Quan sát trực tiếp một sóng âm hình cầu sinh ra bởi tia laser đi qua một bề mặt phân cách rắn-lỏng bằng phương pháp quang đàn hồi

Nội dung text: Quan sát trực tiếp một sóng âm hình cầu sinh ra bởi tia laser đi qua một bề mặt phân cách rắn-lỏng bằng phương pháp quang đàn hồi

1. Nguyen Thi Phuong Thao / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 35-38 35 02(45) (2021) 35-38 Direct observation of a laser-excited circular acoustic wave crossing a solid-liquid interface by photoelasticity imagine technique Quan sát trực tiếp một sóng âm hình cầu sinh ra bởi tia laser đi qua một bề mặt phân cách rắn-lỏng bằng phương pháp quang đàn hồi Nguyen Thi Phuong Thaoa,b* Nguyễn Thị Phương Thảoa,b* aInstitute of Research and Development, Duy Tan University, Da Nang, 550000, Vietnam aViện Nghiên cứu và Phát triển Công nghệ Cao, Trường Đại học Duy Tân, Đà Nẵng, 550000, Việt Nam bFaculty of Natural Sciences, Duy Tan University, Da Nang, 550000, Vietnam bKhoa Khoa học Tự nhiên, Trường Đại học Duy Tân, Đà Nẵng, 550000, Việt Nam (Ngày nhận bài: 10/09/2020, ngày phản biện xong: 20/09/2020, ngày chấp nhận đăng: 10/12/2020) Abstract The interaction of a laser-excited circular acoustic wave in water on an epoxy-resin surface was studied directly and visually by photoelasticity imaging technique. The result shows the direct image of the pressure wave, shear wave, and Scholte waves, which are the three typical acoustic waves excited when an acoustic wave crosses a solid-liquid interface. This technique provides a clear, whole field image of the phenomena; and is a promising method to study the acoustic phenomena at the interfaces. Keywords: Photoelastic images; laser-excited acoustic wave; scholte wave. Tóm tắt Sự tương tác của một sóng âm hình cầu gây nên bởi tia laser trong nước lên một bề mặt epoxy-resin được nghiên cứu trực tiếp và trực quan thông qua phương pháp chụp ảnh quang đàn hồi. Kết quả cho thấy hình ảnh trực tiếp của sóng áp suất, sóng ứng suất, sóng Scholte, là ba loại sóng điển hình sinh ra khi một sóng âm hình cầu đi qua bề mặt phân cách rắn-lỏng. Kĩ thuật này cung cấp một hình ảnh toàn cảnh rõ nét của hiện tượng, và là một phương pháp triển vọng để nghiên cứu các hiện tượng âm học tại bề mặt phân cách. Từ khóa: Hình ảnh quang đàn hồi; sóng âm gây nên bởi tia laser; sóng Scholte 1. Introduction of several research works because of promising The theory of an acoustic wave across a and important applications. It has been known solid-liquid interface, its experimental studies, that when an acoustic wave traveling in a liquid and applications have been vastly investigated interacts with a solid surface, it will excite in for decades. However, they are still the objects the solid the pressure wave (P-wave), shear * Corresponding Author: Nguyen Thi Phuong Thao; Institute of Research and Development, Duy Tan University, Da Nang, 550000, Vietnam; Faculty of Natural Sciences, Duy Tan University, Da Nang, 550000, Vietnam. Email: thaonguyen@duytan.edu.vn
2. 36 Nguyen Thi Phuong Thao / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 35-38 wave (S-wave), and surface acoustic waves focal position was set 0.5~1 mm above the (SAWs) [1], [2]. Although it has been well solid surface. understood, we have yet to have a direct image 2.2. Imaging system of the phenomenon. Conventional methods The imaging system is similar to our used to investigate the bouncing of an acoustic previous report [5], [6] and only a brief wave on the solid-liquid interface represent the description is provided here. We used a pump- acoustic waves as electric signals rather than and-probe system with a polariscope added to the image of real wavefronts [3], [4], thus provide a photoelasticity image. An ICCD unable to provide a direct whole-field image of camera was used together with a set of Neutral- the phenomena. To investigate the laser- Density filters as a recording device. The induced shock process, we have developed a camera gate width was 40 ns. The delay time custom-designed photoelasticity imaging was defined as the interval between the pump technique with the unique ability to visualize and probe pulses and was adjusted by a delay the transient wavefronts inside a solid target generator. With this system, the observable [4]. When applying this technique to observe interval can reach several seconds with a time the coupling of the acoustic wave on a liquid- resolution of 100 ns. solid interface, we were able to provide a direct 3. Results and discussions image of the typical acoustic wavefronts excited inside the solid. Figure 1 shows a typical circular acoustic wave excited by focusing a nano-second pulsed In this experiment, we used a nano-second laser in the water. The black area at the center pulsed laser to excite a circular acoustic wave is the image of a cavitation bubble. The sharp in the water. This circular acoustic wave black circle is the image of the excited acoustic interacted with an epoxy-resin surface. wave. Although water is transparent to the Photoelastic imaging technique was used to wavelength of 1064nm, by tightly focusing the capture the image of acoustic wavefronts nano-second pulsed laser beam inside the excited inside the solid target during the water, the breakdown occurred. After that, the interaction. We report the observed P-wave, S- laser absorption by inverse Bremsstrahlung wave, and Scholte wave with high spatial and occurred, leading to the formation of a high- temporal resolution. temperature, high-pressure plasma. This plasma 2. Material and methods expanded rapidly and induced a shock wave, which slightly changed into an acoustic wave as 2.1. Laser-excited acoustic wave propagating away from the focal region. A circular acoustic wave was excited by tightly focusing a 60mJ, 1064nm laser beam in pure water. The breakdown in the water induced a plasma of which the expansion led to the formation of a circular acoustic wave. This circular acoustic wave traveled in water before interacting with an epoxy-resin target (5.8x20x28 mm3). The target surface was roughed to get the roughness of Ra=1mm. The
3. Nguyen Thi Phuong Thao / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(45) (2021) 35-38 37 Figure 1: A typical image of a laser-excited circular have a good match in impedance, the reflected acoustic wave in water. Delay time: 2000 ns. The laser came from above. wave is much weaker than the transmitted wave. When the target is 1.1 mm below the Figure 2 shows the interaction of a laser- focal point, we hardly observe the reflected excited acoustic wave with an epoxy-resin wave, however, the reflected wave can be seen surface. The delay time is 1500 ns. In Figure when the target is put 0.6 mm below the focal 1a, the target was put 1.1 mm below the focal point. Since epoxy resin has higher water point. In Figure 2b, the target was put 0.6 mm acoustic velocity, there should be a lateral wave below the focal point. When the circular that is tangent to the reflected wave at the total acoustic wave interacted with the solid target, it reflection angle. However, this wave was was partly reflected back to the liquid as the almost not detected in our image. reflected wave. Because water and epoxy resin Figure 2: Photoelastic image of laser-excited acoustic waves in the solid. Delay time: 1500 ns. The laser came from above. The focal point is located (a) 1.1 mm above the target and (b) 0.6 mm above the target. The acoustic waves are numbered as 1: reflected wave,2: P-wave, 3: S-wave. 4: Scholte wave. The acoustic wave transmitted into the solid Scholte wave always exists while the leaky target induced the stress waves inside the Rayleigh wave can only exist if the acoustic target, which were represented by the sharp, velocity in water is lower than the shear black photoelastic fringes. The fastest velocity in the solid. In our experiment, the S- wavefront in the solid is the pressure wave (P- wave velocity is lower than the acoustic wave) which is a longitudinal wave and travels velocity in water, which is 1490 m/s, thus the at the acoustic velocity in the solid. The shear leaky Rayleigh wave does not exist. In our wave (S-wave) is a transverse wave and travels image, the Scholte wave can be detected as at the velocity slower than the P-wave. The S- showed in Figure 2b. From the image, the wave velocity in epoxy-resin was measured in velocity of the Scholte wave was estimated to our previous research to be 1120 m/s[7]. be approximate 1160 m/s, which is close to the At a solid-liquid interface, two kinds of shear velocity in the solid. This result agrees typical surface acoustic waves can be observed. with the reported results [2], [8], [9], thus Scholte wave and leaky Rayleigh wave. The confirming the observed wavefront is Scholte wave.
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