Parent Droplet Research Articles

Atomization by Acoustic Levitation Facilitates Contactless Microdroplet Reactions.

Microdroplet chemistry is now well-known to be able to remarkably accelerate otherwise slow reactions and trigger otherwise impossible reactions. The uniqueness of the microdroplet is attributable to either the air-water interface or solid-liquid interface, depending on the medium that the microdroplet is in contact with. To date, the importance of the solid-liquid interface might have been confirmed, but the contribution from the air-water interface seems to be elusive due to the lack of method for generating contactless microdroplets. In this study, we used a droplet atomization method with acoustic levitation. Upon manipulation of the acoustic field, the levitated parent droplet can be further atomized into progeny microdroplets. With this method, only the air-water interface was present, and a large variety of reactions were successfully tested. We anticipate that this study can be an advance toward the understanding of the air-water interfacial processes of microdroplet chemistry.

Analysis of magnetic field-induced breakup of ferrofluid droplets in a symmetric Y-junction microchannel

This research focuses on the analysis of the breakup of ferrofluid droplets in a symmetric microchannel with a Y-junction microchannel, utilizing computational methods. The study proposes an innovative strategy to enhance the breakup phenomenon by introducing a magnetic field within the branches of the Y-junction microchannel. To verify the obtained results, a comprehensive comparison is conducted, incorporating previous numerical and experimental investigations available in the literature. The outcomes of this comparison demonstrate a significant concurrence between the current findings and the prior studies. The results unequivocally elucidate that the presence of a magnetic field accelerates the fragmentation of the parent droplet in comparison to scenarios without a magnetic field. Furthermore, it is established that the duration required for droplet breakup decreases as the magnetic Bond number increases. Achieved results indicates \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:{\ ext{t}}_{breakup}^{*}$$\\end{document} decreases about 3% and 1.5% for L*=3 and L*=4, respectively. It is worth highlighting that this trend is particularly accentuated in the case of smaller non-dimensional lengths, specifically L∗=3.0.

Child droplet compositions produced by puffing and micro-explosion of two-liquid parent droplets

The paper presents the experimental research findings on the compositions of child droplets produced by puffing and micro-explosion of parent two-liquid droplets. The droplets under study consisted of water and rapeseed oil. The research was carried out with varying concentrations of liquids in droplets, ambient temperatures, heating arrangements and initial sizes of two-liquid droplets. Parent droplets were heated in a flame, in a muffle furnace, and on a heated substrate. Child droplet compositions were identified using Laser Induced Fluorescence. Compositions were separated based on the difference in the fluorescence of liquids in child droplets exposed to laser radiation. Typical distributions of child droplets by size were obtained for water and rapeseed oil during parent droplet fragmentation when there was a group of contributing factors. The child droplet sizes were measured for water and rapeseed oil. Laser Induced Fluorescence was successfully used to identify different liquids in child droplets following the micro-explosive fragmentation of two-liquid droplets. The experimental data were processed to establish functional relationships between the main factors, parameters and integral characteristics of the investigated processes. Dimensionless criterion relationships and mathematical expressions were proposed for transferring the experimental results to different scales and conditions for heating two-liquid droplets. The applicability of this approach was described. Guidelines were provided on developing the proposed approach for heterogeneous compositions of parent droplets.

Component Effects in Binary Droplet Impact Behaviors on the Heated Plate: Comparison Study of Ethanol/Propanol and Ethanol/Water Droplets and Observation of Novel Bubble Shrinkage Phenomenon

This work aims to investigate the effect of liquid physical properties on the behavior of binary droplets impact on the heated smooth aluminum alloy plate with a high-speed imaging system. Two groups of mixed solutions with similar boiling point differences are selected as the working liquid, in which the low-boiling-point components are both ethanol and the high-boiling point components are propanol and water, respectively. Compared to the ethanol/propanol binary droplets, the experimental results show that the ethanol/water binary droplets have diverse impact phenomena and significantly broad transition boiling regimes, as well as the reduced droplet residence time and increased Leidenfrost temperature point. With the decreasing ethanol content in ethanol/water binary droplets, these effects become more prominent. For secondary atomization, the ethanol/water binary droplet undergoes parent droplet breakup into fragment droplets with larger diameters (Ds > 0.3 mm). Both binary droplets produce satellite droplets with small diameters (Ds < 0.3 mm) by puffing and ejection. In terms of the ethanol/propanol binary droplet impact, the probability of puffing is higher and the satellite droplet diameters are small. In the ethanol/water binary droplet impact, the probability of ejection is higher and the satellite droplet diameter distribution is wider. When an ethanol/water binary droplet of 25 vol.% ethanol content impacts the heated wall at Ts = 120 °C, a novel large bubble shrinkage phenomenon occurs at the late stage of droplet evaporation. This phenomenon is proposed to be relevant to the increasing surface tension and saturation temperature with decreasing ethanol content, as well as the decreasing ambient temperature above the top surface of the bubble.

Characterizing boiling behaviors in water/ethanol binary droplet impact on a heated plate

This work experimentally investigates the transition boiling behaviors of water/ethanol binary droplets impact on a heated smooth aluminum alloy plate with a high-speed imaging system, forming a more finely defined impact behavior regime map. The dynamic Leidenfrost point temperature exhibits a non-monotonic variation trend with ethanol fraction. Binary droplets exhibit two characteristic boiling behaviors at specific temperatures (Ts) and droplet compositions (Φe), including corona boiling and prompt boiling, distinguished by the elevation of a crown-shaped thick liquid lamella and the prompt generation of numerous secondary droplets from the contact line, respectively. Both boiling behaviors are featured with the violent breakup of parent droplet into about 10 or more fragment droplets (secondary droplets with D ≥ 0.3 mm) and the short residence times on the wall, while exhibiting different splash angle distributions of satellite droplets (secondary droplets with D < 0.3 mm). Characteristic parameter analysis of the two boiling behaviors is conducted to provide qualitative mechanistic explanations. Corona boiling occurs at Φe = 25% & 50% with a lower temperature range (Ts ≤ 200 ℃), the liftoff of the liquid layer is attributed to the combined effect of decelerated wetting speed caused by evaporation and the vapor-induced lift. Prompt boiling occurs at Φe = 25% & 50% with Ts ≥ 220 °C and Φe = 75% with Ts < 200 °C, the intensity of the bubble cloud determines whether the central part of the droplet forms a jet or a liquid bulk filled with microbubbles. This work will provide some new insights into the boiling behaviors and the regulation of secondary atomization of binary solution droplets impact studies.

Self-Propulsion by Directed Explosive Emulsification.

An active droplet system, programmed to repeatedly move autonomously at a specific velocity in a well-defined direction, is demonstrated. Coulombic energy is stored in oversaturated interfacial assemblies of charged nanoparticle-surfactants by an applied DC electric field and can be released on demand. Spontaneous emulsification is suppressed by an increase in the stiffness of the oversaturated assemblies. Rapidly removing the field releases the stored energy in an explosive event that propels the droplet, where thousands of charged microdroplets are ballistically ejected from the surface of the parent droplet. The ejection is made directional by a symmetry breaking of the interfacial assembly, and the combined interaction force of the microdroplet plume on one side of the droplet propels the droplet distances tens of times its size, making the droplet active. The propulsion is autonomous, repeatable, and agnostic to the chemical composition of the nanoparticles. The symmetry-breaking in the nanoparticle assembly controls the microdroplet velocity and direction of propulsion. This mechanism of droplet propulsion will advance soft micro-robotics, establishes a new type of active matter, and introduces new vehicles for compartmentalized delivery.

Dynamic Oscillation and Motion of Oil-in-Water Emulsion Droplets.

The interplay of interfacial tensions on droplets results in a range of self-powered motions that mimic those of living systems and serve as a tunable model to understand their complex non-equilibrium behavior. Spontaneous shape deformations and oscillations are crucial features observed in nature but difficult to incorporate in synthetic artificial systems. Here, we report sessile oil-in-water emulsions that exhibit rapid oscillating behavior. The oscillations depend on the nature and concentration of the surfactant, the chemical composition of the oil, and the wettability of the solid substrate. The rapid changes in the contact angle per oscillation are observed using side-view optical microscopy. We propose that the changes in the interfacial tension of the oil droplets is due to the partitioning of the surfactant into the oil phase and the movement of self-emulsified oil out of the parent droplets giving rise to the rhythmic variation in droplet contact-line. The ability to control and understand droplet oscillation can help model similar oscillations in out-of-equilibrium systems in nature and reproduce biomimetic behavior in artificial systems for various applications, such as microfluidic lab-on-a-chip and adaptive materials.

Dynamic Oscillation and Motion of Oil‐in‐Water Emulsion Droplets

AbstractThe interplay of interfacial tensions on droplets results in a range of self‐powered motions that mimic those of living systems and serve as a tunable model to understand their complex non‐equilibrium behavior. Spontaneous shape deformations and oscillations are crucial features observed in nature but difficult to incorporate in synthetic artificial systems. Here, we report sessile oil‐in‐water emulsions that exhibit rapid oscillating behavior. The oscillations depend on the nature and concentration of the surfactant, the chemical composition of the oil, and the wettability of the solid substrate. The rapid changes in the contact angle per oscillation are observed using side‐view optical microscopy. We propose that the changes in the interfacial tension of the oil droplets is due to the partitioning of the surfactant into the oil phase and the movement of self‐emulsified oil out of the parent droplets giving rise to the rhythmic variation in droplet contact‐line. The ability to control and understand droplet oscillation can help model similar oscillations in out‐of‐equilibrium systems in nature and reproduce biomimetic behavior in artificial systems for various applications, such as microfluidic lab‐on‐a‐chip and adaptive materials.

Physical and Mathematical Models of Micro-Explosions: Achievements and Directions of Improvement

The environmental, economic, and energy problems of the modern world motivate the development of alternative fuel technologies. Multifuel technology can help reduce the carbon footprint and waste from the raw materials sector as well as slow down the depletion of energy resources. However, there are limitations to the active use of multifuel mixtures in real power plants and engines because they are difficult to spray in combustion chambers and require secondary atomization. Droplet micro-explosion seems the most promising secondary atomization technology in terms of its integral characteristics. This review paper outlines the most interesting approaches to modeling micro-explosions using in-house computer codes and commercial software packages. A physical model of a droplet micro-explosion based on experimental data was analyzed to highlight the schemes and mathematical expressions describing the critical conditions of parent droplet atomization. Approaches are presented that can predict the number, sizes, velocities, and trajectories of emerging child droplets. We also list the empirical data necessary for developing advanced fragmentation models. Finally, we outline the main growth areas for micro-explosion models catering for the needs of spray technology.

Magnetic Beads inside Droplets for Agitation and Splitting Manipulation by Utilizing a Magnetically Actuated Platform.

We successfully developed a platform for the magnetic manipulation of droplets containing magnetic beads and examined the washing behaviors of the droplets, including droplet transportation, magnetic bead agitation inside droplets, and separation from parent droplets. Magnetic field gradients were produced with two layers of 6 × 1 planar coils fabricated by using printed circuit board technology. We performed theoretical analyses to understand the characteristics of the coils and successfully predicted the magnetic field and thermal temperature of a single coil. We then investigated experimentally the agitation and splitting kinetics of the magnetic beads inside droplets and experimentally observed the washing performance in different neck-shaped gaps. The performance of the washing process was evaluated by measuring both the particle loss ratio and the optical density. The findings of this work will be used to design a magnetic-actuated droplet platform, which will separate magnetic beads from their parent droplets and enhance washing performance. We hope that this study will provide digital microfluidics for application in point-of-care testing. The developed microchip will be of great benefit for genetic analysis and infectious disease detection in the future.

Ion emission from nanodroplets undergoing Coulomb explosions: a continuum numerical study

A droplet charged above the Rayleigh limit is unstable. In the resulting dynamical process, referred to as a Coulomb explosion, smaller droplets with higher charge-to-mass ratios are ejected, reducing the charge of the parent droplet below the Rayleigh limit. Furthermore, if the droplet is sufficiently small, the electric field on its surface can promote ion field emission. Ion emission can lower the charge of a spherical droplet below its Rayleigh limit, keeping it stable, or reduce the charge of a deforming droplet, changing its dynamics and potentially preventing the Coulomb explosion. This article develops a continuum phase field electrohydrodynamic model to study the interplay between Coulomb explosions and ion emission, using charged nanodroplets of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-Im) as a case study. In small droplets (diameter $D \lesssim 20$ nm for EMI-Im), the electric field is strong enough to emit ions in the early phase of the droplet's evolution, suppressing the Coulomb explosion. For $20 \lesssim D \lesssim 45$ nm, the electric field on the EMI-Im droplet may not promote significant ion emission; however, as the unstable droplet becomes ellipsoidal, ions are emitted from its vertices, ultimately suppressing the Coulomb explosion while shedding $20\unicode{x2013}40\,\%$ of the initial charge. For larger EMI-Im droplets, $45 \lesssim D \lesssim 100$ nm, the evolution typical of a Coulomb explosion is observed, accompanied by ion emission which is however insufficient to prevent the Coulomb explosion. Ion emission and the smaller progeny droplets account for $24\,\%$ and $16\,\%$ of the initial charge, respectively.

Regime maps of collisions of fuel oil/water emulsion droplets with solid heated surface

The paper presents the experimental research findings for the characteristics of secondary atomization of fuel oil/water emulsion droplets colliding with a solid substrate. This reproduces the interaction of fuel droplets with heated walls of combustion chambers, reactors, mixing units and swirlers. Conditions were identified, when two regimes of the interaction of a substrate with a parent droplet occur consistently and lead to its atomization or sticking. To approximate the experimental conditions to the gas–vapor-droplet technologies, the main parameters were varied in wide ranges: relative water and fuel oil concentrations, droplet sizes (0.1–2.5 mm), droplet velocities (0–8 m/s), temperatures of the fuel (20–90 °C) and substrate (20–300 °C). The key characteristics were recorded: collision regimes, critical Weber, Reynolds and Ohnesorge numbers, child droplet number and sizes, liquid surface area. Interaction regime maps were drawn up. Equations describing the transition boundaries between the interaction regimes were derived. The surface areas of fuel oil/water emulsion droplets after and before the fragmentation were calculated. Guidelines were provided for applying the research findings to the development of secondary atomization technologies of fuel oil/water emulsion droplets.

Electric-field-mediated morpho-dynamic evolution in drop–drop coalescence phenomena in the inertio-capillary regime

When two drops collide, they may either exhibit complete coalescence or selectively generate secondary drops, depending on their relative sizes and physical properties, as dictated by a decisive interplay of the viscous, capillary, inertia and gravity effects. Electric field, however, is known to induce distinctive alterations in the topological evolution of the interfaces post-collision, by influencing a two-way nonlinear coupling between electro-mechanics and fluid flow as mediated by a topologically intriguing interfacial deformation. While prior studies primarily focused on the viscous-dominated regime of the resulting electro-coalescence dynamics, several non-intuitive features of the underlying morpho-dynamic evolution over the intertio-capillary regime have thus far remained unaddressed. In this study, we computationally investigate electrically modulated coalescence dynamics along with secondary drop formation mechanisms in the inertio-capillary regime, probing the interactions of two unequal-sized drops subjected to a uniform electric field. Our results bring out an explicit mapping between the observed topological evolution as a function of the respective initial sizes of the parent drops as well as their pertinent electro-physical property ratios. These findings establish electric-field-mediated exclusive controllability of the observed topological features, as well as the critical conditions leading to the transition from partial to complete coalescence phenomena. In a coalescence cascade, an electric field is further shown to orchestrate the numbers of successive stages of coalescence before complete collapse. However, an increase of the numbers of cascade stages with the electric field strength and parent droplet size ratio is non-perpetual, and the same is demonstrated to continue until only a threshold number of cascade stages is reached. These illustrations offer significant insights into leveraging the interplay of electrical, inertial and capillary-driven interactions for controllable drop manipulation via multi-drop interactions for a variety of applications ranging from chemical processing to emulsion technology.

Native Electrospray Ionization of Multi-Domain Proteins via a Bead Ejection Mechanism.

Native ion mobility mass spectrometry is potentially useful for the biophysical characterization of proteins, as the electrospray charge state distribution and the collision cross section distribution depend on their solution conformation. We examine here the charging and gas-phase conformation of multi-domain therapeutic proteins comprising globular domains tethered by disordered linkers. The charge and collision cross section distributions are multimodal, suggesting several conformations in solution, as confirmed by solution hydrogen/deuterium exchange. The most intriguing question is the ionization mechanism of these structures: a fraction of the population does not follow the charged residue mechanism but cannot ionize by pure chain ejection either. We deduce that a hybrid mechanism is possible, wherein globular domains are ejected one at a time from a parent droplet. The charge vs solvent accessible surface area correlations of denatured and intrinsically disordered proteins are also compatible with this "bead ejection mechanism", which we propose as a general tenet of biomolecule electrospray.

Cascade fragmentation of composite parent and child droplets

This paper presents the experimental findings on the consecutive cascade fragmentation of composite fuel droplets in a high-temperature gas. Using the breakup of typical two-liquid parent droplets as an example, we show the possibility of monotonous evaporation, puffing, and micro-explosion resulting in an array of secondary fragments of different sizes. The characteristics of these processes can be successfully controlled. A certain volume of a parent droplet can break up several times in the alternating puffing and micro-explosion regimes. The experiments were based on two-liquid parent droplets fixed on a designated holder in a high-temperature gas. The fragmentation of moving secondary droplets was recorded using tracking algorithms. The experimental findings were processed to obtain approximations of the experimental dependences of delay times between consecutive breakups, number of resulting droplets, their sizes, and other characteristics. A physical cascade fragmentation model was developed for a two-liquid droplet. Recommendations were formulated on how to ensure the intense cascade fragmentation of two-liquid droplets.

Behavior of child droplets during micro-explosion and puffing of suspension fuel droplets: The impact of the component mixing sequence

The article provides experimental research findings on the behavior of micro-explosion and puffing of two-liquid droplets in different droplet formation schemes: liquid combustible component (rapeseed oil) as the shell with liquid incombustible component (water) as the core and vice versa. The experiments were conducted on fuel compositions that had not been pre-mixed; no coal particles had been added into them. These conditions aimed to reflect industrial production conditions with separate injection and blow-in of components into combustion chambers. Shadow Photography and Planar Laser Induced Fluorescence techniques were used in the experiments. An alcohol burner was used for heating droplets. The temperature of the heating medium was varied in the range of 850–1050 K by adjusting the vertical distance of the droplet from the burner bottom to match the conditions typical for industrial applications, specifically, composite fuel creation technologies and thermal/flame purification of liquids. Conditions have been determined under which droplets of combustible and non-combustible components are atomized during fragmentation thus producing polydisperse and monodisperse aerosols. Effects have been discovered whereby the fragmentation of combustible and noncombustible liquid components produces droplets of different sizes. Correlations between the size and component composition of child droplets with the heating temperature and mixing sequence of the liquids in the parent droplet have been established.

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.

Turbulent droplet breakage in a von Kármán flow cell

Droplet dispersion in liquid–liquid systems is a crucial step in many unit operations throughout the chemical, food, and pharmaceutic industries, where improper operation causes billions of dollars of loss annually. A theoretical background for the description of droplet breakup has been established, but many assumptions are still unconfirmed by experimental observations. In this investigation, a von Kármán swirling flow device was used to produce homogeneous, low-intensity turbulence suitable for carrying out droplet breakage experiments using optical image analysis. Individual droplets of known, adjustable, and repeatable sizes were introduced into an isotropic turbulent flow field providing novel control over two of the most important factors impacting droplet breakage: turbulence dissipation rate and parent droplet size. Introducing droplets one at a time, large data sets were gathered using canola, safflower, and sesame oils for the droplet phase and water as the continuous phase. Automated image analysis was used to determine breakage time, breakage probability, and child droplet size distribution for various turbulence intensities. Breakage time and breakage probability were observed to increase with increasing parent droplet size, consistent with the classic and widely used Coulaloglou–Tavlarides breakage model (C–T model). The shape of the child drop size distribution function was found to depend upon the size of the parent droplet.

Numerical investigations on the deformation and breakup of an n-decane droplet induced by a shock wave

This paper adopts the coupled level-set and volume-of-fluid and the large eddy simulation methods to simulate the deformation and breakup of an n-decane droplet under the action of a shock wave. We aim to investigate the effects of the shock Mach number and droplet diameter on temporary deformation and breakup characteristics at high Weber numbers from 5813 to 22 380. Additionally, special attention is paid to subsequent sub-droplet size distributions, which many researchers generally ignore. The results indicate that the evolution of droplet deformation and breakup in the shear breakup regime generally agrees with the obtained experimental data. Based on the present methods, the physical mechanisms for variations of multiple recirculation zones and the development of Kelvin–Helmholtz instability in wave formation are discussed. Larger shock Mach number and smaller droplet diameter can significantly increase the cross-stream and stream-wise deformations. Moreover, both relaxation and breakup times are directly proportional to the initial droplet diameters but inversely proportional to the shock Mach numbers. Eventually, as the shock Mach number increases, the superficial area and mass ratios of sub-droplets to parent droplets all increase from 5.596 to 8.278 and from 23.38% to 38.38%, while the ratios increase from 2.652 to 18.523 and from 4.63% to 92.7%, respectively, as the droplet diameter decreases.

Deformation behavior of liquid droplet in shock-induced atomization

The purpose of this study is to clarify the deformation behavior characteristics of a single droplet in the high Weber number region corresponding to catastrophic breakup, depending on the Weber number, and show the relationship between the spread of fragments and their initial deformation behavior. Accordingly, we investigated waves generated from the droplet surface by photographing the deformation behavior of the droplet with high spatio-temporal resolution and the correlations between the Weber number and height and thickness of the parent droplet, center-of-mass movement, and movement of the leading and trailing edges. The results show no significant relationship between the spread of the fragment and its initial deformation behavior. We also measured the wavelength of the wave pattern at the upstream interface of the droplet during a catastrophic breakup. As a result, a relationship between the wavelength of the upstream interface of the droplet and the Weber number was found.