Droplet formation in ultrasonic atomization: Influence of film thickness, amplitude and liquid properties

Ultrasonic atomization has emerged as a valuable technique for producing high-quality metal powders with high sphericity and a narrow particle size distribution for Additive Manufacturing (AM) applications. This research aims to study the effects of liquid properties, liquid film thickness and the vibration amplitude of the ultrasonic atomizer on droplet size based on the dynamics of capillary waves. High-speed optical imaging and MATLAB post-processing are employed to study in situ the ultrasonic atomization dynamics and droplet size distributions. Results reveal that below critical liquid film thickness, droplet size is independent of liquid film thickness, because breakup dynamics are governed primarily by capillary and surface tension forces rather than inertial effects, and the capillary wavelength remains constant. In contrast, for film thicknesses above the critical value, increasing liquid film thickness leads to larger droplets with a broader size distribution due to enhanced capillary wave amplitude and stronger cavitation effects, which promote the ejection of larger liquid ligaments. In contrast, higher viscosity results in smaller droplets with a more uniform distribution, as reduced capillary wave growth leads to larger wavelengths and smaller wave amplitudes, limiting the volume of detached liquid. A modified dimensionless correlation is proposed, in which the traditionally used flow rate is replaced by liquid film thickness, a parameter that more accurately represents the local conditions at the atomization surface. This leads to improved prediction of droplet size and provides a basis for optimizing ultrasonic atomization, enhancing its potential for producing fine, uniform metal powders. • Microscopic imaging is used to link droplet size to liquid film thickness, liquid properties, and ultrasonic parameters. • Flow rate is replaced by film thickness in dimensionless analysis. • Below the critical liquid film thickness, droplet size is independent of film thickness due to constant capillary wavelength. • Above the critical liquid film thickness, thicker films, thicker films generate larger droplets and broader distributions due to irregular wave patterns. • Modified correlation with liquid thickness accurately predicts typical droplet size.