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A high-speed imaging technique was used to observe the phase separation process of water (H2O)-20 %succinonitrile (SCN) immiscible solution within ultrasound field. Combining with numerical simulation, the effects of ultrasonic cavitation and acoustic streaming on the fragmentation and migration of secondary droplets were revealed. It was found that the previously spherical or near-spherical secondary H2O-rich droplets formed under static condition were dynamically transformed into several novel forms, such as tadpole-like, string-beads, gourd-like, and threadlike patterns. The calculated results showed that the cavitation could fragment micron-scale H2O-rich droplets because of the produced higher shock wave pressure than the droplets’ Laplace pressure, and the subsequent droplet morphology evolution mainly depended on the liquid ejection volume determined by the distance between the droplets and the collapsing bubbles. Meanwhile, acoustic streaming, which generated shear force exceeding the surface tension of H2O-rich phase, stretched, split and finally fractured millimeter-sized or even larger secondary droplets into several smaller spherical sub-droplets. In comparison, the observed secondary droplet distribution characteristics in H2O-20 %SCN solution were similar to the Bi-rich particles in the ultrasonic solidification microstructures of Al-30 %Bi immiscible alloy, confirming that this work provided a deep understanding of the liquid phase separation mechanism within ultrasonic field.

• Full coalescence and partial coalescence are elucidated. • Interfacial force at the oil droplet bridge affects coalescence process. • Critical line of partial coalescence and full coalescence has been determined. Flotation is an efficient pretreatment technology for oily water. In this study, the coalescence processes of air bubbles and isooctane droplets with different size ratios, 1.0–2.2, were investigated using a high-speed imaging system. Two types of oil droplet-gas bubble interaction have been identified: full coalescence and partial coalescence. Through analysis of interfacial force at the liquid bridge, it has been observed that interfacial force at the oil droplet bridge affects the lateral contraction of the liquid bridge, which in turn affects the coalescence of oil droplets and gas bubbles. Through dimensional analysis, a phase diagram of dimensional parameter coalescence behavior has been constructed, and the critical line of partial coalescence and full coalescence has been determined. This work fills the void in the study of secondary oil droplet coalescence within the oil droplet-gas bubble coalescence in gas flotation, which is potentially applied for industrial air flotation system design and optimization. [ABSTRACT FROM AUTHOR]

The phenomenon of droplet impacting on hot film exists widely in nature and engineering applications. Considering limitations in current researches, an experimental setup to record the process of droplets impacting on hot film was built in this paper. Lots of experiments have been done and then the transitional threshold was established to determine impingement outcomes. The number of secondary droplets and the temperature of the secondary droplet are investigated based on the experimental results. The results show that impingement outcomes can be determined by the splashing parameter provided in this paper. The number of secondary droplets increases with the splash parameter, the probability distribution function of secondary droplet diameters follows the Weibull distribution. The temperature of secondary droplets is close to the hot film and almost all secondary droplets' temperature is above 90% of the hot film.

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• A single drop with different Weber number impacting on a static liquid film of different thickness is recorded by a high-speed camera. • A series of image pre-processing procedures are performed to detect and extract the secondary droplets from the image. • The evolution of the secondary droplet number, diameter and velocity are carried out. • The effect of Weber number and the dimensionless film thickness on the features of the secondary droplets is investigated. In this paper, an experimental study of the generation of secondary droplets produced by a single droplet impacting on a static liquid film is studied. Different factors are investigated through a high-speed camera at 5000 fps, such as the Weber number of the falling droplet, and the dimensionless liquid film thickness. Several technics are used to improve the image quality to detect the secondary droplets. Especially, a particle tracking algorithm is developed to calculate the velocity distribution of the secondary droplets. The influence of time, Weber number and dimensionless film thickness on the number, diameter and velocity distribution of the secondary droplets are performed. The results show that as time increases, the number of the secondary droplets rises up in the dimensionless time with the range of 0 < τ < 2, then decreases during the rest time regardless the Weber number and the dimensionless film thickness. The maximum value of the diameter distribution of the secondary droplets becomes higher, and the range of the velocity distribution becomes narrower over time. As increasing the Weber number, the number of secondary droplets grows up, as well as the peak value of the diameter distribution. Also, the increase of Weber number has a slight effect on the width of the velocity distribution of the secondary droplets. The results also show that, after the droplets impacting on liquid film with different thickness, the number of the secondary droplets changes little at different film thickness. Additionally, the dimensionless film thickness does not have a strong effect on the diameter distribution or the velocity distribution of the secondary droplets. [ABSTRACT FROM AUTHOR]

In order to study the influence of surface roughness on the splash characteristics of water droplets, the experiments of droplets impacting different roughness surfaces are carried out under different temperature conditions. The information of the diameter, reflection angle, and velocity of the secondary droplets impacting different roughness surfaces is obtained by a high-speed camera. The influence of roughness on the splash characteristics of the water droplets is analyzed. The results show that the diameter of secondary droplet is mainly affected by the roughness in a certain impact parameter K range. The rougher the surface is, the larger the average diameter of secondary droplets. The effects of roughness on the normal and tangential velocity of secondary droplets are opposite. The dimensionless angle of the secondary droplet is positively correlated with the roughness St. The temperature effect appears at St > 0.1, while it is smaller compared with the roughness effect. [ABSTRACT FROM AUTHOR]

• Ultrasound modulates static spherical Bi-rich particles into several novel forms. • Shock wave generated by cavitation induces H 2 O-rich droplets fragmentation. • Distance between collapsing bubble and droplet determines sub-droplet morphology. • Shear flow produced by acoustic streaming stimulates H 2 O-rich droplet breakup. A high-speed imaging technique was used to observe the phase separation process of water (H 2 O)-20 %succinonitrile (SCN) immiscible solution within ultrasound field. Combining with numerical simulation, the effects of ultrasonic cavitation and acoustic streaming on the fragmentation and migration of secondary droplets were revealed. It was found that the previously spherical or near-spherical secondary H 2 O-rich droplets formed under static condition were dynamically transformed into several novel forms, such as tadpole-like, string-beads, gourd-like, and threadlike patterns. The calculated results showed that the cavitation could fragment micron-scale H 2 O-rich droplets because of the produced higher shock wave pressure than the droplets' Laplace pressure, and the subsequent droplet morphology evolution mainly depended on the liquid ejection volume determined by the distance between the droplets and the collapsing bubbles. Meanwhile, acoustic streaming, which generated shear force exceeding the surface tension of H 2 O-rich phase, stretched, split and finally fractured millimeter-sized or even larger secondary droplets into several smaller spherical sub-droplets. In comparison, the observed secondary droplet distribution characteristics in H 2 O-20 %SCN solution were similar to the Bi-rich particles in the ultrasonic solidification microstructures of Al-30 %Bi immiscible alloy, confirming that this work provided a deep understanding of the liquid phase separation mechanism within ultrasonic field. [ABSTRACT FROM AUTHOR]

• Secondary droplet formation mechanism after droplet impact on hot wall is discussed. • Quantitative analysis of size and velocity distribution of secondary droplets is presented. • Breaking model of droplet on hot wall is proposed with good prediction. When a droplet impacts a superheated surface, due to the different effects between wall superheat and the initial Weber number of the droplet, secondary droplets will be generated by different mechanisms due to the coupling effect of kinetics and temperatures, such as fracture, spray, atomization, and breakup. In this paper, a visual experimental study is conducted based on the effects when droplets impacting on the superheated wall. The wall temperature ranges from room temperature to 650 °C, and the droplet Weber number ranges from about 10 to 500. Based on the sequence images of droplets taken by a high-speed camera, three of four secondary droplet mechanisms (excluding fracture droplets) are presented and discussed in detail. Furthermore, the size and velocity distribution of the secondary droplets under different mechanisms are quantitatively analyzed through image processing. Finally, an edge-breaking model of the droplet with a high-temperature wall is proposed considering the evaporative momentum force near the triple-phase contact line, and it is in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: The coalescence of an aqueous droplet at an oil–water interface under an electric field has been investigated, with a view to quantify conditions that give rise to secondary droplet formation. Two patterns of drop-interface coalescence may occur: complete coalescence and partial coalescence. The former is obviously the desirable pattern for industrial coalescers. However in practice, the process of coalescence could actually produce smaller droplets, which become more difficult to remove, and hence undesirable. This is caused by either necking, due to extensive elongation of the droplet, or reaction to a fast and energetic coalescence and is referred to as partial coalescence. The volume of the droplets formed in this way has been analyzed as a function of the initial droplet size, electric field strength and the distance between the droplet and the interface. The expansion speed of the neck connecting the droplet and interface at the beginning of the pumping process has also been quantified. These results are useful in optimizing the electro-coalescence process. [Copyright &y& Elsevier]

• Experiments were conducted to measure basic features of early-splash droplets. • Larger drops of slower speed are ejected in more upward direction as time advances. • Size, speed, and angle of secondary drops are expressed as sole functions of time. • Number of secondary drops and splash period depend on excess Weber number. • These correlations can be used to calculate various statistics of secondary drops. Basic features of early-splash droplets produced during single-drop impact onto a quiescent liquid film were explored experimentally using water and three aqueous solutions of glycerol at different concentrations as the test liquids. It was found that the time from the contact of primary drop with a liquid film is a crucial parameter in determining the size, ejection velocity, and ejection angle of secondary drops. It was also shown that the time period for the early splash to occur, the total number of secondary drops, and the number of secondary drops produced per unit time are dependent primarily on the impact Weber number. From the experimental data accumulated in this work, dimensionless correlations were developed for these quantities. These correlations can be used to calculate various quantities such as the total mass, total surface area, and total kinetic energy of the secondary drops. As a typical example, the total mass of the secondary drops was calculated to explore its dependence on the Weber and Ohnesorge numbers. [ABSTRACT FROM AUTHOR]