Fast Surface Tension Measurement of Liquid Iron and 304L Steel using an Aerodynamic Levitation Apparatus and a new Multi-burst Excitation

Liquid metals play a crucial role in strategic industrial processes such as additive manufacturing, casting, and welding. Accurate knowledge of their physicochemical properties, particularly surface tension, is essential for process optimisation. This work presents a fast-multi-burst acoustic excitation method applied through a loudspeaker in an aerodynamic levitation setup for determining the surface tension of high-purity iron (99.95%) and 304L stainless steel. The method is based on oscillating droplet resonance and the Rayleigh theoretical framework. The multi-burst excitation enables the acquisition of multiple dynamic responses from a single experiment, providing several independent surface tension measurements on the same sample. Successive acoustic bursts, delivered at short time intervals, induce oscillations from which only the free vibration phases are analysed. To improve frequency identification, a time frequency analysis using spectrograms is employed in addition to Fourier transform processing. This approach allows precise identification of the central resonance peak associated with the oscillation modes, ensuring accurate determination of the resonance frequency. Surface tension measurements obtained for iron and 304L stainless steel over the temperature range 1 500°C-2 000°C show excellent agreement with literature data. The proposed multi-burst acoustic excitation technique is therefore a rapid, contamination-free, and reliable method for determining the surface tension of liquid metals at very high temperatures.