# Innovative Ultrasonic Spray Methods for Indoor Disinfection
## 2 Development of Ultrasonic and Combined Spraying Devices for Disinfectant Liquids
### Innovative Acoustic-Dynamic Methods and Devices for Spraying Disinfectant Liquids
#### 2.3.2 Cavitation Combined Method  
To overcome the limitations of ultrasonic liquid spraying methods in terms of productivity, we propose to combine the advantages of the hydraulic spraying method (with high productivity) with the capabilities of ultrasonic cavitation (to ensure high droplet dispersion). The main idea is to create a cavitation region in a liquid under excess pressure. This involves creating a cavitation region at the liquid outflow point of the hydraulic nozzle. The ultrasonic emitter placed here affects the volume of liquid above the end surface of its working end with ultrasonic vibrations. An important condition is a sufficiently high vibration amplitude, exceeding the threshold for creating cavitation in the liquid. Schematically, the cavitation combined spraying method is shown in Figure 10.  
Under the influence of excess pressure, the cavitating liquid flows out of the pressure hole in the form of large drops, inside which there are one or more cavitation bubbles. The higher the hydraulic pressure, the greater the flow rate and, accordingly, the spraying performance. The cavitation bubble collapse generates pressure pulses that expand the droplet until the surface tension limit is reached, causing the large droplet to break up into smaller ones. The size of these fragments (new drops) depends significantly on the size of the cavitation bubbles, which, in turn, is determined by the parameters of the ultrasonic action.  
Thus, hydraulic pressure determines the flow rate (and spray performance). Ultrasound characteristics (amplitude and frequency) determine the parameters of the cavitation region and, ultimately, the dispersion of the droplets.  
The proposed method offers several notable advantages. It provides increased spraying performance comparable to hydraulic and vortex atomizers, while significantly surpassing the performance of conventional ultrasonic atomizers with similar technical specifications. Additionally, it facilitates the production of small-diameter droplets through ultrasonic cavitation, resulting in droplets smaller than those produced by hydraulic or vortex atomizers under similar pressure and flow rate conditions. Another benefit is the ability to use low-frequency ultrasonic vibrations to generate small droplets, whereas conventional ultrasonic atomizers require significantly higher frequencies to achieve similar results.  
The proposed cavitation combined spray method is implemented using a two-half-wave ultrasonic emitter. A sketch and a photo of the emitter with a resonant frequency of 22.1 kHz are shown in Figure 11.  
Figure 10: Scheme of cavitation combined spraying method. 1—supply of sprayed liquid, 2—cylindrical volume for liquid under increased pressure, 3—ultrasonic emitter, 4—cavitation area, 5—outlet, 6—large drops, 7—cavitation bubbles, 8—atomized liquid droplets.  
After manufacturing and assembling the emitters and transducers, the oscillation amplitude of the emitting end was measured. The measurement results showed that the maximum oscillation amplitude of the end surface of the developed piezoelectric transducer was 30 \(\upmu\)m. The obtained oscillation amplitude values are sufficient to create cavitation in the liquid.