Self-consistent surface tension measurements for liquid metals/alloys via drop oscillations during levitation

This research presents a novel technique for measuring the surface tension of high-temperature liquid metals in space, using Faraday instability on a spherical droplet. Surface tension is a critical property for advancing technologies like welding, in-situ crystal growth in space, and additive manufacturing processes such as Direct Energy Deposition. The method works by applying a periodic electric field at the droplet's modal natural frequency, causing resonance and generating distinct surface patterns. This frequency depends on the droplet's mass, surface tension, and modal pattern, with the ratio of modal frequencies being independent of the droplet's mass and surface tension for a given sample.

Through image analysis and Fourier decomposition, secondary modal responses are observed when the droplet is over-forced, as energy from the primary mode induces additional responses. By comparing the primary and secondary modal frequencies, a self-consistent approach to surface tension determination is developed.

Experimental validation is provided by data from the Japanese Aerospace Exploration Agency's (JAXA) Electrostatic Levitation Furnace (ELF) and the Marshall Space Flight Center's (MSFC) Electrostatic Levitation Laboratory (ESL), demonstrating the method’s efficacy in both microgravity and terrestrial conditions. The primary focus is on platinum, which showed a strong correlation between experimental and theoretically predicted natural frequency ratios, both on Earth and in space. This highlights the method's precision, reliability, and applicability for measuring surface tension in microgravity environments.