within the outer droplet only has an effect when the value of the Capillary number Ca is varied from 0.01 to 0.08. For Ca < 0.01 corresponding to the merging mode and Ca ≥ 0.16 corresponding to the passing-over mode, the inner droplet position has almost no impact on the collision behaviors of two compound droplets.

Numerical study of collision modes of multi-core compound droplets in simple shear flow

Collisions of multi-core compound droplets have generated substantial interest in recent years because of their applications in the industry and academia. This study uses the front-tracking method to simulate the transition between the two collision modes of multi-core compound droplets in a simple shear flow. Compound droplets initially assumed identical have two sub-droplets of equal size. Given the shear flow, the droplets collide with one another and behave in two main modes, namely, passing-over and reversing. In the passing-over mode, the droplets pass over one another after coming into contact. The reversing mode appears with two compound droplets returning to their initial sides after the collision. During collision, the sub-droplets circulate approximately at the center of their enclosing outer droplets. Some parameters, including capillary number Ca, viscosity ratios μio and μmo, radius ratio Rio of the sub-droplets to the outer droplets, and sub-droplet angle a0, are investigated to determine their impact on these modes of collisions. We find that the transition from a reversing to passing-over mode occurs when we increase the value of Ca from 0.01 to 0.63}, {tiRio from 0.20 to 0.475, and μio and μmo in the range of 0.16-6.3. However, an increase in the value of a0 between -75° and 90° leads to a change from a passing-over to reversing mode. Diagrams of the colliding modes are also presented in this research.

DEVELOPMENT OF MICROEMULSION AND WATER/OIL/WATER MULTIPLE EMULSION CONTAINING BETA-ARBUTIN, LACTIC ACID, AND SODIUM ASCORBYL PHOSPHATE

Objective: In addition to lactic acid and sodium ascorbyl phosphate, which have whitening effects, beta-arbutin is a safe whitening agent for skin.Combining these three substances should reduce the concentration of each one in a formula and achieve an optimal whitening effect. In this study,microemulsions and water/oil/water (W/O/W) multiple emulsions were applied to produce a formula containing these whitening agents.Methods: All the active ingredients were formulated into microemulsions and W/O/W multiple emulsions with different concentrations of Tween 80and Span 80 as emulsifiers to obtain a stable formula. Twelve-week physical stability studies were performed for every formula at low (4±2°C), room(28±2°C), and high (40±2°C) temperatures.Results: The produced microemulsions were transparent with a mean droplet size of 15.50 nm. In addition, the W/O/W multiple emulsions containeddroplets within droplets, which were dispersed in a continuous phase with an inner droplet size of 0.15 μm and an outer droplet size of 0.37 μm. TheW/O/W multiple emulsions showed pseudoplastic thixotropic flow properties. Furthermore, the microemulsions were stable at low (4±2°C) androom (28±2°C) temperatures, while the W/O/W multiple emulsions were stable at room (28±2°C) and high (40±2°C) temperatures.Conclusion: It was concluded that the combination of beta-arbutin, lactic acid, and sodium ascorbyl phosphate was suitable for formulating intomicroemulsions as well as W/O/W multiple emulsions as whitening cosmetic products.

Numerical study of a compound droplet moving toward a rigid wall in an axisymmetric channel

Dynamical behaviors of compound droplets including those interacting with rigid walls in an axisymmetric channel appear in various industrial and natural processes. However, so far, no detailed investigation has been carried out for such interactions of compound droplets. Motivating from this missing gap, we here numerically study the finite deformation and breakup of an initially concentric compound droplet when it moves toward a rigid wall at the bottom of an axisymmetric vertical channel. The method used is a finite difference-based front-tracking method. The numerical results reveal that when the compound droplet is delivered toward the wall, it is deformed and can break up into smaller droplets. For the cases of finite deformation (i.e. non-breakup), while the outer droplet is radially stretched, the inner droplet first moves downward in the direction of the outer flow but then gets back. Thereby, a thin film is created between the outer and inner interfaces at the droplet top and thus prevents the outer droplet further deforming and breaking up. In contrast, if breakup happens, the outer droplet is further stretched, and most of the middle fluid moves outward toward the outer droplet edge to form a blob. Breakup can be available in one of three patterns: off-axis breakup, on-axis breakup, and inner breakup. The off-axis breakup mode only happens with the outer droplet while the inner breakup mode is only for the inner droplet. Various parameters are investigated to show the transition between a non-breakup mode to a mode of breakup. Such parameters contributing the transition include the Capillary number Ca (varied in the range of 0.01–2.5), the channel aspect ratio (varied in the range of 0.4–2.0), the ratio of the inner to outer droplet radii (varied in the range of 0.3–0.8), the droplet size relative to the channel size (varied in the range of 0.2–0.9), the interfacial tension ratio of the inner to outer interfaces (varied in the range of 0.1–4.0), and the viscosity ratio of the middle to outer fluids (varied in the range of 0.16–6.3). In contrast, some others, e.g. the Reynolds number, the viscosity ratio of the inner to the outer, do not induce any transition. From the numerical results, regime diagrams of breakup and non-breakup based on these parameters are proposed.

Preparation of encoded bar-like core-shell microparticles on a microfluidic chip

This work presented a microfluidic multi-phase droplet system for preparing encoded bar-like core-shell microparticles. The integration of T-junction and flow-focusing junction on one microfluidic chip was designed to form water-in-oil-in-water double emulsions. And the double emulsions were used as templates to fabricate the encoded bar-like core-shell particles on the chip. The encoded particles were of small and uniform size. The core droplets were filled with different quantum dots and dyes for providing encoding information, and the outer oil phase droplets of trimethylolpropane ethoxylate triacrylate were solidified into the shell of the encoded particles for confining the core droplets in them and supplying the surface for capturing targets. The sequence of the core droplets could be confined by the configuration of the micro-channels when the core droplets were encapsulated in the outer droplets. Thus a new multi-mode encoding method was defined by combining dual-optical encoding and graphical encoding. Different quantum dots provided the wavelength code and the intensity code, multiple dyes provided the color code, and the single order of core droplets in the shell offered the position code. Compared with the conventional encoding methods, this approach increases the encoding number by hundred times, and has great potential for high-throughput multiplexed analysis.

Investigating swelling-breakdown in double emulsions

Dilute water-in-oil-in water double emulsions are prepared to investigate the release and swelling phenomena during preparation and storage. During storage, a specific attention is paid to detect the overswelling-breakdown phenomenon that is due to overswelling of the outer droplets which causes their breakage and abrupt release of the inner droplets. A swelling ratio of the outer droplets is defined and is found useful to identify the effect of different parameters on the swelling behavior of the double emulsions. Yet, the investigation of the full droplet size distribution remains necessary. Before overswelling-breakdown, the escape is governed by the Brownian motion of the inner droplets and their coalescence with the external phase through the surface of the outer droplets. The swelling rate is mainly governed by the osmotic gradient between the internal and external water phases.

Visualization of features of atomizing circular liquid sheet using simultaneous Volume Laser-Induced Fluorescence

Present work is an experimental investigation of primary atomization of a radially expanding circular liquid sheet produced from a cylindrical vertical water jet issuing out from the convergent nozzle and impinging orthogonally on a horizontally placed cone-disc deflector below the nozzle. The radially expanding thin circular water sheet formed from the peripheral edge of deflector disc. The atomization process of expanding liquid sheet is captured simultaneously for the first time in the top and side views using a high-speed Volume Laser-Induced Fluorescent (VLIF). The experiments were conducted at 6 kHz framing rate for various jet Weber numbers (Wejet) over a range of 996 <Wejet < 7480. The top view VLIF revealed interesting spatiotemporal features of atomizing sheet such as outer edge thickness, liquid threads, ligaments and droplets and formation of perforations punctures (perforations) on the local sites of liquid sheet. Moreover, the evolution of the perforations leading to the breakup into ligaments and droplets was also captured in top view VLIF as well. Results show the liquid sheet outer edge (rim) thickness is unaffected by change in Wejet, whereas the rim velocity is found linearly increases with increase in Wejet. The liquid threads observed at the rim are subjected to end-pinching effect (as Ohnesorge number, Oh ≪ 1) when detached (because of Plateau-Rayleigh instability) from the rim. Ligament size in terms of length (Llig) and thickness (tlig) are measured at two different locations; (i) at rim (~100 mm from nozzle axis) and (ii) at about 130 mm downstream from the rim. Ligaments measured at downstream of the rim were found to be smaller than those measured at the rim. Further downstream, the droplets are formed whose size decreased with increase in jet Weber number (Wejet). Three different sites of origin of droplets namely rim, perforations edges and liquid bridge are identified. The variation of droplet diameter that produced from the rim with the Wejet is found to follow D10~Wejet-1/4 and D32~Wejet-1/5. Various features originating from the perforations are captured in top view VLIF and quantified for the first time using rigorous image processing methods. These features are perforation diameter, perforation’s front and rear edge thicknesses and velocities, liquid bridge, droplets from perforation via ligaments and via formation of liquid bridges. The perforations are formed close to sheet breakup and number of occurrence of perforations increases with increase in Wejet. The rear edge of perforation is seen thicker than the front edge because of larger accumulation of water in the rear edge. Consequently, the rear edge velocity is seen lower than the front edge velocity during the life span of perforations from the origin to their rupture into ligaments and droplets. Liquid bridge formed by merging/coalescing of two perforations is also observed. It is found that liquid bridge thickness increased with increase in Wejet. The droplets originating from the perforations are weak function of Wejet.

Where Do the Ions Reside in a Highly Charged Droplet?

Aqueous droplets in atmospheric and electrosprayed aerosols are charged due to presence of multiple ionic species. We examine the ion spatial distribution and the surface electric field in aqueous charged nanodrops by using atomistic modeling and analytical theory. We find that in nanoscopic liquid drops the concentration of simple ions is higher in the outer droplet shells, reduces gradually toward the drop center, and dies-off toward the vapor-droplet interface. The behavior of the ion spatial distribution is supported by a general analytical theory that takes into account a fluctuating droplet interface, an effective screening length of the charges and the finite size of a solvated ion. We compute the electric potential and the electric field near the droplet surface using a multipole expansion. We emphasize the significance of the fluctuations of the normal component of the electric field in ion evaporation via the Born model. In the presence of a highly charged peptide, we find that the peptide is situated mainly in the droplet interior and occasionally near the droplet surface. The simple ions are mainly near the droplet surface. The study provides insight into droplet chemistry and electrospray ionization mass spectrometry findings.

Modeling droplets swelling and escape in double emulsions using population balance equations

Monitoring the physical stability of double emulsions is considered in this work. A coupled population balance equation model for the inner and outer droplets is developed. The model involves a sub-model describing swelling of the inner, and so of the outer, droplets and a sub-model describing the escape of the inner droplets. The swelling phenomena is governed by the osmotic and Laplace pressures while the inner droplet escape rate is governed by the balance of the surface and viscous forces of the droplets. Experimental investigations demonstrate the usefulness of the model and its capability to describe the evolution of the inner and outer droplets size distributions and the release rate during storage of the double emulsions.

Parametric study of the collision modes of compound droplets in simple shear flow

Compound droplet collision has been found in various industrial and academic applications. Such colliding phenomena of two droplets in simple shear flow are numerically resolved by a front-tracking technique. Initially, each compound droplet, assumed cylindrical, contains one concentric inner droplet. They are separated at the lateral and vertical intervals denoted by Δx0 and Δy0. Because of the shear flow, the compound droplets interact with each other and exhibit three collision modes: passing-over, merging (i.e. coalescence) and reversing. These modes and their transition are affected by many parameters including the Reynolds Re and Capillary Ca numbers (based on the properties of the outer fluid), the viscosity ratios μ13 and μ23, the interfacial tension ratio σ12 of the inner to outer interfaces, the ratio of the radii of the inner to outer droplets R12 and the initial distance between them (in terms of Δx0 and Δy0). It is found that from a merging mode, decreasing Re from 2.51 to a value less than or equal to 1.0 induces a transition to a reversing mode, whereas, increasing Ca from 0.005 to a value greater than or equal to 0.04 leads to a transition to a passing-over mode. A transition from the merging to passing-over modes also appears when varying μ23 in the range of 0.1–10.0 or varying R12 in the range of 0.2–0.8. A transition from a passing-over mode to a reversing one is available when increasing Δx0 or decreasing Δy0. Three modes of collision all occur when μ13 is varied in the range of 0.1–10.0. However, the variation of σ12 does not induce any transition between different modes. Several phase diagrams in terms of Re versus Ca, or Δx0 versus Δy0 are also proposed to show the transitions between these modes.

A dune with a view: the eyes of a neotropical fossorial lizard

BackgroundLizards are excellent models to study the adaptations of the visual system to different scenarios, and surface-dwelling representatives have been relatively well studied. In contrast, very little is known about the functional anatomy of the eyes of fossorial lineages, and properties such as the light transmission by the ocular media have never been characterised in any fossorial species. Some lizards in the family Gymnophthalmidae endemic to the sand dunes of North Eastern Brazil have evolved sand-burrowing habits and nocturnal activity. Lizards in the sister group to Gymnophthalmidae, the family Teiidae, have decidedly diurnal and epigeal lifestyles, yet they are equally poorly known in terms of visual systems. We focussed on the eye anatomy, photoreceptor morphology and light transmittance properties of the ocular media and oil droplets in the gymnophthalmid Calyptommatus nicterus and the teiid Ameivula ocellifera.ResultsThe general organisation of the eyes of the fossorial nocturnal C. nicterus and the epigeal diurnal A. ocellifera is remarkably similar. The lenses are highly transmissive to light well into the ultraviolet part of the spectrum. The photoreceptors have the typical cone morphology, with narrow short outer segments and oil droplets. The main difference between the two species is that C. nicterus has only colourless oil droplets, whereas A. ocellifera has colourless as well as green-yellow and pale-orange droplets.ConclusionsOur results challenge the assumption that fossorial lizards undergo loss of visual function, a claim that is usually guided by the reduced size and external morphology of their eyes. In the case of C. nicterus, the visual system is well suited for vision in bright light and shows specialisations that improve sensitivity in dim light, suggesting that they might perform some visually-guided behaviour above the surface at the beginning or the end of their daily activity period, when light levels are relatively high in their open dunes habitat. This work highlights how studies on the functional anatomy of sensory systems can provide insights into the habits of secretive species.

Electric field-induced pinch-off of a compound droplet in Poiseuille flow

We address the pinch-off dynamics of a compound droplet that is suspended in a carrier fluid in a parallel plate microchannel. The droplet is subjected to a transverse electric field in the presence of an imposed pressure-driven flow. When a concentric compound droplet migrates in a pressure driven flow, the inner droplet deviates from the concentric position and forms an eccentric configuration that finally leads to the pinch-off of the outer shell. Our results reveal that the temporal evolution of droplet eccentricity as well as the kinetics of the thinning of the outer droplet is markedly influenced by the strength of the electric field as well as the electric properties of the fluids. We also bring out the conversion of different modes of droplet pinch-off, such as the equatorial cap breakup or the equatorial hole-puncture mode, by altering the electric field strength and electrical properties of the fluids. We also identify the relevant pointers that dictate the pinch-off time as well as the location of the pinch-off. This, in turn, opens up novel means of modulating the morphology of double emulsion in a confined channel by applying an electric field.

Theoretical and Experimental Investigations of Double Emulsion Preparation by Ultrasonication

Double emulsion preparation by ultrasonication is investigated in order to optimize the outer droplet size and the encapsulation efficiency. The effects of the sonication time, the oil viscosity, a...

Double-Emulsion Drop Evaporation and Formation of a Daughter Droplet.

In this study, we present experimental and theoretical analyses of evaporating a double-emulsion drop resting on a substrate. Multistage evaporation of the outer and inner droplet is witnessed. The complete evaporation of the outer drop and the initialization of the inner drop evaporation demonstrate an interesting transition dynamics. After the apparent completion of evaporation of the inner phase of a double-emulsion drop, surprisingly, formation of a daughter droplet is observed. We further investigated to hypothesize this phenomenon and achieved the formation of the daughter droplet for a single-phase drop as well. While engineering the "daughter drop formation" phenomena, we also proposed a way to obtain prolonged fixed contact line evaporation for a single-phase drop.

Deformation and breakup of a double-core compound droplet in an axisymmetric channel

The present paper numerically investigates the finite deformation (i.e. non-breakup) and breakup of a double-core compound droplet in an external flow confined in an axisymmetric channel. The compound droplet is axisymmetric around the vertical center axis of the channel, and its two inner droplets are located symmetrically around the horizontal center plane of the channel. The problem is solved by an axisymmetric front-tracking method that models the interface by connected elements. Many parameters (such as the Capillary number Ca, the Reynolds number Re, the interfacial tension ratio σ21 of the inner to outer interfaces, the channel size ratio C21, the ratio R21 of the inner to outer droplet radii, the inner droplet location and the outer droplet size R1) are varied to reveal their influences on the dynamic behavior of the compound droplet. Numerical results show that the compound droplet exhibits two distinct modes – finite deformation or breakup – depending on the flow condition. In the finite deformation mode, the inner droplet first moves away from and then moves back to the center. It stays there for the rest of time, resulting in the non-breakup of the outer droplet. However, in the breakup mode, after moving back to the center, the inner droplet again moves away while the outer droplet is continuously stretched and eventually performs necking and breakup at its ends. The transition between these two modes occurs when Ca varies in the range of 0.004–0.1, C21 varies in the range of 0.5–1.4 or the ratio of the outer droplet radius to the channel size varies in the range of 0.3–0.9. Changing Re (from 0.01 to 1.0), σ21 (from 0.25 to 4.0), R21 (from 0.3 to 0.45) or the inner droplet location does not make any transition. The effect of the kinematic viscosity ratios on the transition is also studied. In addition, a phase diagram in terms of Ca and the droplet size, on which the breakup and non-breakup modes are recognized, is proposed.

Generation of Water-In-Oil-In-Water (W/O/W) Double Emulsions by Microfluidics

Abstract Novel compartment microparticles prepared with double emulsion droplets as templates provide a protected internal space for material encapsulation. The effect of three-phase flow rate on the micro-droplet generation of double emulsion mechanism is available for reference to produce precise size and highly monodisperse particles. The influence of three-phase flow rate on the formation mode and size of the emulsion droplets is investigated by combination of experiment and numerical simulation. The size of compound droplets decreases and frequency increases with the increasing outer fluid flow rate. The monodispersity of the double emulsion reduces due to transition from dripping to narrowing jetting regime. Outer droplet size increases with the increasing flow rate of the middle fluid, whereas inner droplet size is the opposite. The frequency increases and then stabilizes, which leads to a widening regime. When Q2/Q1 > 6, the multi-core type double emulsion droplets are produced. Droplet coalescence occurs when surfactants is not involved. As Q1 increases, there is an increasing tendency for inner drop size. The outer drop size is proportional to the sum of the inner and middle flow rate, and that is irrelevant to Q1/Q2. For drop size, the ratio of core-shell and internal structure is precisely controlled by adjusting three-phase flow rate respectively.