Faraday wave instability characteristics of a single droplet in ultrasonic atomization and the sub-droplet generation mechanism

Compared with the traditional mechanical atomization technology, ultrasonic atomization technology has been widely used in industry in recent years because of its higher energy efficiency and environmental friendliness. However, the physical mechanism of ultrasonic atomization of the individual droplet has not been studied, especially the evolution of Faraday instability and the formation mechanism of sub-droplets, leading to a lack of scientific theoretical guidance for the application of this technology. Here, we observed different Faraday wave patterns and evolution processes on the surface of the individual droplet using high-speed camera technology. The wettability of the droplet is significantly enhanced by the vibration effect. Interestingly, the Faraday waves of highly viscous droplets show multiple concentric semicircular ripple shapes. Droplet atomization is the result of Faraday wave mode transition and instability evolution induced by inertial forces, and both viscosity and surface tension play a suppressive role in the development of Faraday instability. The vibration frequency of stable Faraday waves was traced, and the effects of amplitude, surface tension, viscosity, and liquid volume on the Faraday wave wavelength and number were investigated. The characteristic parameters of Faraday wave instability evolution such as atomization amplitude threshold and atomization time were investigated under different experimental conditions. It is found that for low-viscosity liquids, the atomization amplitude threshold depends mainly on the surface tension, while for high-viscosity liquids, the atomization amplitude threshold is influenced by both viscosity and liquid volume. The effects of surface tension, viscosity, and volume on the atomization time are similar to those on the atomization amplitude threshold. We investigated the kinetic process of sub-droplet generation and showed that differences in Rayleigh-Plateau instability and capillary pinch-off mechanisms lead to different patterns of sub-droplet generation, where the former depends on the capillary number threshold and the latter can be divided into the protrusion break mode and the ligament break mode. The proportion of these two modes was investigated under different experimental conditions to quantitatively evaluate the selectivity of the mother droplet atomization mode. For the first time, the dispersion distribution of the sub-droplets was explained by using the length and number characteristics of the ligaments. The correlation analysis of sub-droplet size on ligament length and ligament number was established, and a linear fitting function was obtained.