Atomisation of an acoustically levitated droplet: Experimental observations of a myriad of complex phenomenon

We report the dynamics of a droplet levitated in a single-axis acoustic levitator. The deformation and atomization behavior of the droplet in the acoustic field exhibits a myriad of complex phenomena, in sequences of steps. These include the primary breakup of the droplet through stable levitation, deformation, sheet formation, and equatorial atomization, followed by secondary breakup which could be umbrella breakup, bag breakup, bubble breakup or multistage breakup depending on the initial size of the droplet. The visualization of the interfacial instabilities on the surface of the liquid sheet using both side and top-view imaging is presented. The primary breakup of the droplet precedes with a stable levitation of the droplet. The thinning of the sheet is caused by the differential acceleration induced by the greater pressure difference between the poles and the equator. As the sheet thickness reduces to the order of a few microns, Faraday waves develop at the thinnest region (preceding to rim) which causes the generation of tiny-sized droplets perpendicular to the sheet. The corresponding, hole formation results in a perforated sheet that causes the detachment of the annular rim which breaks due to Rayleigh Plateau (RP) instability. The radial ligaments generated in the sheet possibly due to Rayleigh Taylor (RT) instability break into droplets of different sizes. The secondary breakup exhibits We dependency and includes umbrella, bag, bubble, or multistage, ultimately resulting in complete atomization of the droplets. Both the primary and the secondary breakup of the droplet admit interfacial instabilities such as Faraday instability, Kelvin Helmholtz (KH) instability, RT instability, and RP instability and are well described with visual evidence.