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Research Group of Researcher Song Yanlin, Institute of Chemistry, Chinese Academy of Sciences, Makes Important Progress in Research of Underwater Acoustic Reflective Supersurface
2019-12-25 Source: Polymer Technology

With the full use of marine resources by human beings, underwater communication has become increasingly important. Because electromagnetic waves decay very quickly in water, and water communication can only rely on sound waves, it is of great significance to control the propagation of sound in water. Water sound insulation is of great significance to suppress marine noise pollution and protect marine life. However, due to the difference in the acoustic impedance between air and water, the effectiveness of some sound insulation materials commonly used in air in water will be greatly reduced. However, the traditional theory of mass action requires that the size of the object is equal to the wavelength of the blocked sound waves, which makes it very difficult to block the sound waves in the low frequency region. Therefore, the regulation of low frequency sound waves in water is a long-term problem.

As the simplest acoustic metamaterial, air bubbles in water have unique acoustic properties. In 1933, the Dutch scientist Marcel Minnaert proposed the Minneart resonance phenomenon of a bubble, and pointed out that the acoustic wave wavelength corresponding to the resonance frequency of the bubble is nearly 500 times its radius, so low-frequency sound waves can be adjusted. It is further pointed out that if the bubbles can be made into a three-dimensional lattice arrangement in water, it will have the widest low-frequency acoustic wave band gap, which has important applications in the regulation of low-frequency broadband acoustic waves. However, the controllable preparation of bubbles in water has always been a problem due to the buoyancy of the bubbles and fluid instability.

Researcher Song Yanlin's research group has focused on the regulation and application of fluid patterning such as droplets and bubbles for many years. Realized anti-Oswald ripening control and patterning preparation of bubbles (Nat. Commun. 2017, 8,14110). And the theory of pattern control between any incompatible fluids such as water / gas / oil is proposed (Adv. Mater. 2018, 30 (31), 1802172). Recently, they applied bubble regulation to sonic regulation, and for the first time experimentally measured the phonon band gap of a three-dimensional bubble crystal (Adv. Funct. Mater. 2019, 1906984). Based on these studies, they were inspired by the bubbles trapped in nature, and proposed a method to control the alternate appearance of Cassie state (non-wet) and Wenzel state (wet) to more conveniently prepare patterned bubbles from micron to millimeter level. It is also used for the preparation of acoustic reflective metasurfaces, which has important significance in underwater acoustic detection and underwater sound insulation.

As shown in Figure 1, they proposed a method to control the alternate appearance of Wenzel state and Cassie state by using solid surface microstructures to prepare patterned bubbles. Through theoretical analysis, they obtained the relationship between the critical pressure of water droplets changing from the Cassie state to the Wenzel state on the solid surface and the solid surface structure. By fine-tuning the structure of the solid surface, it is possible to precisely regulate whether or not every part of the solid surface is wetted by the liquid. Where liquid is infiltrated is water, and non-infiltrated areas are bubbles. By designing the distance and arrangement of the microstructures, the position, size and morphology of the bubbles can be precisely controlled.

Figure 1. Wenzel state and Cassie state appear alternately to regulate patterned bubbles

According to the Minneart resonance equation of bubbles, different sizes of bubbles correspond to different resonance frequencies. The bubble spacing affects the resonance coupling between the bubbles. Therefore, by adjusting the size and spacing of the bubbles, they achieved the regulation of the sound wave from 9 kHz to 1.7 MHz. Moreover, because the bubbles are constrained by the microstructure, they do not have to be spherical. Through the preparation of flat bubbles, the wavelength of the sonic wave can reach 3333 times its bubble height, which is much larger than the 500 times corresponding to the Minneart resonance effect, so it can be prepared Thinner reflective metasurfaces than traditional.

Figure 2 shows a full-wave simulation of an acoustic reflective metasurface. In the absence of air bubbles, 60% of the acoustic energy can pass through the sample. However, a layer of bubbles with a height of 50 um and a volume fraction of only 3% can reduce its transmission to less than 0.2%. They have also demonstrated that multi-layered bubbles can be used to achieve wide-band sound insulation. Taking the transmittance below 1% as a boundary, they pointed out that the band gap width of the four-layer bubble is 30 times that of the single-layer bubble, so it can be used for sound insulation applications of broadband sound waves.

Figure 2. Patterned bubbles for acoustic reflective metasurfaces

This research is of great significance in controlling noise in the water and protecting the marine environment. In addition, its total reflection of sound waves is conducive to improving the sensitivity of underwater sound waves detection. For example, after a plane crash, its black box can only be searched by sonar after its battery is exhausted. The layer of bubbles on the surface can enhance the reflection at a specific frequency, making the black box like a mirror in the night, and it is easier to detect when it encounters light.

This paper was published in ACS Applied Materials & Interface (ACS Appl. Mater. Interfaces, 2019, DOI: 10.1021 / acsami.9b15683). The first author of the paper is Dr. Zhandong Huang from the Institute of Chemistry of the Chinese Academy of Sciences. He is currently a post-doctoral researcher of Professor Yang Jun of the University of Western Ontario. The co-first author is Dr. Zhao Shengdong from Qingdao University.

Paper link: http://pubs.acs.org/doi/pdf/10.1021/acsami.9b15683

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