With the pursuit of children, colorful bubbles floating in the air, dancing with the wind, bringing infinite happiness to the curious children. At the same time, bubble formation and rupture also contains rich and basic scientific problems, attracted from the physics, chemistry, mechanics and other fields of scholars of widespread concern. Bubbles play an indispensable role in people's production and life, such as fermentation and puffing in food processing is the process of bubble formation, in pharmaceuticals, cosmetics, mineral flotation and many other fields are also important.
Under the long-term guidance of Academician Bingbo Wei and Professor Dominique Langevin, Professor Zang's team used the ultrasonic field to provide a new method for preparing bubbles. The group found the transformation phenomenon of droplet to bubble induced by sound field, and revealed the vocal cavity resonance mechanism of this transformation. The study was held on September 11th(London time) in the top International journal Nature Communications, entitled "Inducing drop to bubble transformation via resonance in ultrasound" (" Nature • Communications”) is published online. Northwestern Polytechnical University is the first author and sole correspondent unit of the paper, with the cooperation units at Monash University in Australia and the University of Hull in the United Kingdom.
In the past, bubbles were generated by the use of turbulent mixing, flow focusing technology, or the strong shear generated by microfluidic, thus providing the energy needed to generate a bubble interface. While these techniques can produce bubbles efficiently, they are difficult to study the physical problems of the bubbles, especially the physical and mechanical properties of the bubble membranes.
Under the condition of acoustic suspension, by adjusting the sound field intensity, the shape of the suspension droplet undergoes the change from ellipsoid to flat liquid cake to bending liquid film. 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 not be explained by the equilibrium shape theory of acoustic suspension droplets, nor can it be used for reference from the existing phenomenon of droplet instability.
Professor Zang in this paper, through systematic experimental research and in-depth numerical simulation, it is found that one of the key factors in droplet-bubble transformation is the bending of liquid film to form a bowl-shaped cavity. When the empty cavity reaches the critical volume, its volume increases sharply. It is found that the critical volume is independent of the type of liquid and the initial size of the droplet, and depends significantly on the frequency of the acoustic field. This is because the air cavity formed by the bubble bending actually constitutes a Helmholtz resonator. When the volume is appropriate, it will resonate with the ultrasonic field, absorbing a large amount of energy from the vibration source, resulting in a violent expansion of the cavity and rapid closure to form bubbles.
Because the bubble essentially has a high specific surface area, so when the liquid film is punctured, the common phenomenon is that the membrane hole continues to grow, the liquid membrane shrinkage to reduce its surface area. Eventually, it is either broken into many small droplets, or a sub-bubble is formed. In this work, Professor Zang’s group achieved a "reverse process": the area of the liquid film is increasing, bending and rapidly expanding into a bowl shape, and eventually wrapping the air to form a bubble.
The most important finding of this study is that the cavity surrounded by an acoustic suspension bending liquid film can be regarded as an acoustic resonator independent of the properties of a liquid. Once the curved liquid membrane cavity reaches the appropriate volume, whether it is to increase the sound field strength or through the external drag, will produce ultrasonic resonance and suddenly expand to form bubbles.
On the basis of this theory, Professor Zang ‘s group has developed a new technology to control the preparation of bubbles by using acoustic suspension. Acoustic energy can be effectively absorbed and the liquid film area is increased at the millisecond scale. At this point, the sound field provides not only the suspension force, but also the energy that produces the new surface.
The results provide a brand-new idea and method for the study of the field of droplet dynamics manipulation, and also have some reference significance for the preparation of shell-type soft materials, drug packaging and other fields. This work is supported by the National Natural Science Foundation of China (U1732129), the Shaanxi Basic Research Program (2016JM1003) and the Central University scientific Research business Fee (3102016ZY026), and has received long-term support from the school international Cooperation office in academic exchanges and cooperation. Newsweek, New Scientist, Science news and many other well-known media have reported on the research work.
It is reported that Professor Zang’s group has achieved a series of innovative achievements in the field of fluid droplet physics and mechanics, the group has published more than 60 research papers in soft Matter, Langmuir, APL, PRE and other famous journals. The group has also made certain achievements in the training of graduate students and the cultivation of college students’ innovative ability, and many graduate students have won national scholarships, and many national college students' innovative experimental projects have obtained excellent problems. Professor Zang has served as guest editor of journals such as Soft Matter, ADV. Cond. Matter Phys. He is currently one of the representative journals in the field of Soft Matter, EUR. Phys, deputy editor-in-chief, J. E.
words: Jiaqi Si, Photo: Zehui Zhang
Full-text link: https://www.nature.com/articles/s41467-018-05949-0
Media report: https://www.newscientist.com/article/2179212- watch-a-blast-of-sound-turn-floating-drops-into-bubbles/