Drop Breakup Mechanisms Research Articles

Single drop breakup in turbulent flow

The breakup process of a single drop in homogeneous isotropic turbulence was studied using direct numerical simulations. A diffuse interface free energy lattice Boltzmann method was applied. The detailed visualization of the breakup process confirmed breakup mechanisms previously outlined such as initial, independent, and cascade breakups. High‐resolution simulations allowed to visualize another drop breakup mechanism, burst breakup, which occurs when the mother drop has a large volume, and the flow is highly turbulent. The simulations indicate that the type of the breakup mechanism is a strong function of mother drop size and energy input. Large mother drops in highly turbulent flow fields are more likely to burst, producing a large number of drops of the size close to the Kolmogorov length scale. Small drops in moderate turbulence tend to break only once (initial breakup). The interfacial energy of a drop was tracked as a function of time during drop deformation and breakage. The maximum energy level of the deformed mother drop was compared to commonly used estimates of critical energy necessary to break a drop. Our results show that these reference levels of critical energy are usually underestimated. Moreover, in some cases even if the critical energy level was exceeded, the drop did not break because the time of the interaction between the drop and the eddies was not enough to finish the breakup. The numerical insight presented here can be used as a guideline for the selection of assumptions and simplifications behind breakup kernels.

Secondary Breakup of Drops

Secondary atomization of droplets generated from primary atomization is observed in high-speed flows. In natural scenarios, falling raindrops undergo aerodynamic breakup that modifies the resulting drop size distribution on the ground. In this article, we review some aspects of the drop breakup mechanisms, initial deformation, drag characteristics, and time scales of breakup. We also review some of the secondary atomization models, such as TAB and DDB, proposed in the literature. We discuss the role of new numerical algorithms for two-phase flow simulations in providing insights into the exact physical mechanisms involved during drop breakup.

Efficient self-emulsification via cooling-heating cycles

In self-emulsification higher-energy micrometre and sub-micrometre oil droplets are spontaneously produced from larger ones and only a few such methods are known. They usually involve a one-time reduction in oil solubility in the continuous medium via changing temperature or solvents or a phase inversion in which the preferred curvature of the interfacial surfactant layer changes its sign. Here we harness narrow-range temperature cycling to cause repeated breakup of droplets to higher-energy states. We describe three drop breakup mechanisms that lead the drops to burst spontaneously into thousands of smaller droplets. One of these mechanisms includes the remarkable phenomenon of lipid crystal dewetting from its own melt. The method works with various oil–surfactant combinations and has several important advantages. It enables low surfactant emulsion formulations with temperature-sensitive compounds, is scalable to industrial emulsification and applicable to fabricating particulate drug carriers with desired size and shape.

Turbulent spectrum model for drop-breakup mechanisms in an inhomogeneous turbulent flow

Drop-breakup models for liquid/liquid dispersion in turbulent flow mostly derive from Kolmogorov and Hinze analysis. They are of a semi-empirical type, since they are based on power laws involving at least Weber and Reynolds numbers. The pre-factors are determined by fitting the experimental data on droplet diameters in the various configurations. The main cause for the discrepancies in the fitting constant between different flow types is the intrinsic spatial heterogeneity of the turbulent field, especially the turbulent kinetic energy (TKE) dissipation rate ε. This feature explains why there is no universal physical model suitable for breakup prediction throughout the range of flow geometries.In the present work, we investigate the drop size distribution by reference to Hinze's actual theory in a local approach, attempting a direct interpretation thanks to the turbulence spectra measured (by LDA) in the most dissipative locations of a flow which is basically inhomogeneous. This method allows estimation of droplet size with no constant fitting and with acceptable accuracy; however the knowledge of the turbulence field is required. The present study is carried out with low dispersed-phase fraction, so that the coalescence is negligible. Experiments show that the “typical value” for the TKE dissipation rate to fit the raw model of Hinze and Kolmogorov lies between the maximum and the mean value in the flow field. The issue of “typical ε value” hence avoided is discussed by physical arguments for the flow structure.

Scale-down failed – Dissimilarities between high-pressure homogenizers of different scales due to failed mechanistic matching

The high-pressure homogenizer (HPH) is used extensively in the processing of non-solid foods. Food researchers and producers use HPHs of different scales, from laboratory-scale (∼10 L/h) to the largest production-scale machines (∼50 000 L/h). Hence, the process design and interpretation of academic findings regarding industrial condition requires an understanding of differences between scales. This contribution uses theoretical calculations to compare the hydrodynamics of the different scales and interpret differences in the mechanism of drop-breakup.Results indicate substantial differences between HPHs of different scales. The laboratory-scale HPH operates in the laminar regime whereas the production-scale is in the fully turbulent regime. The smaller scale machines are also less prone to cavitation and differ in their pressure profiles. This suggest that the HPHs of different scales should be seen as principally different emulsification processes. Conclusions on the effect or functionality of a HPH can therefore not readily be translate between scales.

Nanoparticles change drop breakup mechanism to obtain icecream-like microparticles in inverse suspension polymerization

Nanoparticle modify particle shaped in suspension polymerization by changing drop breakup. (a) 0.4% initiator, (b) 0.05% initiator.

Tailored Nanostructured Thermoplastic Elastomers from Polypropylene and Fluoroelastomer: Morphology and Functional Properties

Conventional thermoplastic elastomer from rubber–plastic blends has rubber domains of a few micrometers dispersed in a plastic matrix. Nanotailoring of novel thermoplastic elastomeric blends from polypropylene (PP) and fluoroelastomer (FKM) by a special green technique generated a unique morphology in which FKM was dispersed in the nanometer range (50–70 nm) into the continuous PP matrix. The evolution of the unique nanostructure morphology was explained with the help of a viscoelastic drop breakup mechanism. The nanostructured blends exhibited superior physical properties and outstanding oil swelling resistance and thermal stability, which increased upon vulcanization of the rubber phase in situ. For example, volume swell of nanostructured PP/FKM blends was 8–10% in ASTM oil #3 at 100 °C for 72 h, which was the lowest in this type of thermoplastic elastomer. Insights into how the unique morphology influenced the functional properties of thermoplastic elastomers was provided. These new thermoplastic elastomers can be used in the automotive industry, particularly as a liner material for car hoses where superior flexibility, oil resistance, and heat resistance are desirable properties.

Production of W/O/W double emulsions. Part II: Influence of emulsification device on release of water by coalescence

We compare different emulsification devices for the production of W/O/W double emulsions. Rotor-stator devices (colloid mill and tooth rim dispersing machine), high pressure homogenization (standard and modified process) as well as a rotating membrane system were used. For better comparability, we produce double emulsions of different oil drop sizes with each device by changing process parameters. A standard double emulsion recipe was chosen for all emulsions.Encapsulation efficiency of water in the double emulsions, measured by DSC technique, is found to be directly dependent on oil drop size of the double emulsions for each emulsification device. The bigger the oil drops, the higher is the encapsulation efficiency. Double emulsions with comparable oil drop sizes produced with different devices show more or less the same encapsulation efficiency.This finding indicates that coalescence is mainly influenced by geometrical parameters like oil drop size. The differing drop breakup mechanisms in the different emulsification devices do not show any influence on encapsulation efficiency. It can thus be concluded that loss of inner water droplets by coalescence mainly takes place after the production process. The choice of the emulsification device does not directly influence encapsulation efficiency. Conventional high shear devices are as suitable for the production of double emulsions as membrane processes.

Influence of Physical Properties and Process Conditions on Entrainment Behavior in a Static-Mixer Settler Setup

This paper examines the role of physical properties (interfacial tension, the viscosity of the dispersed and continuous phase, density difference, and solubility), and process conditions (flow rate, phase ratio, and temperature) on the drop size and entrainment in a static-mixer settler setup. Two extraction systems were investigated, that is, caprolactam–toluene–water and ethylbenzene (EB)−α-methyl benzyl alcohol (MBA)–water/NaOH (pH = 12). Depending on the system, the entrainment increased by a factor of 5–7, because of a change in the physical properties induced by changing the concentration of caprolactam and MBA. Different entrainment trends were observed for the two systems with phase ratio and temperature. The differences are explained by the drop break-up mechanisms reported in literature. The phase ratio effect resulted from on one hand turbulent attenuation and increased coalescence, and on the other hand increased shear and hindered settling at high phase ratios. The temperature influence was determined by its counteracting effects on the interfacial tension and the viscosities of continuous and dispersed phases.

The effect of interfacial slip on the dynamics of a drop in flow: Part I. Stretching, relaxation, and breakup

Using a numerical method based on the boundary-integral technique, we assess the impact of interfacial slip on the dynamics of deformation and breakup of a single drop subjected to a uniaxial extensional flow under creeping-flow conditions. Interfacial slip is incorporated in our continuum development as a jump in the tangential velocity across the interface. This velocity jump is shown to reduce to the Navier-slip boundary condition to leading order and is characterized by a dimensionless slip coefficient α=(dI/μI)(μ/R), where dI is thickness of the diffuse interface between the liquids, μI is the viscosity of the interfacial region, μ is the viscosity of the suspending fluid, and R is the drop radius. A key contribution of this paper is the development of a stable, boundary-integral formulation to incorporate interfacial slip into existing, no-slip boundary-integral frameworks for drop deformation. Slip has a fourfold impact on the drop stretching, relaxation, and breakup phenomena. First, when the capillary number is small, the steady deformation of the drop with slip is smaller than the no-slip result, and the difference increases with the viscosity ratio and the capillary number. Slip thus leads to larger critical capillary numbers beyond which the drop stretches continuously in the extensional flow. Second, for capillary numbers greater than the critical value, we find that the shape of the deformed drop for the same drop elongation is relatively insensitive to the slip coefficient, but the time required to reach this deformation is a strong function of the slip coefficient—slip slows down the deformation process. Third, the end-pinch mechanism of drop breakup leads to a different number and sizes of droplets with the inclusion of slip. Finally, slip causes the capillary-instability mechanism of drop breakup to produce larger drops at faster rates relative to the no-slip case. In addition to the above results, we also show that slip moderates the viscosity and normal stress differences in a sheared, dilute emulsion. Our study has important implications in the area of blending of immiscible polymers and indicates that the drop size distribution, which ultimately governs the material properties of the blend and composites prepared from it, is influenced strongly by interfacial slip.

Investigation of viscosity effect on droplet formation in T-shaped microchannels by numerical and analytical methods

Both numerical and analytical models have been developed to explore the viscosity effect of the continuous phase on drop formation at a T-shaped junction in immiscible liquids. The effects of the generalized power law coefficient, the power law exponent and the yield stress on the mechanism of drop breakup, final drop size and frequency of drop formation are studied by using the numerical three-dimensional volume of fluid model. Droplets coalescence in Bingham fluids is observed in the beginning transient period. The effect of yield stress on drop extension is also discussed. Predictions of drop size by using an analytical force balance show satisfactory agreement with simulation results for Newtonian and power law fluids with different viscosity ratios. The approximation error associated with the analytical model for Bingham fluids is also acceptable. This analytical model can greatly shorten the prediction time as compared with the numerical model, which is helpful for on-line control.

Drop Breakup in an SMX Static Mixer in Laminar Flow

The mechanism of drop breakup inside SMX static mixers in the laminar flow regime was studied using experimental observations and computational fluid dynamics (CFD). The deformation and breakup of a single drop was simulated using the volume of fluid (VOF) model. It was observed that drops break up after collision with the leading edges and cross-points of the bars in the SMX static mixer. It was found that drop collision with the bar cross-points of the SMX static mixer elements is most effective for drop breakup. Elongation and folding result in drop breakup at the cross-points. Le mécanisme de rupture de gouttes dans les mélangeurs statiques SMX en régime d'écoulement laminaire a été étudié par des observations expérimentales et par simulation numérique des écoulements (CFD). La déformation et la rupture d'une goutte unique ont été simulées avec le modèle du volume de fluide (VOF). On a observé que les gouttes se séparaient lors de la collision avec les bords d'attaque et les points d'intersection des barres dans le mélangeur statique SMX. On a trouvé que la collision des gouttes avec les points d'intersection des barres des éléments de mélangeur statique SMX était plus efficace pour la rupture des gouttes. L'élongation et le pliage entraînent la rupture des gouttes aux points d'intersection.

Does drop size affect the mechanism of viscoelastic drop breakup?

We studied the breakup of viscoelastic drops in Newtonian liquid matrices undergoing simple shear flow. The coexistence of different breakup modes was observed. The drop size determines, to a great extent, the type of drop breakup mechanism and the critical point when the mechanism changes. Small drops break along the vorticity direction, which is the direction perpendicular to the flow direction and the velocity gradient direction, whereas large drops break in the flow direction. The change in mechanism abruptly occurs as the drop size reduces. There is a big jump in the critical shear rate when the mechanism changes from breakup in the flow direction to breakup in the vorticity direction.

Drop breakup in dilute Newtonian emulsions in simple shear flow: New drop breakup mechanisms

We investigate drop breakup in dilute Newtonian emulsions in simple shear flow using high-speed video microscopy over a wide range of viscosity ratio (0.0017 2Cac, breakup dynamics are strongly controlled by λ. For 0.1<λ<1, capillary instability is the dominant drop breakup mechanism, and thread radius and wavelength at breakup are independent of the initial drop sizes. Fairly monodisperse emulsions are obtained, and the average drop size is inversely proportional to the shear rate. For 1.0<λ<3.5, long wavelength capillary instability generates large satellite drops, resulting in emulsions with bimodal distribution. For λ<0.1, a new drop re-breaking mechanism is observed, producing polydisperse emulsions. The polydisp...

Erosion and breakup of polymer drops under simple shear in high viscosity ratio systems

AbstractThe deformation and breakup of a single polycarbonate (PC) drop in a polyethylene (PE) matrix were studied at high temperatures under simple shear flow using a specially designed transparent Couette device. Two main breakup modes were observed: (a) erosion from the surface of the drop in the form of thin ribbons and streams of droplets and (b) drop elogation and drop breakup along the axis perpendicular to the velocity direction. This is the first time drop breakup mechanism (a), “erosion,” has been visualized in polymer systems. The breakup occurs even when the viscosity ratio (ηr) is greater than 3.5. although it has been reported that breakup is impossible at these high viscosity ratios in Newtonian systems. The breakup of a polymer drop in a polymer matrix cannot be described by Capillary number and viscosity ratio only; it is also controlled by shear rate, temperature, elasticity and other polymer blending parameters. A pseudo first order decay model was used to describe the erosion phenomenon and it fits the experimental data well.

Polymer Blends in Co-Rotating Twin-Screw Extruders

Abstract The properties of polymer blends are determined to a decisive extent by the morphological structure of the polymer combinations employed. The design of extruders thus calls for models to calculate the estimated morphology development over the length of the extruder screws in the melt-conveying section. Since the most significant morphological changes are observed in the melting section, however, it is also necessary for the morphology formation and development to be analyzed in this section of the extruder. The melting process of binary material combinations is thus important too. In the context of this research, experimental investigations were conducted using polypropylene/polyamide 6 (PP/PA 6). In the tests, the degree of melting and the morphology development were determined over different screws and compared with calculations. In order to analyze a range of relevant influences, the extruder size, screw configuration, screw speed, weight components and also the viscosity ratio were varied by using different PP types. Apart from the model for calculating the melting of polymer blends, a formulation was developed that can be used to estimate the morphological changes occurring in the melt-conveying section. The model is based on the assumption that morphological changes can be estimated by calculating the probabilities of different drop breakup mechanisms and the coalescence process. The investigations of the blend morphology in the melting section and the melt-conveying section reveal key findings that have to be taken into account for modeling the formation and development of the morphology. First of all, in the melting section, it is very clear that a kind of melt film removal occurs at the surface of the granules of the second component, which melts at higher temperatures, as these granules melt. The drops of second component in the melting section, which are directly adjacent to fractions that have not yet fully melted in some cases, have already assumed dimensions (in the μm range) similar to those seen at the end of the extrusion process. This means that, in the melting section of the twin-screw extruder, small volumes are broken or worn off the already-molten granule surfaces. An evaluation of scanning electron micrographs also shows that, in the melting section of co-rotating twin-screw extruders, virtually all the breakup mechanisms that can essentially be distinguished take place in parallel, such as quasi-steady drop breakup or supercritical breakup, folding, end pinching, tip streaming and breakup through capillary instabilities. Alongside the breakup mechanisms, there are also drops that clearly unite to form bigger drops through coalescence. When comparing the calculations for the melting of polymer blends, relatively good agreement is obtained with the experimental test results. The calculations display a satisfactory level of accuracy, particularly for polymer combinations with similar viscosities and also for bigger extruders. The calculations with the morphology model also show the same trends as the experimental investigations. Hence, for the design or optimization of twin-screw extruders, it is now possible not only to calculate the fundamental process variables (such as pressure, temperature, melting) but also to estimate the morphology that has a decisive influence on the resultant material properties.

Characterisation of charged hydrocarbon sprays for application in combustion systems

Phase Doppler anemometry measurements and flow visualizations are used to measure the structures of electrostatically atomized hydrocarbon fuel sprays, produced by charge injection nozzles. Due to the jet and drop breakup mechanisms that occur for electrostatically charged insulating liquids, these sprays contain relatively large numbers of small drops which are repelled away from the spray core region where the radial electric field component is high. The largest drops remain near the spray centreline and a radial stratification of the average diameter occurs, which can be advantageous for flame stabilisation. Droplet size reduces with increasing specific charge for the spray. Higher values of specific charge are obtained for reduction of orifice diameter, optimum positioning of the nozzle electrode, and increasing liquid flow rate. On the basis of the measurements, descriptions are given of the physics of processes both inside the atomizer and in the spray and the importance of operating the atomizing nozzle at electrohydrodynamically supercritical conditions is described.

Computer Modeling of Diesel Spray Atomization and Combustion

A multi-dimensional code has been developed to study the effects of injection pressure and nozzle hole inlet conditions on diesel engine performance and emissions. The code includes a new liquid core and spray breakup model. The models were validated using spray visualization images obtained from a single-cylinder version of the Caterpillar 3406 heavy-duty truck engine instrumented with an endoscope system. The computational results were also compared with experimental emissions data. The results show that combustion and emissions predictions are controlled by the details of the spray model. With modifications to the spray model to account for Rayleigh-Taylor and Kelvin-Helmholtz drop breakup mechanisms, the predicted liquid and vapor-fuel penetration agrees well with that measured. The models were applied over a wide range of engine operating conditions and were found to provide good prediction accuracy. The simulations also showed that sharp edged-inlet nozzles give significantly lower particulate emissions than rounded-inlet nozzles with the same rate-of-injection profile as has been seen experimentally.

Study of breakup mechanisms in cavity flow

AbstractDrop breakup mechanisms inside a cavity flow are presented for two immiscible fluids. Due to the nonuniform flow condition of the cavity, the breakup mechanism varied along the streamlines. The streamlines were characterized by stream zones A and B, where zone A possessed a methodical transient breakup governed by Tomotika's breakup via capillary instabilities, and the breakup mechanism of stream zone B consisted of tip streaming breakup, an inefficient breakup mechanism. The flow behavior near flight region had a significant role in the drop breakup mechanisms. The study of the evolution of drop dispersion showed that the matrix viscosity is critical in controlling the transient breakup process and that the shear rate increase had little or no effect on the drop breakup.

Development of polymer blend morphology during compounding in a twin‐screw extruder. Part II: Theoretical derivations

AbstractIn Part II of the work, the intermeshing twin‐screw extruder is briefly described and the theoretical procedures used to model its operation are summarized. Based on the microrheological considerations discussed in Part I, a predictive procedure of the morphology evolution during compounding of two immiscible polymers is proposed. In this first generation model, only the shear flow effects are considered. Furthermore, to avoid complications due to coalescence a low concentration of the dispersed phase was assumed. In the procedure, two drop breakup mechanisms are discussed. The first assumes that the drops do not break under flow while the second postulates that breakup occurs under flow. Two dispersion mechanisms are considered, the first postulating continuously increasing polydispersity of drop size and the second postulating that drop polydispersity is inversely proportional to deformation strain. The influence of the screw configuration and operating conditions on blend morphology evolution is studied. It is expected that the computed drop size distribution provides limiting values for the experimental data. Dependency of predicted morphology on operating conditions is also investigated. Increasing screw rotating speed (resulting in increasing energy consumption) and decreasing throughput (resulting in decreasing productivity) lead to prediction of finer drop size. In practice, therefore, a compromise would be required. The proposed procedure is limited to melt flow (excluding the die region) within the region of large capillary parameter values, k &gt; 4kcrit.