# Development of High-Power Ultrasonic System Dedicated to Metal Powder Atomization
## 4 Results and Discussion
### Observation of Ultrasonic Atomization of Water by High-Speed Camera  
Verification of the operation of both ultrasonic systems was also made by the trials, oriented at the atomization of distilled water on the surface of the sonotrode. The selection of water for the initial trials was done for two reasons: (i) ease and safety with its application; but also due to (ii) its lower acoustic impedance when compared to liquid metals. For this reason, if the atomization phenomenon can be effectively induced for water, the more probable that it will occur for liquid metals or metal alloys. The atomization processes were observed with the Phantom v12 high-speed camera. The frequency of registration of the atomization process of the droplet on the sonotrode plate was always higher than the frequency of vibrations of the plate: 22,000 fps for 20 kHz system and 72,000 fps for 70 kHz one. The images were recorded using a PC, connected via Ethernet interface and dedicated software (Phantom Camera Control 3.7). The test site of the atomization area was illuminated by a special reflector with a power of 5 kW. A dedicated image analysis tool allowed determining the time of individual stages of atomization and the particle size distribution of liquids after atomization for various process parameters. During the research, two vibration systems (20 kHz and 70 kHz) were used. For each, the power of the system was changed, and it was checked how the atomization proceeded at selected points on the plate. These points were selected on the basis of vibration amplitude measurements, and it was decided to investigate the phenomenon of droplet atomization both for arrows (amplitude maxima) and for nodes (amplitude minima). Below, the results of atomization tests recorded with a high-speed camera (Figure 9) are presented. The water droplet was placed in the center of the plate, and after 22 \(\upmu\)s of its exposition to ultrasonic waves the phenomenon of cavitation occurs. After another 22 \(\upmu\)s, fine droplets start to be released, while in 66 \(\upmu\)s from when the UT system is switched on, the droplet completely explodes, which is accompanied by the phenomenon of the so-called ultrasonic fountain. Figure 10 shows the individual frames obtained for the 70 kHz system. It can be clearly seen that the time from the moment of switching on the system to the moment of cavitation is twice as long. This is due to the characteristics of the 70 kHz system itself, which cannot be fine-tuned prior it is switched on. So, the delay is caused by tuning the system when turned on. However, from the moment of observing cavitation in a droplet of water (Figure 10b) to the time of triggering the phenomenon of the ultrasonic fountain (Figure 10d) about 44 \(\upmu\)s have passed. Figure 10: Atomization of a liquid droplet on a plate of the system with a resonant frequency of 70 kHz. Figure 9: Atomization of a liquid droplet on the plate of a vibrating system with a resonant frequency of 20 kHz. Selected photos of the process of atomization of water droplets on a macro scale are presented in Figure 11a,b and on the micro scale in Figure 11c,d. Macro photos show a fountain of liquid as a whole, and micro photos allow observing only selected fragments of a column of atomized droplets, but with a much higher resolution. They were recorded from a shorter distance, thanks to which it is possible to analyze the morphology of the droplets and make a histogram of the distribution of their size. The analysis of images obtained from the high-speed camera was carried out in several key stages. In the first one, the single frames were selected for which the atomization process was stable, i.e., in the subsequent moments of registration, the particle size was comparable (Figure 12). Then, it was necessary to extract the area of atomized particles from the image so that the external objects would not interfere with the post-analysis process.