Composite Droplets Research Articles

Analytical Model of Microexplosion of a Hydrocarbon–Water Composite Droplet

Analytical Model of Microexplosion of a Hydrocarbon–Water Composite Droplet

Enhanced control and efficiency in millimeter-scale composite droplets preparation

A microfluidic device utilising the flow focusing method was designed and fabricated for the high-throughput and controllable preparation of millimeter-scale water-in-oil (W1/O/W2) droplets. Two distinct flow focusing modes were employed in order to prepare the composite droplets. In the flow-focused axisymmetric jetting mode, the continuous phase propels the dispersed phase, forming a cone, which subsequently leads to a jet formation downstream of the small hole. When the outer phase is activated, perturbation growth interferes with downstream fragmentation, resulting in composite droplets with excellent monodispersity. In the dripping mode, the dispersed phase fails to form a jet downstream, and the interface disintegrates, resulting in droplet formation at the exit of the small orifice. In the jetting mode, the size and shell-to-core ratio of the generated droplets can be controlled by adjusting the volume of flow of the outer and inner phases, as well as the applied frequency. Furthermore, the applied perturbation allows for precise control of droplet size, which significantly reduces satellite droplet formation and enhances droplet monodispersity. In the dripping mode, the shell-to-core ratio is adjusted by manipulating the ratio of inner to outer flow rates, which is tailored to the specific requirements of the flow-focused dripping mode. The prepared compound droplets exhibit good monodispersity. Finally, the rotational solidification method can be employed to obtain spherical shells with a shell thickness of less than 20 μm. In conclusion, the flow focusing method is an effective, controllable, and stable method for synthesising millimetre-sized composite droplets.

Composite liquids under high-power heating: superheat of water in micro-explosion of water-in-fuel droplets

Abstract The article analyses the degree of water superheating with respect to the liquid-vapour equilibrium line in experiments on the micro-explosion of a composite droplet comprised of two immiscible liquids. The analyses were carried out for water-in-fuel drops under conditions of high-power heating. This degree is compared with the mechanical effect of droplet decay, involving the formation of daughter droplets. Our attention was drawn to the smallness of the degree of superheating preceding the decay. A model of the boiling up of such a droplet is constructed taking into account the sources of premature boiling up of water inherent in micro-explosive experiments. The dependencies of the boiling up temperature of water on the heating rate obtained in the model turned out to be in accordance with the experimental data across a wide range of heating rates. A hypothesis about the local superheating of the transition layer, which is not detected in the experiment, is formulated. Thus, a step has been taken to clarify the essence of the mismatch of the degree of superheating of water recorded by macroscopic equipment along with a completely satisfactory generation of daughter droplets serving as the basis for advanced fuel technology.

Puffing/micro-explosion of two-liquid droplets: Effect of fuel shell composition

Theoretical research into the heat and mass transfer, hydrodynamic and physicochemical processes in combustion chambers of gas turbine engines usually implies that multi-component jet fuels are modeled using single-component liquids (saturated or cyclic hydrocarbons) and their substitutes. Due to an insoluble dispersed phase (e.g., water) in their composition, droplets consist of a noncombustible core and a liquid fuel shell. During heating, water droplets coalesce in fuel droplets to produce explosion-triggering volumes of liquid superheated to the boiling point. When heated, these heterogeneous droplets breakup in the micro-explosion and puffing modes. This study reports the numerical simulation results providing the temporal characteristics of heating and evaporation of heterogeneous droplets until puffing/micro-explosive breakup, when varying the composition of the fuel shell in the homologous series of saturated and cyclic (as illustrated by monocycloparaffins) hydrocarbons from C7 to C16. The conducted research has revealed that the variations in the breakup delay times in the homologous series of saturated and cyclic hydrocarbons are nonlinear. The breakup delay rates were found to increase substantially in the boundary points of the investigated series. Mechanisms to control droplet fragmentation delay time were identified for different initial and boundary conditions. A dimensionless complex reflecting the correlation between the critical conditions of composite liquid droplet breakup and the physicochemical properties of the fuel shell components was proposed.

Analysis of the impact dynamics of water-in-oil composite droplets on metal plates

Composite droplets composed of immiscible liquids often have unique applications in many situations, where splashing and recoil of the composite droplet upon impact with plane surfaces are important. Due to the different properties of the constituent liquids, composite droplets will have different flow morphology than single-phase droplets. This research employs an injection method to generate composite droplets and investigates the impact dynamics of these droplets on a horizontal copper plate at room temperature using high-speed cameras. Four impact heights (H = 10 cm, 15 cm, 20 cm, 25 cm) and seven water volume fractions (α = 0.0625, 0.125, 0.1875, 0.25, 0.3125, 0.375, 0.4375) were considered. The results show that the double squeezing of the oil phase by the water phase and the copper plate is the main reason for the droplet splashing phenomenon. The degree of droplet splashing and the probability of the droplet generating a satellite droplet are directly proportional to the impact velocity. When the impact height is constant, the extent of splashing decreases with an increase in α. Additionally, the maximum value of the jetting height for composite droplets is mainly influenced by the volume of satellite droplets during the recoil process. Overall, the maximum relative jetting height of composite droplets shows an initial increasing trend followed by a decreasing trend with an increase in α. At α = 0.1875 and 0.25, there is a negative correlation between the relative jetting height and impact height. This implies that the maximum value of the relative jetting height is primarily controlled by the volume of the satellite droplets during the recoil process.

A new experimental method for studying the single droplet breakup mode of diesel-water composite droplet fuel

Good atomization of the liquid phase fuel is a prerequisite for efficient combustion and micro-explosion can significantly increase the atomization. In order to study the influence of water content and ambient temperature on the breakup mode and combustion behavior of diesel-water composite droplet, a novel experimental method was proposed. Specifically, the required volume of water was placed at the end of the probe, and diesel was then dropped onto the outer surface of the water by free diffusion to form water-in-oil composite droplet. The experimental results indicated that compared with the combustion of pure diesel, the diesel-water composite droplet additionally experienced a fluctuating combustion stage, in which micro-explosion and puffing occurred. Water content and ambient temperature have dual effects on the breakup mode and combustion characteristics of composite droplet. When the water content exceeded 20%, the breakup mode of the droplet was converted from micro-explosion to puffing, which reduced the atomization performance of the fuel. The ignition time of the composite droplet was 50–60% lower than that of pure diesel, and the burnout time decreased by about 35%. The breakup mode of the droplet and the critical size results indicate that 10%water/90%diesel at 973 K is most conducive to combustion.

A COMBINED ANALYTICAL/NUMERICAL APPROACH TO THE MODELING OF THE PROCESSES LEADING TO PUFFING AND MICRO-EXPLOSION IN A COMPOSITE MULTI-COMPONENT FUEL/WATER DROPLET

The previously developed analytical/numerical model for predicting heat transfer and component diffusion in composite multi-component droplets is adjusted for use in practical engineering applications related to the analysis of droplet heating and evaporation and the onset of puffing and micro-explosions in those droplets. This adjustment allowed us to gain new insights into the previously developed models of these processes. The focus of the analysis is on kerosene/water droplets. It is demonstrated that the number of terms in the series in the analytical solution to the heat transfer equation can be reduced to just one or two to ensure that the maximal error of the model prediction does not exceed 1%, unless we are interested in the processes at the very start of heating. At the same time, the minimal number of terms in the series in the analytical solution to the component diffusion equation should be at least seven to ensure that the errors of the prediction of the numerical code do not exceed 3%. It is shown that, to ensure that the analytical/numerical code predicts physically consistent results, the maximal absolute error of calculation of the eigenvalues based on the bisection method cannot exceed 10<sup>-7</sup>. It is shown that using these limiting values for each of these input parameters leads to about 50%-75% reduction in the CPU time required to obtain results close to those which were obtained using the nonoptimized version of the numerical code. The overall reduction in CPU time can be up to about 95%. The predictions of the adjusted analytical/numerical code are validated against in-house experimental data and data available in the literature.

Controllable generation of core-shell composite droplets in a conical inlet double-focus microchannel

In this paper, a double-focused conical inlet axisymmetric structure with double acceleration is proposed and the evolution of composite core-shell droplets is calculated using the volume of fluid (VOF) method, focusing on the size and droplet formation frequency of composite core-shell droplets as well as the quality control method. The results show that dripping and jetting regimes occur in the channel, and the generation period and flow distribution under the two mechanisms are investigated accordingly. The effect of local geometry on the evolution of composite droplets in the core-shell is investigated and analyzed using the composite droplet size, frequency, maximum extended length, and core droplet relative offset rate at the initial stage. It is found that changing the entrance angle can achieve the regulation of droplet size to a certain extent. With the increase of dimensionless width of the collection channel, the core-shell composite droplets size shows an overall increasing trend and is inversely proportional to the outer phase capillary number. Finally, a quality control method for composite droplet formation based on the maximum relative offset rate of core droplets is proposed for the purpose of controlling the integrity of composite droplets.

Self-propulsion of an elliptical phoretic disk emitting solute uniformly

Self-propulsion of chemically active droplets and phoretic disks has been studied widely; however, most research overlooks the influence of disk shape on swimming dynamics. Inspired by experimentally observed prolate composite droplets and elliptical camphor disks, we employ simulations to investigate the phoretic dynamics of an elliptical disk that emits solutes uniformly in the creeping flow regime. By varying the disk's eccentricity $e$ and the Péclet number $Pe$ , we distinguish five disk behaviours: stationary, steady, orbiting, periodic and chaotic. We perform a linear stability analysis (LSA) to predict the onset of instability and the most unstable eigenmode when a stationary disk transitions spontaneously to steady self-propulsion. In addition to the LSA, we use an alternative approach to determine the perturbation growth rate, illustrating the competing roles of advection and diffusion. The steady motion features a transition from a puller-type to a neutral-type swimmer as $Pe$ increases, which occurs as a bimodal concentration profile at the disk surface shifts to a polarized solute distribution, driven by convective solute transport. An elliptical disk achieves an orbiting motion through a chiral symmetry-breaking instability, wherein it repeatedly follows a circular path while simultaneously rotating. The periodic swinging motion, emerging from a steady motion via a supercritical Hopf bifurcation, is characterized by a wave-like trajectory. We uncover a transition from normal diffusion to superdiffusion as eccentricity $e$ increases, corresponding to a random-walking circular disk and a ballistically swimming elliptical counterpart, respectively.

Droplet Evaporation Process of a Fluorobenzene + n-Octane + Polystyrene Mixture.

The vapor-liquid equilibrium of the fluorobenzene-polystyrene binary polymer solution at 303.15 K was measured using a static pressure device. The vapor-liquid equilibrium of the fluorobenzene-n-octane-polystyrene ternary solution in a partial concentration range under normal pressure was determined using an improved Othmer equilibrium still, in which the octane concentration was low. Three activity coefficient models, poly-NRTL, UNIQUAC, and M-UNIQUAC-LBY, were utilized to correlate the experimental data of binary and ternary solutions, and the component activities of the fluorobenzene-n-octane-polystyrene solution at 303.15 K were predicted. A mathematical model based on the Stefan flow was developed to simulate the evaporation process of composite spherical droplets. The activity predicted by the activity coefficient model was used for numerical simulations, and compared with simulations using the activity following Raoult's law. The comparative analysis revealed that simulations based on Raoult's law and activity coefficient models yielded similar results when the mass fraction of fluorobenzene exceeded 0.6. However, in the later stages of evaporation, the calculations based on Raoult's law predicted a 10% shorter drying time for fluorobenzene. The activity coefficient models provided a better approximation and exhibited similar droplet diameter shrinking behaviors to the actual evaporation process.

Pattern-Distortion Technique: Using Liquid-Lens Magnification to Extract Volumes of Individual Droplets or Bubbles within Evaporating Two-Dimensional Arrays

We present an experimental ``pattern-distortion'' (PD) technique which connects the shape of a liquid lens to its magnification. We demonstrate how to optimize the technique for arbitrary droplet sizes and optical configurations, and demonstrate its widespread utility in three distinct situations. Firstly, we consider multiple sessile droplets. Although ubiquitous in nature, understanding of their complex interactions is limited, partly due to experimental limitations in determining individual droplet volumes for arbitrary configurations. We use the PD technique to overcome these limitations, and we find excellent agreement between our experimental data and three recent theoretical models. Secondly, we show how our technique can be used to inform the design of liquid lenses. Thirdly, we extend the method to composite droplets systems, using it to extract the size of an air bubble trapped inside a liquid droplet.

Tribological characteristics of nanofluid composite electrostatic spraying (NCES) in sliding friction

As a new improvement technology of minimum quantity lubrication (MQL), the tribological properties of nanofluid composite electrostatic spraying (NCES) are highly affected by type of base oil and adding location of nanoparticles. This work focused on the tribological characteristics of NCES with different base oils and adding locations (base oil, water, and both base oil and water) of muitiwall carbon nanotube (MWCNT) by sliding friction experiments with cemented carbide sheet and aluminum disc as the friction pair on the tribotest setup developed on the basis of lathe. The electrowettability of composite droplet was investigated and the effect mechanism of base oil type and adding location of MWCNT on tribological properties of NCES was discussed. The findings demonstrated that compared to NCES with castor oil as base oil, NCES with sunflower oil as base oil lowered the coefficient of friction and wear amount of cemented carbide sheet by 18.8–29.2% and 4.6–12.5%, respectively. In terms of tribological properties of NCES, the best adding location of MWCNT was both base oil and water, followed by base oil. NCES with accession of MWCNT in both sunflower oil and water exhibited the best tribological properties.

Flow-induced transition of compound droplet to composite microfiber in a channel with sudden contraction

The deformation behavior and hydrodynamic stability of a three-dimensional Newtonian single-core compound droplet during flow in a channel with sudden contraction were studied by numerical modeling. This research was motivated by the quest for conditions of the steady transition of a compound droplet into a composite microfiber, whose core is stretched as much as the shell. With this aim, the dynamics and morphology evolution of the compound droplet were analyzed in detail as functions of capillary number, core-to-shell relative viscosities, interfacial tensions, and the relative initial core radius. It was found that the effective elongation of the core occurs either with a significant increase in the shell viscosity relative to the ambient fluid or with a decrease in the core viscosity with respect to the shell. In this case, as the composite droplet advances into the narrowing zone of the canal, it continues to stretch, becoming a bullet-shaped composite microfiber. A new mechanism of disintegration of the compound droplet was revealed, which is caused by the core destabilizing effect and manifests itself either with an increase in the relative core/shell interfacial tension or the relative core viscosity.

Nucleation and bubble growth during puffing and micro-explosions in composite droplets

Heating of droplets composed of water and fuel is known to lead to internal nucleation and bubble growth that can eventually lead to their puffing and to micro-explosions. The time to puffing/micro-explosions includes times spent on: heating (time to nucleationPleasestriketheSymbolt_Nfromtheabstract), bubble growth Pleasestrike:tgrfromtheabstract. In the present paper, we examine the effect of different aspects of bubble growth on the puffing and micro-explosions. Specifically, we address the effects of nucleation temperature and the relative positions of the inner water sub-droplet and the bubble within it. The nucleation temperature of the water sub-droplet is higher than its normal boiling temperature yet lower than its spinodal temperature in most realistic cases. The degree of superheating and the nucleation time depend on the heating rate and the nucleation site density. Higher nucleation temperatures imply larger driving force for the bubble growth. Bubble growth rate is dominated by the degree of superheating, while growth time is dominated by both the degree of superheating and the location of the bubble with respect to the inner and outer interfaces of the composite droplet. It is found that the inertial bubble growth regime is dominant for micron-sized droplets, and thus sensitivity to the modelling of the inertial regime can be of crucial importance to the evaluation of the breakup time for the droplets. The model for puffing and micro-explosion presented in the paper considers an isolated bubble growing at the water/fuel interface at various degrees of superheating, and for a wide range of Jakob numbers. This analysis allows us to assess the sensitivity of bubble growth time to the initial bubble location, and to generalise the previously developed model of the phenomenon taking into account the effect of finite time of bubble growth during the development of puffing/micro-explosion.

Spatial Heterogeneity of Aerosol Effect on Liquid Cloud Microphysical Properties in the Warm Season Over Tibetan Plateau

AbstractThe effect of aerosol on liquid cloud microphysical properties over the Tibetan Plateau (TP) during the warm season is investigated by using aerosol index (AI) and cloud property parameters data. Distinct differences in aerosol effect on liquid cloud microphysical properties have been found between the northern Tibetan Plateau (NTP) and southernTibetan Plateau (STP). The composite liquid cloud droplet effective radius liquid cloud droplet effective radius (LREF) anomalies for positive AI events are positive in the NTP and negative in the STP. In both NTP and STP, when the AI anomalies are positive, the LREF anomalies are also positive, which suggests that the increased aerosol loading reduces the solar radiation reaching the ground and thus enhances the atmospheric stability, which reduces the cloud base height and makes the liquid cloud area thicker, which gives cloud droplets more space to grow by collision‐coalescence. This indicates that the aerosol radiative effect is not likely the reason causing the distinct differences of aerosol effects on liquid cloud properties between NTP and STP. Further analysis shows that in the STP, the LREF first increases and then decreases with the increase of AI, while in the NTP, the LREF always increases with the increase of AI, suggesting a spatial difference in aerosol microphysical effect. In the STP, the influence of aerosol on liquid clouds is mainly dependent on liquid water path and convective available potential energy, while in the NTP, the influence of aerosol on liquid cloud is more likely related to large aerosol particles.

Micro-Explosion Phenomenon: Conditions and Benefits

Adding water to fuel droplets is known to lead to puffing and micro-explosion. Puffing and micro-explosion lead to a rapid increase in the liquid fuel surface area. This, in turn, leads to an increase in the fuel evaporation rate and the formation of a homogeneous fuel vapor/air mixture. The latter is important for improving the efficiency of combustion technologies, including those used in internal combustion engines. The effects produced by puffing and micro-explosion lead to a reduction in fuel consumption, improved fuel/air mixing, and a reduction in harmful emissions. The contributions of puffing and micro-explosion to fire extinguishing have also been discussed in many papers. In this paper, we review the state of the art in the investigation of composite droplet micro-explosion and discuss the sufficient conditions for the start of puffing/micro-explosion as well as child droplet characteristics.

Computational Fluid Dynamics (CFD) Modelling of ZnO-SiO2 Composite Through a Consecutive Electrospray and Spray Drying Method

Modelling of the droplet formation and drying process in the synthesis of Zinc Oxide-Silicon Dioxide (ZnO-SiO2) composite have been conducted through a CFD modelling. In general, modelling of spray drying only focused on exploring the drying chamber section. The phenomenon builds in a consecutive electrospray and spray drying method has been successfully studied in this paper. The influence of carrier gas flow rate (2 to 10 L/min), precursor flow rate (1 to 10 ml/h), drying chamber temperature and applied voltage (12 to 14 kV) were investigated systematically. Numerical modelling was conducted to describe the mechanism of the composite droplet formation through the jet shape of the electrospray. Evaporation of the initial composite droplet was considered in the modelling, accounting for its size evolution. Simultaneous mass transfer modelling due to the composite evaporation was solved in combination with a general dynamic equation solution. The modelling results show that the applied voltage and the precursor flow rate effectively affected the composite droplet size. While the carrier gas flow rate and the drying chamber temperature, influenced the effectiveness of the composite particle formation in the spray drying synthesis.

Collective effects during the formation of child droplets as a result of puffing/micro-explosion of composite droplets

Puffing and micro-explosion are among the major phenomena behind the industrial secondary atomization of composite droplets. These effects significantly reduce the average size of secondary droplets formed during the jet breakup to 5–10 times of the original droplet size in the case of puffing and to 100–200 times in the case of micro-explosion. The initial droplet sizes (radii) ranged from 0.5 to 1.5 mm. Here we present the research findings on the collective effects during the formation of child droplets as a result of puffing/micro-explosion of composite droplets. We have analyzed the characteristics of child droplets formed during the micro-explosive fragmentation of a group of three composite droplets. We used two fuel blends: 90 vol% of Diesel fuel, 10 vol% of water; 10 vol% of Diesel fuel, 90 vol% of water. Typical sizes of child droplets formed during the fragmentation of each droplet in a group were determined by shadowgraphy. The number and size of secondary fragments practically do not change when the distances between parent droplets do not exceed 8–10 radii. In these conditions, the integral fragmentation characteristics of a group of droplets are in acceptable agreement with those of isolated droplets. With shorter distances between droplets, there are considerable differences in the characteristics of secondary droplets formed during the puffing/micro-explosion of composite droplets. Conditions were recorded in which puffing or micro-explosion can occur as a result of collisions between secondary fragments and parent droplets.

Coacervate Formed by an ATP-Binding Aptamer and Its Dynamic Behavior under Nonequilibrium Conditions.

Although numerous protocell models have been developed to explore the possible pathway of the origin of life on the early earth, few truly fulfill the roles of the DNA/RNA sequence and ATP molecules, which are keys to cell replication and evolution. The ATP-binding aptamer offers an opportunity to combine sequence and energy molecules. In this work, we choose the coacervate droplet as the protocell model and investigate the interaction of the DNA aptamer, poly(l-lysine)(PLL), and ATP under varying conditions. PLL and aptamers form solid precipitates, which spontaneously transform to coacervate droplets as ATP is introduced. The selective uptake and sequestration of exogenous molecules is achieved by the ATP-containing coacervates. As an electric field is applied to expel ATP, the portion of the droplet deficient in ATP becomes solid. The solid/liquid phase ratio is tunable by varying the electric field strength and excitation time. Together with the vacuolization process, a solid head with a soft mouth periodically opening and closing is created. Moreover, the composite coacervate droplet gradually grows as it is treated with an electric field and cannot recover to the original liquid phase after the power is turned off and replenished with ATP. Our work highlights that the proper integration of the DNA sequence, ATP, and energy input could be a powerful strategy for creating a protocell with certain living features.

Ratio of water/fuel concentration in a group of composite droplets on high-temperature heating

The paper presents the experimental research findings on the water/Diesel fuel concentration ratio in a group of composite droplets after passing through a heated gaseous medium. The research was carried out using the optical methods of Planar Laser Induced Fluorescence and Shadow Photography. The experimental method involved measurements carried out while a group of composite droplets (water/Diesel fuel) was free-falling through a tubular muffle furnace. The findings have shown combined effects of droplets on the water and Diesel fuel concentration ratio at the furnace outlet. We have also shown how droplet collisions with each other and with a heated chamber wall affected the number and size of droplets of water and combustible liquid as they left the high-temperature zone. The limiting temperatures of the gaseous medium have been determined that are sufficient for the breakup of a group of composite droplets free-falling through a fixed-length operating section with heated gases. We have also calculated the required heating time and the length of the channel with a heated gas, so that parent composite drops break up into individual droplets of water and a combustible liquid. The numbers and sizes of fragments with different component compositions after passing through a heated gaseous medium have been determined as well. The relative proportion of composite droplets containing vapor bubbles at the furnace exit.