Dynamics of droplets entering ultrasonic standing wave field at different angles

We, herein, present dynamic behaviors of droplets entering an ultrasonic standing wave field (19 800 Hz) at different angles. In experiments, droplets' motion is recorded by using a high-speed camera, and an in-house Python program is used to obtain droplet positions and morphological characteristics as functions of time. The experimental results indicate that when the sound intensity is lower than the instability intensity and higher than the levitation intensity, the vertically falling droplet will oscillate up and down based on the equilibrium position. Although the oscillation amplitude decays from 0.52T(l) to 0.01T(l) (T-l = lambda/2, lambda is the wavelength) under the action of viscous resistance, the oscillation frequency of the droplet remains unchanged. Meanwhile, as the droplet's position oscillates, the acoustic radiation force on the droplet also periodically fluctuates, resulting in the acoustically forced oscillation of the droplet shape. In addition, when the droplet enters the sound field with a horizontal tilt angle theta of 15 degrees, it undergoes a V-shaped translational motion, first descending and then ascending. As the sound pressure amplitude increases, the rebound position of the droplet advances. When the sound pressure amplitude reaches the instability value (7900 Pa), the droplet undergoes right-hand and left-hand disintegration during its descent and ascent, respectively. This instability is due to the acoustic radiation pressure distribution and the droplet's V-shaped trajectory. This work comprehensively discussed the complex motion of moving droplets in the acoustic standing wave field, which may inspire revealing the spray motion in the liquid engine with high-intensity resonance.