Along with the children's pursuit, colorful bubbles float in the air and dance with the wind, bringing infinite happiness to the curious children. It has attracted extensive attention from scholars in different fields such as physics, chemistry, and mechanics. This is because bubbles play an indispensable and important role in people's production and life. For example, fermentation and puffing in food processing are the process of bubble formation, and bubbles are also indispensable in many fields such as pharmaceuticals, cosmetics, and mineral flotation. .
Under the long-term guidance of Academician Wei Bingbo and Prof. Dominique Langevin, Prof. Zang Duyang's group has provided a new method for preparing bubbles using ultrasonic fields. Shifting acoustic cavity resonance mechanisms. The research results were published online in the top international journal Nature Communications on September 11 (London time) under the title "Inducing drop to bubble transformation via resonance in ultrasound". Northwestern Polytechnical University is the first author and the only corresponding author of the paper, and the cooperative units are Monash University in Australia and the University of Hull in the United Kingdom.
The previous methods of generating bubbles mostly rely on turbulent mixing, flow focusing technology or strong shear generated by microfluidics to provide the energy required to generate the bubble interface. Although these techniques can efficiently generate bubbles, it is difficult to study the physical problems of the bubble itself, especially the physical and mechanical properties of the bubble film.
Under acoustic levitation conditions, by adjusting the intensity of the acoustic field, the shape of the suspended droplets undergoes changes from ellipsoid-like to flat liquid cakes to curved liquid films. Interestingly, when the bending degree of the liquid film reaches a certain critical value, the liquid film suddenly expands and closes to form bubbles (Fig. 1). This phenomenon can neither be explained by the equilibrium shape theory of acoustically suspended droplets, nor can it be learned from existing droplet instability phenomena.
Through systematic experimental research and in-depth numerical simulation, Prof. Zang Duyang's group found that one of the key factors for the droplet-bubble transition is the bending of the liquid film to form a bowl-shaped cavity. When the cavity reaches the critical volume, a sharp increase in its volume is induced. It has been found experimentally that the critical volume is independent of the liquid species and the initial size of the droplet, but is significantly dependent on the frequency of the acoustic field. This is because the cavity formed by the bending of the bubble actually constitutes a Helmholtz resonant cavity. When the volume is appropriate, it will resonate with the ultrasonic field and absorb a large amount of energy from the vibration source, resulting in the violent expansion of the cavity and rapid closure to form bubbles.
Due to the inherently high specific surface area of ​​bubbles, when the liquid film is punctured, a common phenomenon is that the film pores continue to grow and the liquid film shrinks to reduce its surface area. Eventually, either break up into many small droplets or form sub-bubbles. In this work, Professor Zang Duyang's research group realized an "inverse process": the area of ​​the liquid film increases continuously, bends and rapidly expands into a bowl shape, and finally wraps the air to form a bubble.
The most important finding of this study is that the cavity surrounded by the acoustically suspended curved liquid film can be regarded as an acoustic resonator independent of the liquid properties. Once the curved liquid-film cavity reaches the proper volume, either by increasing the sound field strength or by external drag, an ultrasonic resonance occurs that suddenly expands to form a bubble.
On the basis of this theory, Professor Zang Duyang's research group has developed a new technology for the controllable preparation of bubbles using acoustic levitation. Acoustic energy can be efficiently absorbed on the millisecond scale and increase the area of ​​the liquid film. At this point, the sound field provides not only the levitation force, but also the energy to generate new surfaces.
This achievement provides a new idea and method for the research in the field of droplet dynamics manipulation, and also has certain reference significance for the preparation of shell-core soft materials, drug encapsulation and other fields. This work was supported by the National Natural Science Foundation of China (U1732129), the Basic Research Program of Shaanxi Province (2016JM1003) and the Scientific Research Fund of the Central University (3102016ZY026), and received long-term support from the International Cooperation Office of the school in terms of academic exchanges and cooperation. Newsweek, New Scientist, Science News and many other well-known media reported the research work.
It is reported that Professor Zang Duyang's research group has achieved a series of innovative results in the field of droplet physics and mechanics. The research group has published more than 60 research papers in famous journals such as Soft Matter, Langmuir, APL, and PRE. Some achievements have also been made in the cultivation of innovative ability of college students. Prof. Zang Duyang has served as the guest editor of Soft Matter, Adv. Cond. Matter Phys. and other journals, and is now the associate editor of Eur. Phys. J. E, one of the representative journals in the field of soft matter.
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