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The hydrodynamics and mixing during the nonaxisymmetry impingement of a micro-droplet and a sessile droplet of the same fluid are investigated by many-body dissipative particle dynamics (MDPD) simulation. In this work, the range of the impingement angle (θi) between the impinging droplet and the sessile droplet is 0°–60° and the contact angle is set as 45° or 124°. The droplets impingement and mixing behavior is analyzed based on the droplet internal flow field, the concentration distribution and the time scale of the decay of the kinetic energy of the impinging droplet. The dimensionless total mixing time (τm) is calculated by a modified mixing function. With the Weber number (We) ranging from 5.65 to 22.7 and the Ohnesorge number (Oh) ranging from 0.136 to 0.214, we find τm hardly changes with We and Oh. Whereas, θi and surface wettability are found to have a significant effect on τm. We find that θi has no clear effect on τm on a hydrophobic surface, while on the hydrophilic surface, τm increase with the θi. Thus, reducing the impinging angle is a valid method to shorten the τm.

It has been urgently required to develop ionic liquid electrospray thruster (ILET) for rapidly expanding micro-/nanosatellites. In this work, ILETs with high-density array emitters are fabricated t...

The behavior of single water drop impacting an oil layer on the deep water pool was recorded by using a high-speed camera. Heptane was used as the target liquid layer on the water pool. A particular phenomenon of double crown can be observed. The formation of double crown is not only related to the impact Weber number of the water droplet, but also the thickness of the oil layer. The critical Weber number for the formation of double crown decreases sharply as the decrease of the oil layer thickness. With the increase of Weber number and the decrease of oil layer thickness, there are four typical phenomena: single crown-jet, single crown-bubble, double crown-bubble and double crown- jet. Moreover, the effects of oil layer thickness and impact Weber number on surface bubble lifetime and the geometry size of crown, crater and jet were quantitatively analyzed.

This paper proposes a comparative study of two techniques of granulation of a submicronic alumina powder with high binder content slurries, by spray drying and freeze granulation for the preparation of mixed granules. First, the viscosity and flow index of the suspensions are given as a function of dispersant, solid and binder contents versus alumina content, and the data are analysed in order to give a predictive model in a wide range. Suspensions with varying viscosities (7–208 mPa.s), densities (1.31–1.76) and surface tensions (23–40 mN/m) were then granulated. The first observations reveal the importance of the content and the molar mass of the binder: when they are too high, the freeze granulation fails and filaments are produced instead of granules due to extensive stretching of the molecular chains of the binder during spraying. Then through a theoretical analysis of the phenomena leading to granulation, an original dimensionless number is proposed to describe the evolution of the granule size as a function of suspension formulation. This number is related to the Reynolds and Weber number and is able to predict the granule size over a wide range (20–500 μm for freeze granulation, and 5–30 μm for spray drying). Spray drying leads to smaller granules with various shapes, from full shape to hollow or donut-like, whereas freeze granulation leads to bigger but spherical granules with a microporosity, and a size easier to predict as no drying shrinkage is observed.

The deposition of droplets inside the deformable surfaces has attracted researchers for decades due to its application in direct-writing microporous polymer architectures, printing embedded flexible wires, patterning functional nanoparticles, etc. Herein, a patterned microstructure method based on droplets, named as the embedded droplet printing (EDP), is proposed. The experiments were conducted at a Weber number of 5.49–17.7 to explore the impact outcome, spreading laws and embedded morphology of the droplets. The rebound of droplets impacting the viscous surface was suppressed under appropriate conditions. The spreading factor of the droplet impacting on the high viscous surface followed the power law d*∝ t α. However, the exponent α was observed to be in the range 0.042–0.031, much smaller than the reported values (0.1–1), which could be explained by the Oh number. The diameters of droplets wrapped with viscous PDMS were only about 1/6th as compared to the spreading on the surface of PDMS precured for 30 min. In particular, a domain map was plotted in which patterns of solid bracts, plates and coffee rings were printed. Overall, EDP is a promising candidate to tailor the size, depth and morphology of droplets for preparing the patterned microstructures inside the soft materials.

The spatial variation in the wettability of a surface can have a significant effect on the spreading and retraction behavior of an impacting droplet and hence the overall impact dynamics. Although composite surfaces have proven applications, there is a lack of understanding of droplet impact on surfaces with a sudden jump in wettability. Here, we study the behavior of a liquid drop impacting a composite surface having a superhydrophilic (SHL) spot surrounded by a superhydrophobic (SHB) region. We find that the droplet exhibits different regimes: no-splitting, jetting, and splashing, depending upon the spot size (βs) and the Weber number (We). At a smaller βs, the behavior shifts from the stable to jetting regime and then to the splashing regime, with increasing We. We find that by increasing the value of βs, one can avoid the undesirable splashing and jetting regimes and attain a stable regime even at a higher We. Our study reveals that βs has a significant influence on the maximum spreading diameter βmax at a smaller We but a negligible effect at a higher We. We show that the dominance of capillary energy at a smaller We and viscous energy at a higher We underpins the phenomena. We employ an energy conservation approach to develop an analytical model to predict βmax on a composite SHL-SHB surface by considering the total energy of the system before the impact and at the maximum spread position. We find K = (Re1/2/We) emerges as a key parameter in the model that accurately predicts the experimentally measured βmax. Our study reveals the existence of an inertia-viscous dominated regime at a smaller K and an inertia-capillary dominated regime at a larger K. The outcome of our study may find applications in stable and precise positioning of impacting droplets.

In this study, the dynamic characteristics of water droplets impinging on a heated cylinder were investigated using a high-speed camera regarding the diameter ratios between droplet and cylinder. Water droplets generated by a needle in the range of 2.5 mm to 5.0 mm in diameter were impinged onto a cartridge heater with surface temperature of 25 °C and 300 °C. The diameter of the cartridge heater was 6.0 mm. The recording rate of the high-speed camera was set to 10,000 Hz, and the Weber number was controlled from 17.4 to 217 by adjusting the free-fall height of the water droplets. As a result, the dynamic characteristics of water droplets impinging on cylinder were different depending on the diameter ratio and Weber number. In the high-temperature region, the water droplets were detached from the cylinder after the impingement by Leidenfrost effect. Under conditions of low Weber number and low diameter ratio, the water droplets were rebounded in the opposite direction of the impingement, maintaining its shape. However, as the Weber number increases, the water droplets impinging on cylinder were broken-up and detached by the Leidenfrost effect regardless of the diameter ratio. This study is expected to be utilized to determine the dynamic characteristics of droplet, and be applied such as phenomenon analysis of the heat transfer and cooling.

Droplet wettability and dynamic behavior of working fluids with low surface energy are widely used in various industrial and advanced technology fields, such as refrigerant spray cooling and microfluidic chip. Their wettability and dynamic behavior play a critical role in improving the efficiency of industrial processes. Some scholars have realized the wettability control of droplets of low surface energy fluids by changing the characteristics of micro-nano structures on solid surfaces. In addition, many external incentive ways, such as electrowetting, magnetic field, thermal stimulation, and supersonic vibration, have been proposed to control the droplet wetting state and motion behavior. However, such methods have limitations and no significant effect on low surface energy working fluids. Inspired by the above methods and the Leidenfrost phenomenon, this study made use of the action of the microporous airflow, which can regulate the wettability and motion behavior of low surface tension droplets without adding additional components to the droplets and without special requirements for the droplets themselves. A visual experimental system for wetting and dynamic behavior of droplets regulated by the microporous airflow was set up, and the experimental section of the regular array micropore was machined by laser. The momentum force generated by the microporous flow was used to balance droplet gravity, which overcomes the wetting behavior of the low surface tension droplets FC3283 and FC40 and the realizes the quasi-superhydrophobic levitation state of droplets with low surface tension. This study aimed to explore the regulatory mechanism of the microporous flow additional momentum force on the wettability and dynamic behavior of droplets with low surface energy. Experimental results showed that the motion behavior of droplets in the quasi-superhydrophobic levitation state could be divided into four stages (i.e., hovering, bobbing, bouncing, and detaching) with the decrease in droplet radius. The motion behavior of droplets was related to the dynamic balance of gravity, microporous flow momentum force, inertia force, and surface tension of droplets. In the bouncing stage, mutual conversion of energy was observed in the process of droplet falling and rising. A positive correlation was found between gravitational potential energy and surface energy. This kind of energy conversion made droplets oscillate periodically at a stable frequency. In addition, the deformation degree and amplitude of droplets with two kinds of low surface energy working fluids during the bouncing stage were compared, and the effect of viscosity on the droplet energy conversion process was analyzed. For wettability research, analogous to Young’s equation, combined with the force analysis of droplets at the inflection point during maximum deformation, the concept of pseudo contact angle was proposed to describe the wetting behavior of droplets. The pseudo contact angle of droplets in different oscillation stages was analyzed. In the hovering stage, the pseudo contact angle of droplets reached 153.6°. In the bouncing stage, as the droplet radius and falling velocity increased, the inertia of droplets increased and the surface tension decreased. Therefore, with the increase in Weber number, the pseudo contact angle of droplets decreased gradually. This method can supplement the external field force droplet manipulation method, and it provides a new method for potential applications in low surface tension droplet manipulation, lossless rapid directional transport, and microfluidic system design.

Background Droplet impact on a superhydrophobic cylinder differs from that on a flat surface. After bouncing, once re-touch takes place, the contact time τc would increase, which is unfavorable for some applications. The increased τc may strongly depend on the Weber number and radius ratio, R*, of cylinder to droplet. Methods The impact is investigated via lattice Boltzmann method simulations. The particular emphasis is placed on re-touch rebound patterns and τc. Significant findings Rebound patterns and τc both strongly depend on a combined parameter, α=We/R*, characterizing the asymmetry of droplet spreading and retraction. As α increases, upward rebound and stretched breakup take place sequentially for the first bouncing, whereas rebound patterns change as intact re-touch rebound and separate re-touch rebound for the second bouncing. Increasing α enhances the asymmetry, which promotes the first rebound, thereby reducing τc regardless of rebound patterns. The enhanced asymmetry also accelerates rebound and thus reduces τc in the separate re-touch rebound regime, whereas it hinders rebound, leading to a significantly increased τc. The power-law correlations of τc vs α are developed for the first and second bouncing. Besides, a method is proposed to suppress or prevent the re-touch, which is proven to effectively reduce τc.

Droplet impact on a smooth solid surface at atmospheric pressure was experimentally studied and physically interpreted. A particular emphasis of the study is on the effects of liquid viscosity on the transition between droplet deposition (or droplet spreading without breakup) and droplet disintegration (including droplet splash and receding breakup). Specifically, the critical Weber number separating droplet deposition from droplet disintegration decreases and then increases with increasing Ohnesorge number (Oh). The splash in the low-Oh region and the receding breakup in the high-Oh region were analyzed qualitatively based on the unbalanced forces acting on the rim of the spreading or receding liquid film. A semiempirical correlation of droplet deposition/disintegration thresholds is proposed and well fits the experimental results from previous and present studies over a wide range of liquid viscosity.

Well-wetting liquids exiting small-diameter nozzles in the dripping regime can partially rise up along the outer nozzle surfaces. This is problematic for fuel injectors and other devices such as direct-contact heat and mass exchangers that incorporate arrays of nozzles to distribute liquids. We report our experimental and numerical study of the rising phenomenon for wide ranges of parameters. Our study shows that the interplay of three dimensionless numbers (the Bond number, the Weber number, and the Ohnesorge number) governs the capillary-driven rise dynamics. In general, as the flow rate or the viscosity increases, the capillary-driven rise height over each dripping period becomes smaller. We identify liquid flow rates below which the temporal evolution of the meniscus positions can be well approximated by a quasistatic model based on the Young-Laplace equation. Our analysis reveals two critical Bond numbers that give nozzle sizes, which correspond to the maximum meniscus rise and the onset of capillary-driven rise cessation. These critical Bond numbers are characterized as a function of the contact angle, regardless of the fluid type. Our study leads to a more efficient and optimized nozzle design in systems using wetting liquids by reducing both the risks of contamination and high pressure drop in such devices.

Previous studies reported that single-phase liquid curtains become more stable with increasing Ohnesorge number. Many liquid films used in the coating industry are types of emulsion samples. Despite this fact, the effect of multiphase liquids on the dynamics of the curtain breakup has not yet been considered. This study explores the stability of emulsion curtain coating via high-speed visualization. The critical condition at the onset of curtain breakup was determined by finding the flow rate below which the curtain broke. Curtain breakup was observed via a hole initiation within the curtain. The results reveal that curtain breakup dynamics is governed by the characteristic dynamic viscosity and the surface tension. The emulsion curtain stability, defined by Weber number, increases as Ohnesorge number rises, similar to the single-phase liquid (i.e., Newtonian and pure shear thinning) curtain stability. The critical web speed at which the contact line moves upstream of the curtain, a phenomenon called heel formation, and that at which air entrainment occurs, were determined for emulsion solutions at different flow rates. The results reveal that the surface tension increase delays the onset of air entrainment which could help to conduct faster curtain coating substrates with emulsion liquid films.

In this article, we propose, with the aid of detailed experiments and scaling analysis, the existence of magneto-elastic effects in the impact hydrodynamics of non-Newtonian ferrofluid droplets on superhydrophobic surfaces in the presence of a magnetic field. The effects of magnetic Bond number (Bom), Weber number (We), polymer concentration, and magnetic nanoparticle (Fe3O4) concentration in the ferrofluids were investigated. In comparison to Newtonian ferrofluid droplets, addition of polymers caused rebound suppression of the droplets relatively at lower Bom for a fixed magnetic nanoparticle concentration and We. We further observed that for a fixed polymer concentration and We, increasing magnetic nanoparticle concentration also triggers earlier rebound suppression with increasing Bom. In the absence of the magnetic nanoparticles, the non-Newtonian droplets do not show rebound suppression for the range of Bom investigated. Likewise, the Newtonian ferrofluids show rebound suppression at large Bom. This intriguing interplay of elastic effects of polymer chains and the magnetic nanoparticles, dubbed as the magneto-elastic effect, is noted to lead to the rebound suppression. We establish a scaling relationship to show that the rebound suppression is observed as a manifestation of the onset of magneto-elastic instability only when the proposed magnetic Weissenberg number (Wim) exceeds unity. We also put forward a phase map to identify the various regimes of impact ferrohydrodynamics of such droplets and the occurrence of the magneto-elastic effect.

The role of liquid viscosity on the spreading for an airflow-assisted impact of drops on a surface is investigated. The spreading diameter is found to increase with the Reynolds number of the airflow (Reair) for a given viscosity and impact Weber number (We) compared to the still air. The increment is higher at a low We for viscous drops, whereas the effect of Reair dominates at the intermediate We as the viscosity decreases. Two extra forces, the normal imposed pressure and shear force of air, act on the drop and influence the spreading along with the viscous dissipation. The drop's curvature decreases depending on the viscosity and impact velocity while spreading. Large-scale eddies near the drop-surface region are observed due to the separation of the incident airflow. The formation of eddies signifies low-pressure zones, which extract the trapped air, causing the spreading diameter of the viscous drop to increase at a low We. With the increase in the We, the lamella thickness of low-viscosity drops decreases and is pushed out by the air shear causing the spreading factor to increase. The boundary layer thickness is estimated using the energy balance method to predict the maximum spreading factor. The prediction compares well with the experimental one for higher viscosities. The accuracy improves when the effect of low pressure is incorporated. To confirm, the experimental spreading is compared with that obtained from three existing models, and one, which considers the influence, is observed to provide a better prediction.

The critical parameters of vapor and liquid, corresponding to flooding of the distillation column, were measured experimentally (flooding means the process of partial suspension of liquid in a packing, accompanied by an exponential increase in the pressure drop and subsequent flooding of the vapor space between the packing and the liquid distributor). The experiments were carried out in a wide range of determining parameters: pressures and concentrations of the mixture, ratios of the mass vapor and liquid flows, geometric characteristics of the packing, and physical properties of Freons. The experimental results were processed in dimensionless coordinates and generalized by single dependence We = f(N), where the Weber number We is derived according to the vapor velocity in the packing cell, and the Froude number (included in complex criterion N) is derived by the average liquid velocity in the packing cell.

Jet fuels with various properties are desirable for different operational or environmental advantages, necessitating fuel-flexible technology. For example, JP-5 (A-3) and alternative jet-fuels with high flash points are favored for safe handling and other advantages. The study investigates the effects of fuel properties on the near-field spray characteristics of the relatively viscous A-3 and a more viscous alternative fuel blend (C-3) compared to the baseline fuel—Jet A (A-2) with the least viscosity, using the relatively novel twin-fluid flow blurring (FB) injection. Prior studies have acquired fine FB atomization with high viscosity tolerance compared to conventional jet-based injectors, where atomization is suppressed by high viscosity. The study aims to further quantify the FB fuel flexibility using shadowgraph and particle image velocimetry to potentially enable effective utilization of the desired jet fuels. Results show that the FB injection achieved fine sprays immediately at the injector exit rather than typical jets by conventional atomization. Nearly the same atomization completion length, within 4 D downstream the injector exit with the diameter D of 1.5 mm, was acquired for all three fuels regardless of the varying properties. Compared to A-2, the final droplets of the viscous A-3 are only 6% larger and the most-viscous C-3 are only 10% larger. It is also observed that the density of fuel affects the droplet travel distance from the injector exit to a downstream location before achieving a terminal or peak velocity. Droplet size distribution, velocity distribution, and Weber number estimate consistently reveal more turbulent bag breakup mode at the spray periphery resulting in smaller droplets, compared to the oscillatory mode yielding larger ones in the center.

By releasing liquid drops in turbulent jet flows, we investigated the transformation of single drop breakup from binary to ternary and multiple. Silicone oil and deionized water were the dispersed phase and continuous phase, respectively. The probability of binary, ternary, and multiple breakup of oil drops in jet flows is a function of the jet Reynolds number. To address the underlying mechanisms of this transformation of drop breakup, we performed two-dimensional particle image velocimetry (PIV) experiments of single-phase jet flows. With the combination of drop breakup phenomenon and two-dimensional PIV results in a single-phase flow field, these transformation conditions can be estimated: the capillary number ranges from 0.17 to 0.27, and the Weber number ranges from 55 to 111.

Controlling droplet deposition on a hydrophobic surface has received much attention due to its wide applications. Addition of certain elements into a working droplet is a feasible way to improve drop deposition, which, however, often leads to a significant change in droplet spreading properties. In this work, we show that adding a small amount of hydrophilic TiO2 nanoparticles without any surfactant can significantly suppress the droplet rebound and even generate a whole contact line pinning on the hydrophobic surface. The whole contact line pinning is positively related to the Weber number (i.e., impact velocity) and suspension concentration. Specifically, when the suspension concentration exceeds a critical value, the pinning and droplet deposition occur in the same We range. A mechanism is proposed to explain the observed unique pinning and depinning behaviors, according to which the agglomerated TiO2 particles depositing at the triple line can change the wettability of the local surface, which leads to pinning, while the disturbance of capillary oscillation leads to depinning. Interestingly, a long-time whole contact line pinning for more than a second was observed under certain conditions. This work can be of value for many practical applications such as pesticide deposition and spray cooling.

This paper numerically analyzes the crown formation due to a plane two-dimensional drop impact onto a pre-existing film in a viscoelastic fluid. The finite volume method is applied to solve the governing equations, and the volume of fluid technique is utilized to track the liquid’s free surface. Here, the non-linear Giesekus model is used as the constitutive equation for the viscoelastic phase. The formation and temporal evolution of the crown’s parameters are evaluated using fluid elasticity and non-linear viscometric functions. The results show that a rise in the Weissenberg number, the viscosity ratio, the Weber number, and the mobility factor of the viscoelastic fluid leads to an increase in both the dimensionless height and radius of the crown, while increasing the Bond number leads to a decrease in the growth of the crown’s dimensions. Moreover, by increasing the film’s thickness, the crown’s height increases, while the crown’s radius decreases. One of the main findings of the present study is that the fluid’s elasticity and surface tension forces enhance the crown’s propagation.

The droplet/wall interaction in the gas channel (GC) of proton exchange membrane fuel cells (PEMFCs) has significant influence on water management, which may affect the performance and durability of the cell. In this paper, a coupled level-set and volume-of-fluid (CLSVOF) method is used to simulate the dynamic behaviours of a micro-droplet impacting on a vertical wall in the GC. The effects of impact velocity (expressed by the Weber number, We), and impact angle on the spreading dynamics are evaluated. Next, the critical conditions for the transition from spreading to rebound are further discussed. For 0

In this paper, the displacement and deformation of a two-dimensional droplet on a vertical wall are examined using a modified pseudopotential model and imposing the multi-relaxation time into the collision term. The selected model guarantees the thermodynamic consistency adjusting proper values for the constant of the potential function k and the weighting factor for the force term, A. Also, it is possible to add adhesion forces and create different contact angles defining the index function for the solid points. Accordingly, non-dimensional parameters of Reynolds number, Weber number, Froude number, and density ratio are defined under the influence of the gravitational force and the uniform vapor flow. Results show that the change of the each dimensionless numbers affects the slip ratio between the droplet and vapor. It is also observed that in addition to the effects of viscous and gravitational forces, the change of the contact angle plays a significant role in the displacement and velocity of the droplet. The results show that reducing the contact angle increases the surface wettability and decreases the droplet velocity. It is seen that by decreasing the intermolecular forces at the Weber number of 62.36 and contact angle of 150°, the droplet begins to decay. Also, it is concluded that the droplet rubbing on the solid surface weakens the slip ratio in all cases.

A procedural model is urgently required for optimization of slag foaming in metallurgical processes. Through physical modeling following the similarity criterion of the Morton number (Mo), this paper investigates the effects of superficial gas velocity, dynamic viscosity, and surface tension on the foaming height. From the physical modeling results, several dimensionless numbers relevant to the slag foaming were identified, and this includes the Reynolds number (Re), Weber number (We), Froude number (Fr), and Mo. In addition, the relationships between Mo and Re for the various types of slag foaming were derived via dimensional analysis. Furthermore, assuming that these dimensionless numbers follow a power law, correlations of foaming height as a function of the slag physical properties and the superficial gas velocity were obtained based on the physical modeling results. Consequently, this study proposes a dynamic mathematical model for predicting the liquid height variation with time during slag foaming according to the gas balance.

This report investigates the influence of microstructure topography on the restitution coefficient, maximum spreading diameter, and contact time of oblique drop impacts on superhydrophobic surfaces. The five surfaces tested allow for comparison of open- versus closed-cell structures, feature size and spacing, and hierarchical versus nanoscale-only surface structures. By decoupling the restitution coefficient into a normal (en) and tangential component (et), it is demonstrated that both en and et are largely independent of the microstructure topography. Instead, the restitution coefficient is governed almost exclusively by the normal Weber number. Next, a new model is presented that relates the maximum spreading diameter to an adhesion coefficient that characterizes the overall adhesive properties of the superhydrophobic microstructure during drop rebounding. Through this analysis, we discovered that surface geometries with greater microstructure roughness (i.e., overall surface area) promote a higher maximum spreading diameter than flatter geometries. Furthermore, the contact time of drop impacts on flat surfaces is positively correlated with the impact velocity due to penetration of the liquid into the porous nanostructure. However, this trend reverses for oblique impacts due to the presence of stretched rebounding behavior. Finally, substrates patterned with sparse pillar microstructures can exhibit pancake bouncing behavior, resulting in extremely low contact times. This unique bouncing mechanism also significantly influences the restitution coefficient and spreading diameter of oblique impacts.

Highly amphiphobic (repelling both water and low surface tension liquids) and optically transparent surface treatments have widespread demand. By combining a rational growth of metal-organic frameworks (MOFs) with functionalization by environmentally safe, flexible alkyl groups, here we present surfaces with nanohierarchical morphology, comprising two widely differing nanoscale features. These nanohierarchical MOF films show excellent amphiphobicity. We further present three key features. First, we demonstrate the need to use flexible alkyl chains to achieve low drop sliding angles and self-cleaning. Second, our thin (∼200 nm) MOF films display excellent optical transparency and robustness. Third, the nanohierarchical morphology enables a unique combination of additional desirable properties, e.g., resistance to high-speed liquid impact (up to ∼35 m/s, Weber number >4 × 104), thermal stability up to 200 °C, scratch resistance, low ice adhesion for >10 icing/deicing cycles, stability in harsh acidic and basic environments, and capability to remove carcinogenic pollutants from water.

This article investigates the formation process of double emulsions, controlling their sizes, and manipulating the encapsulation process in a three nested microcapillary tube configuration. In the proposed geometry, as a result of the interaction of liquids using dripping instability, double emulsions are generated. Governing equations are discretized using the finite-element method utilizing COMSOL Multiphysics 5.4 software, employing axisymmetric assumption, and the interface position is tracked by solving additional transport equations of the phase-field method. Influence of phase flow rates in the form of inner phase Weber number, Wein, intermediate phase capillary number, Caint, and outer phase capillary number, Caout, are studied on the flow regimes and characteristics of double emulsions. Results exhibit that the outer phase flow rate has the most influence among other phase flow rates. Additionally, three distinct regimes, named single emulsion, decussate, and double emulsion, are observed by changing intermediate phase flow rare. Furthermore, satellite droplets are detected at relatively high flow rates of the outer phase due to the second instability induced by the continuous phase to the intermediate phase jet. A comparison between the formation frequency of the present study with some similar works, including both capillary-based and channel-based devices, unveils that the existing configuration has a higher throughput of double emulsion generation in a broad range of flow rates.

Water droplet impacting on a slanted dusty hydrophobic surface is examined in relation to dust mitigation from surfaces. Impacting droplet characteristics including droplet spreading/retraction rates, slipping length, and rebound heights are analyzed via high-speed recording and a tracker program. The environmental dust characteristics in terms of size, shape, elemental composition, and surface free energy are evaluated by adopting the analytical methods. The findings reveal that the dynamic characteristics of the impacting droplet on the slanted hydrophobic surface are significantly influenced by the dust particles. The maximum droplet spreading over the dusty surface becomes smaller than that of the nondusty surface. The presence of the dust particles on the slanted hydrophobic surface increases energy dissipation, and the water droplet slipping length over the surface becomes less than that corresponding to the nondusty surface. Impacting droplet fluid infuses over the dust particle surface, which enables mitigation of dust from the surface to the droplet fluid. A dust-mitigated area on the slanted surface is larger than that corresponding to the horizontal surface; in which case, the area ratio becomes almost six-fold, which slightly reduces with increasing Weber number. The optical transmittance of the dust-mitigated surface by the impacting droplet remains high.

This paper aims to analyze dimensionless parameters, Weber number and Reynolds number, to describe inception of droplets at the interface of liquid–liquid pipe flow. Experiments are conducted in a 15.9 mm diameter and 6 m long stainless-steel pipe using high viscous oils and water. An empirical Weber number correlation, adapted from gas–liquid flow and modified for liquid–liquid flow, is proposed to identify the formation of droplets at the interface. The analysis of the proposed Weber number correlation indicates that the droplets originate when Weber number ≈ 300 and higher. The experimental data from the literature of low and high viscous oil–water flows is also considered for the analysis. The analysis suggests that the Kelvin–Helmholtz instability singularly does not produce droplets at the interface but only when the fluids inertial forces exceed the interfacial tension forces by 300 times. The data from the literature provide sufficient evidence on the existence of wall originated fluctuations in at least one of the fluid phases before the advent of interfacial waves or interfacial droplets. Accordingly, the investigation of Reynolds number indicates that the droplets are found to form when either oil or water phase Reynolds number Reo or Rew attains ≈ 12,000 with the respective fluid mass fraction ≥ 0.2. However, for fluid mass fraction

An experimental study on the contact angle dynamic during the impact of single water droplet onto a hot flat practical stainless steel surface under medium weber number was performed. The tested solid material was practical stainless steel of SUS 304. The surface roughness of the hot surface, the static contact angle at ambient pressure, and the diameter of the solid materials were respectively Ra = 0.2 µm, 85.7o, and 30 mm. The surface temperatures were varied from (60–200) oC. The liquid droplet diameter was 2.4 mm. The Weber numbers were 52.1, 57.6, and 63.1. The dynamic contact angles of single droplet during the impact were determined by implementing the developed image processing technique of the obtained video images from experiments. As a result, it was found that (1) the time needed to reach a maximum diameter of single droplet on a flat hot solid surface decreases with the increase of surface temperature, (2) the increase of the Weber number will postpone the spreading of the droplet from its receding, (3) there are three regions of the dynamic contact angle during the liquid evaporation, those are transient spreading region (region 1), a quasi-mechanical equilibrium region (region 2), and transient evaporation region (region 3), and (4) The contact angle in quasi-mechanical equilibrium region the under the nucleate boiling is insensitive to the surface temperature.

The dynamic characteristics of droplet obliquely impact on a liquid film are investigated numerically by using the Coupled Level Set and Volume of Fluid method (CLSVOF), with the impact angle ranging from 0° to 60°. The influence factors as film thickness and Weber number on the splashing formation and the crown evolution are also systematically investigated. Results showed that the crown radius on the behind side of the advancing droplet decreases as the impact angle increases, while the crown radius on the front side of the advancing droplet appears to be an opposite trend. Furthermore, the crown diameter is almost unchanged when the impact angle increases from 30° to 45°. However, when the impact angle increases to 60°, the crown lamella on the front side of the advancing droplet is completely suppressed. It is found that the crown height does not linearly change with the film thickness. With an increase in the film thickness, the right crown height first increases and then decreases, indicating the presence of a critical film thickness under the studied conditions. An increase in Weber number results in a transition from non-splashing to partial splashing and even to entire splashing.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2020-06-24, 資料番号: PA2110026000

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2021-06-08, 資料番号: PA2120019000

Understanding the lower stability limit of gas turbine combustors is imperative for safe operation as well as for the approval of novel sustainable aviation fuels. Previous results have indicated that at relatively low temperatures and pressures (Tair = Tfuel = 238 and 258 K, P = 101 kPa, ΔP/P = 2%), the lean blowout (LBO) limits correlated to fuel physical properties, while at relatively high temperatures and pressures (Tair = 394 K, Tfuel = 322 K, P = 207 kPa, ΔP/P = 3%) chemical properties were highly correlated. This suggests that a transition occurs between physical and chemical property dependency. In the present effort, a single nozzle swirl-stabilized combustor was used to study the LBO for fuels with varying chemical and physical properties as well as operating conditions to study a transition from physical property dependence at low pressures and temperatures to chemical property dependence at higher temperatures and pressures. Four fuels with a wide range of physical and chemical properties were tested at combustor inlet conditions of Tair = Tfuel = 305–355 K, P = 107 kPa, and ΔP/P = 2–6%. It was found that the transition of LBO dependence from fuel physical to chemical properties does not happen at a single point. Rather, a transition range of conditions was observed for changes made to the pressure drop across the combustor dome (ΔP) as well as changes made to air and fuel temperature. Changes in ΔP/P were found to change the relative LBO performance across fuels and demonstrated a change in physical to chemical property dependence as ΔP/P increased. Analysis via random forest regression using dimensionless parameters such as an approximate Weber number and Ohnesorge number further highlighted the transition from physical to chemical property dependency with increasing ΔP/P.

The mechanism of fuel surface cooling with water was studied experimentally by visualising a water droplet impacting on surfaces of burning wood. This droplet impact condition was investigated experimentally for the first time. The main focus of this current investigation is to provide a behaviour description of a water droplet impacting a burning solid surface, with the intent to clarify the differences between impingement on cold surfaces and burning real application materials such as wood. Wood was used in the tests to simulate the temperature and surface characteristics of burning specimens. Sizes of water drops were in a range between 0.95 mm and 2.1 mm, resulting in a Weber number from 52 to 264. Visualisation of impact proved to be a challenge because of the optical characteristics of wood undergoing pyrolysis, the required high shutter speed and the very small droplet size. Images were improved significantly and used for quantitative measurements through careful post-processing and the application of a sequence of enhancement techniques. A new mechanism of fire extinguishment is proposed based on the sealant effect of the water liquid layer. The results showed a clear divergence in the behaviour of water splash on burning wood compared to cold surfaces. Prompt splash, receding breakup, and deposition were observed to occur at the initial stage of the droplet impact. The critical Weber number at which the impact drop breaks up and splashes with sub-droplets was shown to be lower for the tests on burning surfaces. Different types of wood also showed the dependence of droplet dynamics. Furthermore, the increase in droplet impact velocity revealed that most of the wetting was achieved during the first few milliseconds after impact and resulted in faster evaporation. Finally, a conceptual model is presented to summarise the behaviour of a water droplet impact on wood surfaces.

Liquid marbles are gaining increased attention because of their added advantages such as low evaporation rates, less friction, and ease of manipulation over the pristine liquid drop. Their functionalities could be further enhanced by incorporating different types of particles (size, hydrophobicity, chemical properties, etc.), commonly called Janus liquid marbles (JLMs). However, their fabrication process remains a challenge, especially when we require continuous production. Here, we present a simple and fast approach for the fabrication of JLMs covered with nano- and microparticles in an additive-free environment based on the controlled impact of a water drop over the particle beds. The fabrication process involves collection of polyvinylidene difluoride particles (PVDF, particle type 1) by a water drop followed by its impact over an uncompressed bed of black toner particles (BTP, particle type 2). The whole process takes a time of approximately 30 ms only. The drop impact and the condition of the JLM formation were explained based on the Weber number (We) and maximum spread (βm) analysis. A theoretical model based on the energy balance analysis is performed to calculate the maximum spreading (βm), and the experimental and theoretical analyses are found to be in good agreement. Tunability in particle coverage is demonstrated by varying the droplet volume in the range of 5-15 μL. We further extend this strategy for the fast and continuous production of nearly identical JLMs, which could enhance the capabilities of open-surface microfluidic applications.

In this study, the bubbles in rice flour batter were investigated under a constant temperature, because the bubble size distribution is important for the control of food texture. We obtained experimental data using a hand mixer and compared the properties of doughs prepared using six rice flours; each flour was prepared through a different milling process. We also added the size effect of the rice flour particles as the Bond number. Furthermore, we performed a dynamic wettability test to estimate the wettability of the rice flour surface. The results of this test were described well by the Washburn equation, and dccosθ/dp was calculated as a wettability parameter (where, dc = effective diameter of a capillary in a powder bed, cosθ = the contact angle, dp = mean particle diameter of rice flour). If bubble sizes depend mainly on the inertial force, viscous force, surface tension, and gravity, then the normalized mean bubble diameter should be a function of the Reynolds number, Weber number, and Froude number. The mean bubble diameter (dbm) generated by whipping was expected to be affected by the thickness (d) of the rod of the mixer, its movement speed, and physical properties of the material. Therefore, dimensionless mean diameter (dbm/d) was expressed based on a dimensionless equation. In the three‐phase dispersion, different empirical equations were obtained depending on the amount of rice flour added, and the bubble diameter could be predicted using dimensionless parameters. In addition, the equations were generally applicable to the various materials selected for this study. Practical Application The powder properties of rice flour were investigated, and dimensionless parameters were analyzed to construct an appropriate process control system for rice flour‐based food products. Although the process method optimized for flour products is also used for rice flour products in practical situations, the comprehensive evaluation based on dimensionless parameters leads to optimization of the process for rice‐flour based products. Moreover, this optimization might strongly support the creation of a new texture, and thus, the potential for market expansion of rice‐flour based products is considerable.

The solute mixing phenomena of filler and base metal melts during dissimilar-filler metal-inert gas (MIG) welding are simulated by a multi-phase lattice Boltzmann (LB) model for predicting macrosegregation formation in the weld joint. The LB models incorporates the calculations of fluid field and solute transport, and the quantitative capalibity is validated through the problem of one-dimensional liquid diffusion. For the MIG welding of AA6063 sheets with an ER5183 filler wire, the simulation results reveal that the periodic impingement of heterogeneous filler metal droplets leads to symmetrical vortex flows accompanied by the unmixed zones formed in the center. These unmixed zones locate in the bulk weld pool and would develop into macrosegregation in the weld joint. In addition, the fluid flow near the fusion boundary remains almost stagnant, which gives rise to the appearance of the thereby continuous unmixed zone. The predicted distribution of unmixed zone in the weld pool agree well with the macrosegregation patterns in the weld joint observed in the experiments. For partially mimicking the effect of external magnetic field on droplet movement recorded by a high-speed camera, random droplet flight directions are arranged in the LB simulations. With the increase of droplet impact speed (Weber number) and the arrangement of random droplet flight direction, the simulated Mg concentration in the weld pool becomes more uniform, implying that the risk of macrosegregation formation is reduced. The LB simulations provides insight for better understanding the formation of weld defects during MIG welding with a dissimilar filler metal.

Background When spraying simple liquids through a hydraulic nozzle, the mechanisms that affect breakup into droplets have recently been described. However, it is unknown how the droplet size distribution changes when surfactant‐based adjuvants are added to the spray. Results When spraying different surfactant‐based solutions containing commercial adjuvants, the breakup of the different liquids behaves in the same way as for water‐only sprays, but the droplet sizes are smaller. By replacing the equilibrium surface tension with the dynamic surface tension at a surface age of ~20 ms, the volume mean droplet size variation with the Weber number, flow rate and nozzle geometry follows the predictions established for pure water sprays. When we rescale the droplet size distribution with the mean droplet size, all distributions collapse onto a single curve and can be described by a compound Gamma function. Conclusion Addition of a number of surfactant‐based adjuvants to an agricultural spray is observed to lead to a slight decrease in the volume mean droplet size. We show that the effects of these adjuvants on the drop size can be understood by taking into account the nozzle geometry, the flow rate, the liquid and air densities and the dynamic surface tension of the surfactant solutions at a surface age of approximately 20 ms. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., Surfactant additives are often used for agricultural sprays; we show that this leads to smaller droplets due to the dynamic surface tension. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The pre-film breakup in a high-performance gas turbine combustor is simplified into a two-dimensional and three-dimensional liquid sheet breakup, and the liquid sheet breakup processes, mechanisms and characteristics are investigated under nearly representative operating conditions. First, the two-dimensional flat liquid sheet breakup processes are simulated using a Reynolds-averaged Navier–Stokes (RANS) model and a VOF (volume of fluid) method. The effect of airflow velocity on the liquid sheet breakup distance is analyzed under various high-speed airflow velocities of 80 m/s, 180 m/s and 280 m/s. Then, the three-dimensional liquid sheet breakup processes are simulated using the large eddy simulation (LES)/VOF method, and the variations of the characteristics of both the horizontal liquid sheet and the oblique liquid sheet, such as the breakup process, breakup mode, breakup distance, wave characteristics and liquid sheet thickness, are analyzed. It’s found that for the two-dimensional flat liquid sheet breakup, the liquid sheet breakup distance decreases with the increase of airflow velocity or Weber number (We). When the airflow inlet velocities are 80 m/s (We = 6.13), 180 m/s (We = 220.5) and 280 m/s (We = 741.13), the liquid sheet breakup distances are 22.2 mm, 16.0 mm and 10.5 mm, respectively. An empirical formula of the liquid sheet breakup distance was thus summarized. For the three-dimensional liquid sheet breakup, as the airflow inlet angle increases, the breakup distance is significantly shortened, which is 2.21 mm at 0° and 1.85 mm at 15°, and the oscillation angle of the liquid sheet becomes larger. For the oblique liquid sheet breakup, the liquid sheet breakup mode becomes more complicated and the surface wave fluctuation amplitude increases. The empirical formula prediction values of the liquid sheet breakup distance developed in this study are in good agreement with the numerical results.

In this paper, the influence of the liquid droplet temperature on thermo–hydrodynamics of a single droplet impinging on surfaces having different hydrophobicities is experimentally investigated. Variation in the liquid temperature typically results in alteration of properties such as the density, the viscosity, the surface tension, and the enthalpy, consequently, the droplet dynamics gets to be modified. Employing high-speed imaging technique, the morphology and spreading pattern are investigated for water droplet collision on hydrophilic, hydrophobic and super-hydrophobic surfaces. The droplet deformation is monitored qualitatively and quantitatively for drops in the temperature range from 5 to 85°C and the Weber number between 14.5 and 160. It is observed that with an increase in the liquid temperature the spreading factor increases owing to the combined effect of reduction in the density, the surface tension, the viscosity and the contact angle of the solid surface. The differences in extension of droplets under the extreme temperatures for hydrophilic, hydrophobic and super-hydrophobic surfaces are noted to be 62.7, 27.76, and 20.52%, respectively. At the low temperature, the surface tension force dominates and the Cassie–Baxter state prevails on a textured super-hydrophobic surface and the droplets bounce off. In contrast at elevated temperatures, the liquid–solid interface ruptures and liquid penetrates into the cavities and results in the Wenzel state. Furthermore, the drop which exhibits multiple bounces in the low temperature regime is found sticking on a super-hydrophobic substrate at the high droplet temperature irrespective of the Weber number.

Drop impact on a heated surface not only displays intriguing flow motion but also plays a crucial role in various applications and processes. We examine the impact dynamics of a water drop on both heated flat and nanostructured surfaces, with a wide range of impact velocity (V) and surface temperature (Ts) values. Via high-speed imaging and temperature measurements, we construct phase diagrams of different impact outcomes on these heated surfaces. Like those on the heated flat surface, water drops can deposit, spread, rebound, or break-up with atomizing on the heated nanostructures as V and Ts are increased. We find a significant influence of nanostructures on the impact dynamics by generating particular events in specific parameter ranges. For example, events of splashing, gentle central jetting, and violent central jetting are observed on and thus triggered by the heated nanostructures. The heated nanotextures with high roughness can easily trigger the splashing and the central jetting. Our data of the normalized maximum spreading diameter for the heated surfaces display distinct trends at low and high Weber number (We) ranges, where We compares the kinetic to surface energy of the impacting droplet. Finally, compared with the flat surface, the dynamic Leidenfrost temperature (TLD) for We ≈ 10 is decreased (by ≈60 °C) by the high-roughness nanotextures. In addition, our experimental data of TLD is consistent with a model prediction proposed by balancing the droplet dynamic and vapor pressure.

Boiling phenomena are interested in many fields of engineering such as piping and cooling electronic systems. Usually, the phase change through the heat transfer process is avoided as it makes the fluid flow unstable and complex. In the current study, the effect of using porous media in the channel to control the thermo-hydraulic characteristics of the system is studied. To do this aim a steady-state one-dimensional system of equations is solved by numerical method. The gas and liquid Navier–Stokes equations coupled with energy equations are solved simultaneously. The developed method validated with a well-known benchmark problem and the comparison shows a good agreement. The results show that Sauter bubble diameter, forces on the cross-sectional area of fluid, Froude number, fluid wall forces, velocity of mixture, Reynolds number of vapor, fluid velocity, square root of Froude number divided by Weber number, base volume fraction, vapor volume portion, maximum vapor volume portion, critical vapor volume fraction decrease by an increase in Darcy forces, while mean root square of the applied force over the cross section of fluid and heat transported by thermal radiation increased by an increase in Darcy friction up to 1000. The porous media parameters have negligible effects on Eotvos number, speed of sound, liquid Reynolds number, bubble departure frequency, convection heat transfer coefficient, and critical pipe diameter.

The laws of drop formation are experimentally studied in narrow horizontal microchannels with rectangular section with heights from 50 to 150 μm. It is shown that there exists a new flow regime when the drops in the form of vertical liquid bridges move along the microchannel. Three mechanisms of formation of such drops are distinguished: formation directly near the liquid nozzle, departure of drops from liquid moving at the lateral sides of channel, and drop formation due to destruction of severely deformed drops and horizontal liquid bridges. It is established that the deformation of drops increases with increasing Weber number. It is demonstrated that the drops begin to deform when the first critical value of the Weber number is achieved and begin to be destroyed when the second one is achieved.

Twin fluid atomizers allow for two different spray forming modes, flow focusing and flow blurring, depending on the operating, geometric, and thermophysical properties of the working liquids. In flow focusing mode, the liquid jet breaks outside the injector, whereas in flow blurring mode, the liquid jet breaks inside the atomizer. Operating conditions are believed to play an essential role in determining the size and velocity of droplets with specific geometric and fluid properties. This work investigates the effect of air flow rate and liquid flow rate on spray characteristics by varying them independently. The geometric parameters, height (H = 0.3 mm) and orifice diameter (D = 1.5 mm) of the atomizer are kept constant (with H/D = 0.2). Spray mode is identified using high-speed images. Phase Doppler particle analyzer is used to measure droplet size and velocity simultaneously. Regardless of liquid flow rate, flow focusing mode is observed at low air flow rate and flow blurring mode is observed at high air flow rate. In general, flow blurring mode produced smaller droplets at higher velocities than flow focusing mode. The Joint probability distribution function of droplet size and velocity shows unimodal near the exit of the atomizer and bimodal distribution around 50 mm downstream due to the completion of the secondary breakup around that position and the subsequent loss of momentum due to the resistance of the surrounding air from the injector outlet. Flow focusing mode showed that sauter mean diameter and mass median diameter tend to increase initially due to coalescence of the spray and begin to decrease due to secondary breakup. The joint probability distribution of droplet Weber number and the droplet Reynolds number indicate that the flow blurring spray produced finer and faster droplets than the flow focusing spray. Overlapping of two Gaussians representing smaller and larger droplets best fits the droplet probability distribution as compared to lognormal or gamma or beta distribution.

Jet breakup during droplet impacted on heated surface has received wide concerns due to its regularity. The Weber number (We) is a common used dimensionless parameter to classify and analyze the phenomenon, but it is unable to summarize all detailed phenomenon variations and related theory. Thus, the effect of droplet size on jet breakup was investigated by considering its significant difference in various scenarios. The behavior of n-butanol droplets with diameter range from 1.71 mm to 2.84 mm impacts a heated surface with jet breakup was recorded by backlit technology. Three parameters, the jet breakup time, the jet breakup length and the jet ligament diameter, were analyzed to illustrate the phenomenon. The results showed that these parameters are affected by the droplet size largely at 10 < We

Manipulating droplets by tailoring the surface wettability has recently attracted a great interest of research. In this work, controllable splitting of impacting droplets was realized by a hybrid-wettability surface consisting of a hydrophilic background surface and two intersecting hydrophobic stripes. A three-dimensional VOF-based (volume of fluid) numerical model had been developed to investigate the splitting dynamics, and a dynamic contact angle model had been employed to capture the droplet interface near the triple line. Several key parameters including surface hydrophilicity, stripe hydrophobicity, stripe width, and impact Weber number were examined. On the basis of these parameters, complete impact phase diagrams were constructed. The results showed that there are four possible pathways for impacting droplets, namely no splitting I, no splitting II, splitting I, and splitting II. The conditions for the four pathways were distinguished. Finally, some new methods had been proposed to further control volume ratios of split satellite droplets and splitting time. The results reported in this work provided useful guideline for designing hybrid-wettability surfaces to realize controllable droplet splitting.

Flow dynamics of two-dimensional liquid sheets discharged into low-speed gaseous crossflow were experimentally investigated. The flow characteristics of liquid sheets were visualized by taking advantage of diffused backlight shadowgraphy and high-speed photography. Three injectors with an equal thickness of 0.35 mm and aspect ratios of 30, 60 and 90 were manufactured and tested at different flow conditions. A full discussion about the flow characteristics of two-dimensional liquid sheets in the presence of transverse airflow is provided. Visualizations revealed that the liquid sheets represented a unique concave-like structure that was named as inflated sheet. This special characteristic was not previously seen on any other circular or non-circular liquid jets and therefore made the flow dynamics of liquid sheets in subsonic crossflow very distinguished. The inflated sheet was found to transform from an enclosed structure into an open structure. The open inflated sheet was disturbed by different breakup mechanisms including sheet rupture, bag breakup, and Rayleigh–Taylor instability. Based on the observed phenomena, the flow was grouped into five regimes including biconvex, enclosed inflated sheet, open inflated sheet, bag breakup/sheet rupture, and multimode breakup. Furthermore, it was found that the droplet region was bifurcated due to the different breakup mechanisms acting simultaneously upon the sheet. Measurements of sheet trajectory were performed and the effects of momentum ratio and Weber number were studied. It was found that Weber number was only effective at low values, while momentum ratio remarkably impacted the trajectory.

The experimental research into water droplet collisions resulting in their secondary atomization has been performed. The temperature of the first and second droplets was 20°С and 90°С, respectively. The interaction regime maps using the dimensionless linear collision parameter and the Weber number have been plotted. We also determined the integral characteristics of child droplets: size and number. When droplets with a temperature of 90°С were used, the critical Weber numbers went down by 10–30% for disruption, 5–15% for separation, and 15–35% for coalescence. For bounce, they increased by 50–80%. The number of newly formed child droplets for droplets with a temperature of 90°С increased by 5–20% relative to the droplets with a temperature of 20°С. The integral characteristics of child droplets and interaction regime maps of droplets with different temperatures can be used to optimize heat and mass transfer technologies.

Extensive investigations have been carried out on the thermo-hydrodynamics of nanofluid droplet interaction with heated and non-heated flat surfaces. However, the influence of shape of nanoparticles on the dynamics of droplet impingement on heated flat surfaces is yet to be explored in detail. In this study, hydrodynamics of nanofluid droplet impingement process on heated and mechanically polished aluminum substrate was studied using dissolved Al2O3 nanoparticles having spherical as well as cylindrical shapes. Nanofluids of 0.3% volume fractions were prepared from spherical Al2O3 particles of mean size less than 50 nm and from cylindrical Al2O3 particles of 2–6 nm in diameter and 200–400 nm in length. It was observed that, the impact dynamics is different from that of base pure fluid owing to the presence of nanoparticles. Leidenfrost temperatures of both nanofluids were dropped drastically in comparison with pure liquid. Further, the residence time, spread factor as well as retraction height also exhibit a different behavior against the base fluid. Detailed investigations were carried out for different Weber numbers (We = 18–159) and surface superheat and resul ts obtained were compared with de-ionized (DI) water.

The authors have presented results of experimental investigations into the process of collisions of water droplets in a gas medium with variation of the basic parameters in wide ranges corresponding to advanced gas-vapor-droplet technologies: sizes (radii) 0.1–1 mm, velocities of travel 0–10 m/s, angles of interaction (attack) 0–90o, relative volume concentration 0.1–10–3–8.5·10–3 m3 of water/m3 of the gas, velocity of motion of the flow 0–10 m/s, and temperature 20–1000oC; directions of flows: opposing, codirectional, and lateral. In processing the experimental results, the emphasis was on the influence of the relative volume concentrations of the droplets on the characteristics of their interaction. The relative probabilities of four typical regimes of interaction of the droplets have been calculated: recoil, coagulation, spread, and breakup (reduction in size). Parametric ranges have been shown for droplets for which the concentration growth of the latter in an aerosol leads to an increase in the probability of coagulation, recoil, and size reduction. Also, the authors have presented ranges of variation of the concentration of droplets in a gas medium, in which the probabilities of all the investigated interaction schemes are high. The experimental results have been processed with the existing approaches to generalization by calculating the angular factor of interaction and the Weber number.

We show the ranges of droplet size reduction depending on the relative velocity before interaction, initial dimensions of each of the droplets and their ratio, as well as the Weber, Ohnesorge, Reynolds, and capillary numbers. Experiments are conducted for water, Diesel fuel emulsion, and castor oil emulsion. The ranges of a prospective increase in the liquid surface area due to droplet collisions as a function of the variable parameters are determined. We single out three Weber variation intervals: 150 (the number and dimensions of child droplets remain the same with an increase in the Weber number). The number and average size of child droplets are influenced by the relative velocity of the two parent drops, the ratio of their dimensions, as well as the angular and linear collision parameters.