# The Development and Analysis of a Multistage Spraying Method for Liquids in an Ultrasonic Field
## 4 Results and discussion  
### Droplet Disintegration Dynamics  
To study the dynamics of the first stage of liquid droplet disintegration, the video camera was focused on the area on the tubular emitter axis located 20 mm below the upper end of the emitter (the first stage of droplet disintegration). The records of the deformation and disintegration of the droplet with indicated time marks are shown in Figure 5. The sound pressure level inside the tubular emitter was 175 dB. Figure 4: Test bench structure and photo: 1—peristaltic pump; 2—liquid supply tube; 3—ultrasonic tubular emitter; 4—ultrasonic generator; 5—atomized liquid; 6—droplet size analyzer; 7—high-speed video camera; 8—vacuum aspirator for collecting droplets after the first stage of spraying. The presented sequence of high-speed video frames indicates that the droplet undergoes a series of deformations while in the ultrasonic field before breaking into smaller droplets. The following phases of deformation and disintegration of the droplet can be distinguished in the first stage:  
1. First phase: The initial droplet possessing initial momentum (obtained by crushing the liquid jet at the nozzle outlet) deforms as it approaches the region of the velocity  
Figure 5: Images of primary droplet disintegration. anti-node in the standing wave. The droplet impacted by radiation pressure assumes the shape of a concave membrane (disk). 2. Second phase: As the diameter of the liquid membrane increases and its concavity degree increases, the liquid film thickness decreases. Faraday capillary waves [28] are generated on the surface of the liquid disk under the impact of ultrasonic vibrations. These waves reach the highest amplitude at the edges of the disk. When the Faraday waves reach their critical amplitude, disintegration of the liquid disk begins, starting from the outer edge. The smallest droplets are generated owing to the implementation of this mechanism. 3. Third phase: The instability of the liquid film increases due to a further increase in the disk size and decrease in its thickness. When the liquid membrane reaches a certain limiting size, the liquid film thickness decreases below the critical value. This results in the structural destruction of the liquid disk with the formation of large droplets. This completes the first stage of droplet breakdown. The formed polydisperse droplets are subjected to the structuring effect of secondary nonlinear effects arising in the ultrasonic field. Large droplets (formed at the first stage), possessing, as a rule, a lower emission rate (upon liquid film destruction), move in the direction of the next vibration velocity anti-node in the standing wave. This motion originates from the impact of the resultant radiation force, which is proportional to the squared droplet radius. Thus, sufficiently large (more than 100 \(\upmu\)m) liquid droplets formed at the first stage of spraying, with a higher velocity (because the force effecting the droplet is proportional to the squared droplet diameter), move into the vibration velocity anti-node, where they undergo secondary and subsequent disintegration. Accordingly, the smallest droplets move in the direction of the vibration velocity nodes, where the ultrasonic vibrations do not have a significant effect on them. Further, such drops leave the ultrasonic exposure domain (in this case, from the internal volume of the tubular emitter) via the action of gravity or with the flow of transporting air. In this case, due to the arrangement of small droplets in the nodes of vibrational velocity, their coagulation under the impact of ultrasonic vibrations does not, in fact, occur. Thus, in the proposed technique of multistage spraying, ultrasonic vibrations have not only a dispersing effect (the breakdown of liquid droplets) but also a structuring effect (the selective breakdown of the largest droplets) [29, 30, 31].