Synergistic Effects of Initial Volume and Physical Properties on Instability and Bubble Formation in Acoustically Levitated Droplets

This study systematically investigates instability evolution and bubble formation in acoustically levitated droplets. Experiments using working fluids with distinct physical properties - deionized water, SDS solutions, glycerol solutions, and ethanol solutions across various initial volumes identified four distinct instability modes: edge-splash, central-fountain, mixed atomization, and collapse-induced bubble formation. Results demonstrate droplet instability is governed by synergistic effects of physical parameters. Initial volume determines deformation patterns, surface tension controls collapse depth, while viscosity influences deformation rate and bubble stability. Low surface tension accelerates collapse and high viscosity suppresses internal flows. Bubble formation requires a critical initial volume that decreases with lower surface tension but varies nonmonotonically with volatility. Stable secondary bubbling occurs in pure ethanol droplets (25–60 µL), demonstrating roles of liquid-film integrity and curvature. Larger droplets show reduced acoustic responsiveness, while elevated ethanol concentrations enhance both bubble growth and levitation stability. This research provides fundamental insights into coupled parameter effects governing acoustically levitated droplets, supporting advances in acoustic levitation control and containerless processing technologies.