Surface Tension Measurement Using Resonance of Levitated Molten Droplets

Space operations benefit by contactless levitation technologies that can be used to simulate/utilize reduced gravity environments on earth. This work employs the spherical analogue of the Faraday instability, where the interface(s) of a stacked multi-fluid system are subjected to an oscillating external forcing that may resonate with one of the system’s natural frequencies and form standing or breaking interfacial waves. Analogously, when a levitated spherical liquid drop is subjected to a continuous periodic electric field at a frequency equal to one of its natural frequencies, it can undergo resonance and form modal structures at its surface. The natural frequencies of a liquid sphere directly depend on the modal structure, the mass of the liquid, and the surface tension. By deliberately resonating a sphere at its natural frequency we can therefore obtain the surface tension. This thermophysical property is imperative for the modeling of processes such as crystal growth (on Earth but particularly in space), welding, and additive manufacturing processes like Direct Energy Deposition (DED).

The work presented herein compares the analytical result for natural frequency of a liquid sphere in a “self-gravitational field” by Rayleigh (1879) to experimental observations in several metals and alloys of high importance to space and terrestrial fabrication applications. We present findings on two normal modes of oscillation and their respective resonant frequencies, the projection method used in quantifying resonance magnitude, and comparison with existing measurement methods.

Acknowledgments: NASA 80NSSC18K1173, NASA NNX17AL27G, FSGC08/NNX15025, UFIC Research Abroad for Doctoral Students Award