RIIST, resonance induced instability for surface tension measurement, a new technique with experiments in microgravity

This study reports a new method of analysis to measure the surface tension of high melt-temperature liquid metals using levitation in microgravity. The method, which leads to a self-consistent benchmarking technique to determine surface tension, requires forced oscillations of a levitated drop until the drop responds by deforming at a target mode, say the first fundamental mode of the natural frequency of the drop. Decomposition of the drop shape into Legendre modes, followed by time-domain analysis, reveals that the response to a target-mode forcing is composed of multiple modes that oscillate at frequencies, commensurate with the natural frequencies of those modes. We refer to the multiple modes that emanate from the target mode forcing as subordinate or ancillary modes. This then means that multiple modal shapes constituting the deforming drop's response co-exist. It is found that the ratios of the experimentally determined ancillary modal frequencies correlate well with the theoretical ratios predicted by the Rayleigh formula, thereby providing a self-consistent benchmark method for surface tension determination for any given sample, regardless of its composition. Validation of this method has been performed using experiments on the Electrostatic Levitation Furnace (ELF) in the KIBO module aboard the International Space Station (ISS) demonstrating accuracy and precision.