Single Droplet Research Articles

Crater radius analysis after dual droplets successive oblique impact on liquid film

Droplet impact is a common occurrence in nature, agriculture, and industry. The research on the multi‐droplet impact is fundamental to understanding the tangled nature of reality. This paper numerically studies the successive oblique impact of dual droplets on the liquid film by building an effective three‐dimensional model. The leading and trailing droplets are set to pass a certain impact point with the same velocity. The main contribution of this paper is the investigation of the effects of Weber number, liquid film thickness, impact angle, and impact time interval on the interface morphology evolution after the droplet impact. Results show that splash pattern conversion of the primary or secondary crown occurs with the change of these factors. Besides, the variations of the maximum crater radius in upstream, lateral, and downstream directions with time are quantitatively analyzed. The crater radius analysis is carried out from three perspectives, the variation during the single droplet impact, the change during the dual droplets impact, and the comparison between them. It is found that the crater of dual droplets impact exhibits shape distortion in the deformation period and appears a marked dimensional increase in the secondary expansion period.

A non-equilibrium evaporation model for a droplet and spray under diesel engine-relevant conditions

Most continuum-based models are constructed based on thermodynamic equilibrium assumption. Nevertheless, the time scale in high-pressure injection is extremely short, and whether there is sufficient time to reach the equilibrium state has not been confirmed. In this study, a non-equilibrium evaporation model is proposed to investigate the non-equilibrium effect on spray evolutions. First, a non-equilibrium isobaric-isothermal flash is established based on the molecular dynamic simulations and Peng-Robinson equation of state. Then, a non-equilibrium evaporation model incorporating the non-equilibrium isobaric-isothermal flash model is implemented into the Eulerian-Lagrangian spray model. Good agreements with the measured penetration lengths of sprays have been achieved. The results reveal that the evaporation time of a single droplet and the liquid penetration length of spray are underestimated by 7.3 % and 8 %, respectively, due to an overestimation in the interfacial vapor molar fraction under the equilibrium assumption at 800 K and 2.0 K. Besides, the non-equilibrium effect is found to significantly alter the formation of lean and moderately fuel-rich mixture, while it only has a slight influence on the fuel-rich mixtures. Finally, the thermodynamic non-equilibrium effect in sprays is suggested to be considered when the ambient pressure exceeds 3.0 MPa or the ambient temperature surpasses 800 K.

Acoustic vaporization of perfluorohexane droplets is induced by heterogeneous nucleation at 1.1 MHz

We investigated the ADV of perfluorohexane (PFH) nano and micro-droplets at a frequency of 1.1 MHz, at conditions where there is no superharmonic focusing. Our experiments were performed on suspensions of droplets in glycerol to avoid sedimentation. The ADV pressure threshold was defined as the pressure for which there is half a chance to observe a vaporization event. Our experiments showed that the threshold depends on the number of insonified droplets and the data were fitted by a statistical model, that is independant on the mechanism leading to ADV. The fit allows the derivation of the threshold in the case of a single insonified droplet. We further observed that the value of the ADV pressure threshold of a single PFH droplet decreases as its radius increases. In addition, at a constant droplet radius, the threshold did not vary when the PFH volume inside the droplet decreases, thanks to the encapsulation of an increasing number of water droplets. These results are incompatible with a model of homogeneous nucleation (where a nucleus can appear anywhere into the PFH volume). However, we developed a heterogeneous nucleation model, where the nucleus appears at the inner droplet surface, that successfully predicts our experimental ADV results.

Experimental study of liquid aluminum droplet breakup characteristics based on a Drop-on-demand (DOD) magnetohydrodynamic actuation

In magnetohydrodynamic (MHD) drop-on-demand (DOD) actuation, Lorentz force generated in different excitation stages are exerted on the liquid metal jet to form a "push-pull" mechanism, which is crucial for generating accurate and repeatable metal droplets. In this study, the important parameters of the excitation current waveform in the magnetohydrodynamic (MHD) process, including the influence of the excitation voltage pulse width ratio and current amplitude on the velocity, energy, volume, and breakup length of aluminum droplets are studied. The conversion process of surface energy and kinetic energy during the formation and fall stage of liquid droplets is analyzed. The results show that the negative and positive excitation voltage pulse width ratio of the MHD pump has a significant effect on the droplet velocity, breakup time, size, and sphericity. The amplitude of the excitation current and Hartmann number has a significant impact on the generation of stable droplets. The results show that the single droplet ejection state can be achieved within the Ha number between 319.5 and 391.1. As the Ha number increases, the volume, length, kinetic energy and surface energy of the droplets at breakup time also increase. The size of droplets can be adjusted by changing the amplitude of excitation waveform.

Harnessing HetHydrogel: A Universal Platform to Dropletize Single-Cell Multiomics.

A universal platform is developed for dropletizing single cell plate-based multiomic assays, consisting of three main pillars: a miniaturized open Heterogeneous Hydrogel reactor (abbreviated HetHydrogel) for multi-step biochemistry, its tunable permeability that allows Tn5 tagmentation, and single cell droplet barcoding. Through optimizing the HetHydrogel manufacturing procedure, the chemical composition, and cell permeation conditions, simultaneous high-throughput mitochondrial DNA genotyping and chromatin profiling at the single-cell level are demonstrated using a mixed-species experiment. This platform offers a powerful way to investigate the genotype-phenotype relationships of various mtDNA mutations in biological processes. The HetHydrogel platform is believed to have the potential to democratize droplet technologies, upgrading a whole range of plate-based single cell assays to high throughput format.

Single Condensation Droplet Self-Ejection from Divergent Structures with Uniform Wettability.

Coalescence-induced condensation droplet jumping has been extensively studied for anti-icing, condensation heat transfer, water harvesting, and self-cleaning. Another phenomenon that is gaining attention for potential enhancements is the self-ejection of individual droplets. However, the mechanism underlying this process remains elusive due to cases in which the abrupt detachment of an interface establishes an initial Laplace pressure difference. In this study, we investigate the self-ejection of individual droplets from uniformly hydrophobic microstructures with divergent geometries. We design, fabricate, and test arrays of truncated, nanostructured, and hydrophobic microcones arranged in a square pattern. High-speed microscopy reveals the dynamics of a single condensation droplet between four cones: after cycles of growth and stopped self-propulsion, the suspended droplet self-ejects without abrupt detachments. Through analytical modeling of the droplet in a conical pore as an approximation, we describe the slow isopressure growth phases and the rapid transients driven by surface energy release once a dynamic configuration is reached. Microcones with uniform wettability, in addition to being easier to fabricate, have the potential to enable the self-ejection of all nucleated droplets with a designed size, promising significant improvements in the aforementioned applications and others.

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.

Describing the Drying and Solidification Behavior of Single Suspension Droplets Using a Novel Unresolved CFD-DEM Simulation Approach

With increasing computational capacities and advances in numerical methods, a simulation-based process and product design approach for spray dried products has become the focus of many researchers. In this context, a novel unresolved CFD-DEM simulation approach for single suspension droplets is presented, which models the drying and solidification process in a hot gas environment. The solidification process is described by the formation of bonds between the primary particles in the suspension droplet, showing a plausible solidification behavior, which starts when a critical solid concentration is reached at the droplet surface. The drying conditions for the single-droplet simulations are determined from a large-scale spray dryer simulation for different droplet size classes. The resulting solid particles from the CFD-DEM simulation show a higher tendency to form hollow particles at high drying rates when the locking point is reached at earlier stages of the drying process. Using these results, suitable operating conditions for the spray dryer can be determined, which favor the formation of particles with a targeted morphology.

Electrochemical Characterization of Neurotransmitters in a Single Submicron Droplet.

Single-entity electrochemistry, which employs electrolysis during the collision of single particles on ultramicroelectrodes, has witnessed significant advancements in recent years, enabling the observation and characterization of individual particles. Information on a single aqueous droplet (e.g., size) can also be studied based on the redox species contained therein. Dopamine, a redox-active neurotransmitter, is usually present in intracellular vesicles. Similarly, in the current study, the electrochemical properties of neurotransmitters in submicron droplets were investigated. Because dopamine oxidation is accompanied by proton transfer, unique electrochemical properties of dopamine were observed in the droplet. We also investigated the electrochemical properties of the adsorbed droplets containing DA and the detection of oxidized dopamine by the recollision phenomenon.

High-fidelity numerical simulations in Eulerian/Lagrangian framework for liquid fuel jets in crossflow with atomization and evaporation: Effect of aerodynamic Weber number

The effect of the aerodynamic Weber number Weg on the atomization-evaporation process of liquid fuel jets in crossflow is investigated by detailed numerical simulations accounting for evaporation from both the gas–liquid interface and dispersed particles in the Eulerian/Lagrangian framework. The accuracy of the evaporation model on the gas–liquid interface treated in the Eulerian manner is verified by applying two-dimensional (2D) numerical simulations to the evaporation of a single droplet and comparing the computational results with the theoretical solution and psychrometric data. The results show that the present detailed numerical simulations reasonably reproduce the primary and secondary atomization characteristics of the liquid jets in crossflow under different breakup modes. Additionally, considering both evaporations from the Eulerian gas–liquid interface and Lagrangian dispersed particles is essential for precisely predicting the fuel mixing with the oxidizer and the ignition. As Weg increases, the Sauter mean diameter (SMD) of generated fuel droplets decreases in the whole region. Furthermore, as Weg increases, the tendency that the fuel vapor mass fraction is high in the upstream region and decreases downstream becomes more evident. However, the temperature behind the liquid column do not decrease despite the increasing heat loss due to evaporation. These behaviors can be well explained in terms of the differences in the atomization, evaporation, and heat transfer between the liquid jet and hot air crossflow against Weg.

Numerical simulation of droplet dispersion within meso-porous membranes

Analysis of membrane processes in fluid processing, and their main influencing operating conditions are relevant in a variety of industrial applications. Increasing regulatory scrutiny and environmental considerations are forcing industries across all sectors, from food and pharma to oil and gas, to further understand and optimise the handling and formulation of liquid systems for efficient process design. In a generic setup for emulsification and liquid formulation the flow and dispersion behaviour of a liquid oil droplet on its way through a porous water filled membrane is analysed. A set of high-resolution numerical simulations of a single oil droplet dispersed in water through a porous membrane structure with varying contact angles is performed. In this work cluster analysis of volume-of-fluid simulation results to obtain statistical droplet size distributions is conducted and further analysed to highlight the effect of the contact angle as well as pressure drop on the dynamics of the system. It is observed that based on the membrane surface activity the droplet behaviour changes from filtration with coalescence when the membrane is lipophilic to emulsification with droplet break-up when the membrane is lipophobic. Furthermore, the pressure drop is identified as a key factor for the dynamics of the droplet process and the frame in which it occurs. These results highlight that the membrane wettability is a determining factor for the emulsification or filtration effectiveness of a membrane for various applications.

Electric-field induced droplet stable vertical vibration: Experiments and numerical simulations

In this study, stable vertical vibration of a single droplet was stimulated through Electrowetting on Dielectric (EWOD) on a Polydimethylsiloxane-coated Indium Tin Oxide glass (PDMS-ITO) surface in an oil atmosphere. The mechanism of the vibration is clarified as a self-adjusted energy earning and losing equilibrium, and a small friction at the contact line is crucial for this stable vibration. We investigated the impact of needle tip electrode height, different voltage forms (AC and DC voltages), and voltage magnitude on the droplet vibration state. As decreasing the needle tip electrode height, there is a transition of the droplet from a vibrating state to a static equilibrium state. During the vibration, the vibration period remains constant regardless of tip height, but the contact angle changes synchronously with the voltage. The electric signal in the circuit is also measured to provide solid evidence for this synchronicity. Finally, the saturation voltage of the contact angle and amplitude is about 120 V. The intrinsic connection between contact angle saturation and amplitude saturation is clarified by studying the surface energy of the droplet. A theoretical model is constructed to numerically simulate the shape and amplitude of the droplet’s vibrations.

Repeatable Acoustic Vaporization of Coated Perfluorocarbon Bubbles for Micro-Actuation Inspired by Polypodium aureum.

Polypodium aureum, a fern, possesses a specialized spore-releasing mechanism like a catapult induced by the quick expansion of vaporized bubbles. This study introduces lipid-coated perfluorocarbon droplets to enable repeatable vaporization-condensation cycles, inspired by the repeatable vaporization of Polypodium aureum. Lipid-perfluorocarbon droplets have been considered not to exhibit repeatable oscillations due to bubble collapse of the low surface tension of lipid layers. However, a single lipid-dodecafluoropentane droplet with a diameter of 9.17 µm shows expansion-contraction oscillations over 4000 cycles by changing lipid composition and applying a low-power 1.7 MHz ultrasound to induce the partial vaporization of the droplets. The optimal combinations of shell composition, droplet fabrication, and acoustic conditions can minimize the damage on shell structure and promote a quick recovery of damaged shell layers. The highly expanding oscillatory microbubbles provide a new direction for fuel-free micro- or nanobots, as well as biomedical applications of contrast agents and drug delivery.

Features of numerical modeling electrical coalescence in a droplet-layer system using the arbitrary Lagrangian-Eulerian method

The electrocoalescence of a water drop at the water-oil interface was numerically analyzed using the finite element method by solving a set of equations including hydrodynamics and electrostatics. The simulation was performed using COMSOL Multiphysics software, the interface movement was implemented using an arbitrary Lagrangian-Eulerian method (also called the moving-mesh method). The influence of the initial distance from the drop to the layer, as well as the size of the entire domain on the outcome of the process was studied. The electrical coalescence and non-coalescence of a single uncharged droplet with a layer under the action of the DC field were calculated in a wide range of electric field strength. The created computer model makes it possible to calculate the voltage dependence of the detached droplet volume and charge. Also, it can help to determine features of non-coalescence, including the stretching of the droplet along the interelectrode gap and spraying.

A second-order phase field-lattice Boltzmann model with equation of state inputting for two-phase flow containing soluble surfactants

In this paper, we propose a phase field-lattice Boltzmann (LB) model with an equation of state (EOS) inputting for two-phase flow containing soluble surfactants. In this model, both the order parameter for the phase field and the surfactant concentration are described by second-order partial differential equations, along with Navier–Stokes equations for the flow field. Changes in surfactant concentration do not affect the order parameter distribution; hence, an unwanted sharpening effect cannot arise. Most importantly, in the existing models, the EOS of surface tension is determined by posterior simulation tests instead of being directly set as an input parameter before the simulations. Hence, it is difficult to determine the model parameters in practical applications. To address this issue, we systematically develop a fully analytical EOS for surface tension based on the Gibbs–Duhem equation. Subsequently, an approximate explicit form for EOS is provided by utilizing the Jacobi–Gauss quadrature rule. Furthermore, a multiple-relaxation-time LB scheme is utilized to numerically solve the governing equations of three physical fields. Two benchmark examples are simulated to validate the accuracy of the present model. The consistency between the numerical results and the analytical EOS is verified. Moreover, the dynamics of droplets with surfactant in simple shear flow is investigated, unveiling the profound impact of various factors, such as surfactant bulk concentration, capillary number, and viscosity ratio, on single droplet deformation and two equal-sized droplets interaction. A detailed exploration of the fluid mechanism involved in two-phase flow with soluble surfactants is presented.

Influence of tea leaf morphology on water interception for droplet impact

AbstractIt is common for plant canopy to intercept water during rainfall or sprinkler irrigation. How tea leaves intercept water droplets are unknown. The objective of this study is to investigate the influence of different shapes of tea leaves on the water droplet retention and distribution. First, the shapes of different tea varieties (Zhongcha 108, Maolu, Longjing 43, Huangjinya, and Anji Baicha) were statistically classified, and a high‐speed camera (with a frame rate of 10 000 fps) was used to record and analyse the interception of single droplet. The results showed that the morphology of tea leaves is flat, V‐shaped and hemispherical. The percentage of flat shaped leaves is the highest in each variety (45.38%–63.20%), the proportion of V‐shaped leaves is 30.40%–42.97%, and the proportion of hemispherical leaves is the least. When the impact velocity is 1.35 ± 0.05 m/s, it takes only 20–26 ms for droplet to start hitting each morphology of leaves and be retained as a static liquid. Droplets were kept as a film of water on the flat leaf, and similar situation happened to the hemispherical leaf, but they slid downward due to gravity. When impact position is near the midrib or leaf surface angle is smaller, retained droplets tends to converge to the bottom of the V‐shape with a symmetrical distribution. This study aids to better understand and model water interception within tea canopies for future practice of irrigation and chemical spraying.

Identification and postnatal developmental characteristics of brown adipose tissue in yak calves

AbstractBackgroundBrown adipose tissue (BAT) provides newborn mammals with a critical capacity for non‐shivering thermogenesis and enables their adaptation to the cold of the extrauterine environment. The domestic yak (Bos grunniens) has evolved on the Qinghai–Tibet Plateau and has adapted well to extremely cold and hypoxic environments. However, the location, characterization and postnatal development of BAT in yak calves remain largely unknown.MethodsWe identified different types of adipocytes and explored the postnatal development of interscapular and subcutaneous adipose tissue in yak calves at 1, 7 and 30 days old using morphological and molecular biology methods.ResultsThe results show that multilocular and unilocular adipocytes were found in two depots. Multilocular adipocytes contained small lipid droplets packed with numerous mitochondria. However, unilocular adipocytes contained a single unilocular lipid droplet and few mitochondria. The number of multilocular adipocytes significantly decreased, whereas the number of unilocular adipocytes significantly increased from day 1 to 30. Furthermore, uncoupling protein 1 (UCP1) expressions in both multilocular and unilocular adipocytes, as well as UCP1 mRNA and protein expression, were significantly decreased from day 1 to 30. In addition, peroxisome proliferator‐activated receptor‐γ coactivator‐1α and peroxisome proliferator‐activated receptor alpha mRNA expression were significantly decreased.ConclusionsThe results of the research indicate that adipocytes in both newborn yak calf interscapular and subcutaneous adipose tissue are multilocular adipocytes. Many multilocular adipocytes were transformed into unilocular adipocytes after birth, but some multilocular adipocytes remained at day 30; this transition occurred at different rates in different depots. These results provide a basis for further study of BAT in yak calves.

Photothermal-driven flow with water droplets for effective removal of indoor fine particulate matters

Airborne particulate matter (PM) causes hazardous problems in human health, and water droplets have been utilized for PM removal in the air. However, since conventional wet scrubbing techniques mostly rely on mechanically generated water droplets, they have intrinsically limited in collection efficiencies of a single droplet. In addition, PM capturing strategies for removal of fine PM need to be established to enhance PM removal performance based on flow dynamics. Here, we propose a steam flow of water droplets generated by a photothermal membrane for air purification. The generated flow captures PM efficiently by the aid of the coupled effects of water droplets and hydrodynamic flow. The flow achieves high and sustainable deposition constants of 0.458 and 0.472 h−1 for PM1.0 and PM2.5 within 1 h at 4 solar intensity, which is 2.20 and 2.24 times higher than those of without solar irradiation, respectively. The deposition constants of PM using steam flow were higher than that of previous wet scrubbing methods despite a low volume of water usage. Our photothermal-driven flow with water droplets could be utilized as effective indoor air purification system that is low cost, energy-efficient, and environmentally-friendly.

Evaporation Behavior of a Single Isobutane Droplet in the Effluent Spray Process of Isobutane Alkylation Catalyzed by Composite Ionic Liquid

Evaporation Behavior of a Single Isobutane Droplet in the Effluent Spray Process of Isobutane Alkylation Catalyzed by Composite Ionic Liquid

Experimental study of lube oil pre-ignition in an optical gas engine test rig

Lean-premixed gas combustion is an attractive option for meeting future emission standards toward considerably lower particulate matter and emissions while obtaining an efficiency comparable to diesel combustion. However, ignition processes still pose considerable challenges, with pre-ignition being a major issue. The pre-ignition phenomenon might result from the self-ignition of lube oil in hot zones, making it challenging to observe in real engines. To study single or sparsely separated lube oil droplets under engine-like conditions, the “Flex-OeCoS” experimental test facility was used. To be able to induce such pre-ignitions artificially a novel piezo droplet injector was developed and manufactured, which enabled the metering of minor amounts of lubricating oil or even the injection of single droplets. High-speed cameras and specially developed and partially automatable and adaptive evaluation algorithms were used to record and track the behavior of the injected droplet using overlaid schlieren and OH* chemiluminescence. During the measurement campaigns, relevant operating parameters were varied, and optical investigations were carried out with an ignitable mixture on the Flex-OeCoS test facility under engine-relevant operating conditions. Spontaneous ignition of the vaporized lubricating oil was observed, which subsequently led to ignition of the whole air-fuel mixture in the optical combustion chamber. This was especially evident for lean methane-air mixtures up to an air-fuel ratio of 2.0. With this approach, information could be gathered about the necessary internal engine conditions that lead to pre-ignition in lean mixtures and where their limits lie.