Mechanism Of Droplet Formation Research Articles

Three dimensional simulations of droplet formation in symmetric and asymmetric T-junctions using the color-gradient lattice Boltzmann model

Asymmetric T-junctions have recently emerged as a promising tool in microfluidics. However, previous studies of the droplet formation mechanism are largely limited to symmetric T-junctions. In this work, the droplet formation in universal T-junctions, including both symmetric and asymmetric T-junctions, is investigated by a three-dimensional color-gradient lattice Boltzmann model. A three-dimensional color-conserving boundary condition is developed to model fluid–surface interactions, which suppresses the spurious velocities near the contact lines and improves numerical accuracy. Model verification is conducted by the partial wetting test and the droplet formation in a symmetric T-junction. Then, an in-depth study is performed for universal T-junctions. In both symmetric and asymmetric T-junctions, the droplet length is linearly dependent on flow rate ratio at low capillary number, and the droplet formation successively undergoes squeezing, dripping and jetting regimes as the capillary number increases. By investigating the local pressure and velocity field, we find that the upstream pressure and viscous force respectively dominates the droplet formation in squeezing and dripping regimes. In squeezing regime, the pressure fluctuates significantly, and the fluctuation amplitude and frequency decrease with the channel width ratio; while in dripping regime, the pressure fluctuation is negligibly small, and the viscous force is found to decrease with the channel width ratio. Consequently, the droplet size increases with the channel width ratio in both regimes. In addition, the viscosity ratio and surface wettability are found to be influential to the formation regime, droplet shape and size for various channel width ratios and capillary numbers, and play important roles in droplet formation.

Time-resolved mixing and flow-field measurements during droplet formation in a flow-focusing junction

Highly monodispersed emulsions can be produced in microfluidic flow-focusing junctions (Anna et al 2003 Appl. Phys. Lett. 82 364–6, Baroud et al 2010 Lab Chip 10 2032–45). This is the reason why many industrial processes in the medical industry among others are based on droplet manipulation and involve at some point a step of dripping within a junction. However, only a few studies have focused on the flow field inside and outside the droplet, even though it is a necessary step for understanding the physical mechanism involved and for modeling the droplet formation process. Water-in-oil emulsions are produced in flow-focusing junctions of square cross sections. The fluids constituting the emulsion are (i) a 5.0 mPa·s silicon oil for the oil phase and (ii) distilled water containing 2.0 wt% of sodium dodecyl sulfate surfactant for the aqueous phase. Time-resolved shadow particle images are acquired using a microscale particle image velocimetry (µPIV) system and flow fields are calculated using an adaptive PIV algorithm in combination with dynamic masking. Inside the microchannel and in the permanent regime, the droplet has an internal circulation that has been well established by Sarrazin et al (AICHE J. 52 4061–70). But during the formation of a droplet in a flow-focusing junction, the flow field is not so well known, and the circulation in the finger flows forward along the sides and returns along the center. The mechanism can be described in terms of four distinct steps: droplet growth, necking, rupture, and recoil. The liquid expelled from the neck just before rupture is also well observed. The flow field and mixing are measured in detail during a complete cycle of formation of a main droplet and satellite droplets using high-speed imaging. This allows us to develop a better understanding of the different forces that are present and of the physical mechanism of droplet formation.

Droplet Formation Study in Emulsification Process by KSM using a Novel In Situ Visualization System

The semi real-time observations of oil-in-water emulsification process in a Kenics static mixer were performed using a novel in situ visualization system. The homogenization processes and emulsion characteristics were analyzed using images taken periodically in fixed time intervals during the emulsification process. Morphological evolution of droplets was monitored and the mechanism of droplet formation was studied, both experimentally and theoretically. A wide range of dispersed phase concentration as well as surfactant concentration were used in the experiments and their impacts on emulsion characteristics were determined. Different droplet formation mechanisms occurred during the experimental results are presented. The relationship between droplet sizes, flow rate, surfactant concentration, and other impact factors was visually shown, and their role in controlling the emulsification process was revealed. The minimum droplet size obtained from the dispersed phase in emulsions was shown to be perfectly monitored and controlled by this technique.

Automobile diesel exhaust particles induce lipid droplet formation in macrophages in vitro

Exposure to diesel exhaust particles (DEP) has been associated with adverse cardiopulmonary health effects, which may be related to dysregulation of lipid metabolism and formation of macrophage foam cells. In this study, THP-1 derived macrophages were exposed to an automobile generated DEP (A-DEP) for 24h to study lipid droplet formation and possible mechanisms. The results show that A-DEP did not induce cytotoxicity. The production of reactive oxygen species was only significantly increased after exposure for 3h, but not 24h. Intracellular level of reduced glutathione was increased after 24h exposure. These results combined indicate an adaptive response to oxidative stress. Exposure to A-DEP was associated with significantly increased formation of lipid droplets, as well as changes in lysosomal function, assessed as reduced LysoTracker staining. In conclusion, these results indicated that exposure to A-DEP may induce formation of lipid droplets in macrophages in vitro possibly via lysosomal dysfunction.

Alternating Force Based Drop-on-Demand Microdroplet Formation and Three-Dimensional Deposition

Drop-on-demand (DOD) microdroplet formation and deposition play an important role in additive manufacturing, particularly in printing of three-dimensional (3D) in vitro biological models for pharmacological and pathological studies, for tissue engineering and regenerative medicine applications, and for building of cell-integrated microfluidic devices. In development of a DOD based microdroplet deposition process for 3D cell printing, the droplet formation, controlled on-demand deposition and at the single-cell level, and most importantly, maintaining the viability and functionality of the cells during and after the printing are all remaining to be challenged. This report presents our recent study on developing a novel DOD based microdroplet deposition process for 3D printing by utilization of an alternating viscous and inertial force jetting (AVIFJ) mechanism. The results include an analysis of droplet formation mechanism, the system configuration, and experimental study of the effects of process parameters on microdroplet formation. Sodium alginate solutions are used for microdroplet formation and deposition. Key process parameters include actuation signal waveforms, nozzle dimensional features, and solution viscosity. Sizes of formed microdroplets are examined by measuring the droplet diameter and velocity. Results show that by utilizing a nozzle at a 45 μm diameter, the size of the formed microdroplets is in the range of 52–72 μm in diameter and 0.4–2.0 m/s in jetting speed, respectively. Reproducibility of the system is also examined and the results show that the deviation of the formed microdroplet diameter and the droplet deposition accuracy is within 6% and 6.2 μm range, respectively. Experimental results demonstrate a high controllability and precision for the developed DOD microdroplet deposition system with a potential for precise cell printing.

Simulation of the main physical processes in remote laser penetration with large laser spot size

A 3D model is developed to simulate remote laser penetration of a 1mm Aluminum metal sheet with large laser spot size (∼ 3x3cm2), using the ALE3D multi-physics code. The model deals with the laser-induced melting of the plate and the mechanical interaction between the solid and the melted part through plate elastic-plastic response. The effect of plate oscillations and other forces on plate rupture, the droplet formation mechanism and the influence of gravity and high laser power in further breaking the single melt droplet into many more fragments are analyzed. In the limit of low laser power, the numerical results match the available experiments. The numerical approach couples mechanical and thermal diffusion to hydrodynamics melt flow and accounts for temperature dependent material properties, surface tension, gravity and vapor recoil pressure.

A droplet entrainment model for horizontal segregated flows

One limitation in simulating horizontal segregated flows is that there is no treatment of droplet formation mechanisms at wavy surfaces. For self-generating waves and slugs, the interfacial momentum exchange and the turbulence parameters have to be modeled correctly. Furthermore, understanding the mechanism of droplet entrainment for heat and mass transfer processes is of great importance in the chemical and nuclear industry.The development of general computational fluid dynamics models is an essential precondition for the application of CFD codes to the modeling of flow related phenomena. The new formulation for the interfacial drag at the free surface and turbulence parameters within the algebraic interfacial area density model (AIAD) represents one step toward a more physical description of free surface flows including less empiricism. The AIAD approach allows the use of different physical models depending on the local fluid morphology inside a macro-scale multi-fluid framework.A further step of improving the modeling of free interfaces lies within the consideration of droplet entrainment mechanisms. In this paper a new sub-grid entrainment model is proposed, which assumes that due to liquid turbulence the interface gets rough and wavy leading to the formation of droplets. Therefore, the droplet entrainment model requires the consideration of an additional droplet phase, which is described with an own set of balance equations in the spirit of the particle model. Two local key factors determine the rate of droplet entrainment: the liquid turbulent kinetic energy as well as the outward velocity gradient of the liquid relative to the interface motion. The new droplet entrainment approach is included into CFD simulations for attempting to reproduce existing horizontal two-phase flow data from the HAWAC channel. The computational results are in reasonable agreement with the experimentally observed data.

Oil-in-Water Droplets Generation in Dripping Regime with Channel Partial Wetting

We generated silicone oil micro-droplet in deionized water with a micro uidic T-junction devices made on siliconglass with di erent cross-sections (depth and width) of the continuous phase and the dispersed phase micro-channels. We experimentally show that the size of the droplet decreases when the width or the velocity of the dispersed phase micro-channel decreases but is almost insensitive to the channel depth. For describing the observed behaviour, we proposed a modi ed mechanism of droplet formation consisting of three stages, each with start and end precisely identi ed. Based on this mechanism, we developed an analytical model for obtaining the droplet diameter in dripping regime when there is partial wetting at the channel boundary. This model is in better agreement with the experiments than other analytical models from the literature, suggesting the e ect of channel wetting is signi cant. We also discuss the use of the capillary number in models, and suggests that the velocity would be a better metrics for comparing di erent T-junction geometry. In the experiment, the generated droplet diameter is varied between 28 μm and 196 μm.

Breakup of Oil Jets into Droplets in Seawater with Environmentally Benign Nanoparticle and Surfactant Dispersants

During deep-sea oil leaks, dispersants may be used to break up the oil into droplets smaller than about 70 μm, which may then be bioremediated by bacteria before they reach the ocean surface. To investigate the mechanism of droplet formation as a function of dispersant type, concentration, and jet velocity, a flowing oleophilic stream containing amphiphiles was mixed with flowing dodecane and then atomized through a 0.25 mm circular nozzle. The minimum droplet diameters were 2.2, 4.5, and 24 μm for only 5 w:v % amphiphile in the oil phase for Corexit 9500A, Tergitol 15-S-7 (C12H25CH(OCH2CH2)7OH), and a silica nanoparticle/Span 20 mixture, respectively. For Tergitol 15-S-7, the droplet size exhibited the expected scaling with Weber number (We) at low viscosity numbers (Vi 50), where inertial forces overcome viscous forces. However, in the case of the silica nanoparticle/Span 20 mixture, the magnitude...

Numerical modeling of a horizontal annular flow experiment using a droplet entrainment model

One limitation in current simulating horizontal annular flows is the lack of treatment of droplet formation mechanisms. For self-generating annular flows in horizontal pipes, the interfacial momentum exchange and the turbulence parameters have to be modelled correctly. Furthermore the understanding of the mechanism of droplet entrainment in annular flow regimes for heat and mass transfer processes is of great importance in the chemical and nuclear industry.A new entrainment model is proposed. It assumes that due to liquid turbulence the interface gets rough and wavy and forms droplets. The new approach is validated with HZDR annular flow experiments. Important phenomena like the pressure drop, the wave pumping effect, the droplet entrainment, the liquid film formation and the transient flow behavior could be calculated, analyzed and some of the phenomena compared with the measurement.

Enhancement of aqueous stability of allyl isothiocyanate using nanoemulsions prepared by an emulsion inversion point method

Allyl isothiocyanate (AITC), an organosulfur compound in cruciferous vegetables, is a natural antimicrobial and potential chemopreventive agent. However, the instability of AITC in aqueous systems restrains its applications. In this study, oil-in-water AITC nanoemulsion was prepared by the emulsion inversion point (EIP) method, aiming at improving the aqueous stability of AITC. The optimal hydrophilic–lipophilic balance (HLBop) value of surfactants containing Tween 80 and Span 80 was established at 11.0–13.0, yielding nanodroplets with diameters of 137–215nm. The mechanism of droplet formation within the HLPop region was discussed in terms of the possible structure of adsorbed surfactant layers at the oil–water interface in multiple emulsion droplets. In a 6.5-month storage test, the droplet sizes and the count rates (intensity of scattered light) of nanoemulsions decreased only slightly by 4–13% (depending on surfactant-to-oil ratio), even in highly diluted status, indicating the desirable stability of the nanoemulsions. Moreover, the nanoemulsion demonstrated superior protection against AITC degradation (78% remaining after 60d at 30°C), compared with protein nanoparticles as well as non-encapsulated aqueous dispersion. This work shows for the first time that AITC can be formulated into nanoemulsions and thus obtains satisfactory aqueous solubility and chemical stability.

Droplet formation via squeezing mechanism in a microfluidic flow-focusing device

In this work, the formation of droplets in a microfluidic flow-focusing device is studied and presented. Spurious velocities at the fluid–fluid interface were found to be very small in the lattice Boltzmann simulations. Using this technique, simulations have been performed to study the effect of geometry on formation of droplets for a microfluidic flow-focusing device. The effect of orifice width, orifice length and distance of the orifice on the mechanism of droplet formation and size of droplets is presented for different Capillary numbers. It is shown that, for Ca≪1, creation of droplets proceeds through the squeezing process that has earlier been observed in the T-junction configuration. The size of droplets increases with an increase in the (a) width, and (b) distance of orifice from the inlet. On increasing orifice length, droplet size first decreases to a minimum and increases thereafter. This study also reveals that the size of droplets becomes independent and approaches a constant beyond a critical value of the orifice length.

Numerical Analysis of Junction Point Pressure during Droplet Formation in Y-Junction Microchannel

Junction point pressure changes during droplet formation in Y-junction microchannels with differed Y-angles, wetting property and capillary number of the liquid by using a three dimensional numerical simulation. The pressure of the junction point fluctuates throughout the droplet formation process, and it can be used to depict exactly and directly different stages of droplet in microchannels. And the pressure of junctions with different Y-angles of microchannel, different contact angles of dispersed phase with the surface, and different capillary numbers of continuous phase could thus be investigated via the droplet formation mechanism.

Nanoparticles influence droplet formation in a T-shaped microfluidic

Droplet formation in the presence of nanoparticles was studied in a T-shaped microfluidic device numerically. Nanoparticles in continuous phase did not influence droplet formation dynamics obviously. Contrarily, the presence of nanoparticles in dispersed phase will influence evidently droplet formation dynamics, likely reasons are the accumulation of nanoparticles at the liquid–liquid interface leading to the variation of interfacial tension and the anisotropy of nanoparticles’ movement at interface. The droplet size decreases almost linearly with increasing of the volume fraction of nanoparticles in dispersed phase when the volume fraction of nanoparticles not exceeding a critical value (about 0.2 %), because very high concentration of nanoparticles results in particle aggregation so as to not decrease interfacial tension so obviously any more. A complicated mechanism of temperature influences on droplet formation may exist combining the variations of effective viscosity and interfacial tension. Discussions on microscopic mechanism of droplet formation in the presence of nanoparticles were carried out.

Novel High-Temperature Experimental Setup to Study Dynamic Surface Tension Phenomena in Oxide Melts

In many pyrometallurgical applications, subprocesses such as emulsification, droplet, bubble or jet formation, coalescence, and surfactant adsorption occur at small time scales (typically milliseconds to fractions of seconds), both at slag/metal and slag/gas interfaces. These phenomena are surface tension driven and—due to the high-temperature environment—very difficult to investigate in a quantitative manner. Under these dynamic conditions, the instantaneous surface tension of slags may vary in time as well as along its surface and may change dramatically the rate of the involved processes. This paper presents a new high-temperature experimental setup to study and measure the dynamic surface tension of slags, the mechanisms of slag jet and droplet formation, and the capillary breakup of molten slag jets. It features a three zone furnace with optical access, and a droplet generation device incorporating a back-pressure system in combination with a stopper for precise slag flow control. The first successfu...

Controlled production of double emulsions in dual-coaxial capillaries device for millimeter-scale hollow polymer spheres

Capillaries-based co-flowing microfluidic devices were designed and then fabricated for the controlled production of water-in-oil-in-water (W/O/W) double-emulsion droplets, where polystyrene dissolved in fluobenzene was employed as the oil phase. Two configurations of the co-flowing devices were employed in the experiments. In the so-called one-step device, the inner capillary tip locates at the same cross-section of the middle capillary tip. Thus, the core drop and the shell drop depart from their capillaries ends simultaneously, forming a double-emulsion drop in one step. While in the so-called two step device, the inner capillary tip locates upstream to the middle capillary tip. The core drop and the shell drop break off from their respective capillaries ends successively, forming a double-emulsion drop in two steps. The effect of the fluids flow rates on the double-emulsion droplets size as well as their formation mechanism was studied comprehensively in the proposed devices. Experimental results implied that slight difference in device configurations would lead to significant difference in droplet formation mechanisms and thus size-control laws. In the two-step device, the inner stream of fluid had little influence on the breakage and the outer diameter of the double-emulsion drops, while in the one-step device the effect was significant. With the produced double-emulsion droplets, millimeter-sized polymer capsules with single hollow cavity and ultra-thin shells were obtained by solidifying the oil layers of the compound drops using the solvent evaporation method.

Emulsification with microfiltration ceramic membranes: A different approach to droplet formation mechanism

Oil-in-water (O/W) emulsions were prepared by membrane emulsification (ME) in a large scale stirred tank with a submerged cell using flat membranes, and in a cross-flow tubular unit. Results obtained with flat and tubular membranes devices were compared. Common ceramic microfiltration (MF) membranes (ZrO2 supported on TiO2) were used in both cases. The main advantage of these membranes is their low cost compared to others frequently used in ME (SPG membranes), which implies a reduction up to 60%. The effect of operation parameters on droplet size distributions was studied to understand the droplet formation mechanism that takes place in both devices.Monodisperse O/W emulsions were obtained using flat membranes with a droplet-to-pore diameter ratio (Dd/Dp) in the range 2.1–2.9. Span values of 0.58, 0.66 and 0.81 corresponded to membrane pore diameters of 0.45, 0.80, and 1.4μm, respectively, being the active pores fraction in the range of 27–36%. The cross-flow tubular unit allowed for production of monodisperse O/W emulsions but with larger droplet sizes, having droplet-to-pore diameter ratios (Dd/Dp) in the range 3.9–4.7. It was observed a major influence of pore activation by dispersed phase pressure on droplet size distribution.Shear stress had little influence on the droplet size, with both flat and tubular membranes. This suggested that droplet formation mechanism was not shear stress-based. A spontaneous emulsification mechanism was proposed.

Water distribution and removal along the flow channel in proton exchange membrane fuel cells

Distribution expressions of total gas pressure and partial water vapor pressure along the channel direction were established based on lumped model by analyzing pressure loss in the channel and gas diffusion in the layer. The mechanism of droplet formation in the flow channel was also analyzed. Effects of the relative humidity, working temperature and stoichiometry on liquid water formation were discussed in detail. Moreover, the force equilibrium equation of the droplet in the flow channel was deduced, and the critical flow velocity for the water droplet removal was also addressed. The experimental results show that the threshold position of the liquid droplet is far from the inlet with the increase of temperature, and it decreases with the increase of the inlet total pressure. The critical flow velocity decreases with the increase of the radius and the working pressure.

High- and Low-Energy Emulsifications for Food Applications: A Focus on Process Parameters

High-energy emulsification is traditionally used to produce food-grade emulsions. However, energy input, design of the device and the type of surfactant should be carefully evaluated to achieve the desired emulsion properties. The low-energy processes, as spontaneous emulsification and phase inversion temperature, are alternative methods for producing systems with high stability and smaller particle sizes. Nevertheless, the surfactants and cosurfactants frequently used to produce emulsions from the low-energy process are not food grade or require a higher concentration than is allowed in food products. In this review, the characteristics of emulsions produced from low- and high-energy emulsifications, the mechanisms of droplet formation and their stability are reviewed with a particular focus on recent studies addressing the effects of process parameters on the properties of food emulsions. Knowing the principles and limitations of high- and low-energy processes, adequate process conditions and future trends are suggested depending on the system composition and the desired properties of the final product.

Mechanisms of droplet formation and Bi incorporation during molecular beam epitaxy of GaAsBi

We have examined the mechanisms of droplet formation and Bi incorporation during molecular beam epitaxy of GaAsBi. We consider the role of the transition from group-V-rich to group-III-rich conditions, i.e., the stoichiometry threshold, in the presence of Bi. For As-rich GaAsBi growth, Bi acts as a surfactant, leading to the formation of droplet-free GaAsBi films. For films within 10% of the stoichiometric GaAsBi growth regime, surface Ga droplets are observed. However, for Ga-rich GaAsBi growth, Bi acts as an anti-surfactant, inducing Ga-Bi droplet formation. We propose a growth mechanism based upon the growth-rate-dependence of the stoichiometry threshold for GaAsBi.