Effect of Static Magnetic Field on Recalescence and Surface Velocity Field in Electromagnetically Levitated Molten CuCo Droplet in Undercooled State

Abstract

TSUBASA KITAHARA, KOKI TANADA, SHOYA UENO, KEN-ICHI SUGIOKA, MASAKI KUBO, TAKAO TSUKADA, MASAHITO UCHIKOSHI, and HIROYUKI FUKUYAMA The recalescence events of phase-separated Co-rich phases in undercooled molten CuCo droplets electromagnetically levitated under various static magnetic fields were observed directly using a high-speed camera, and also the surface velocities on the levitated droplets were measured by tracing the trajectories of the phase-separated Co-rich phases as tracer particles. In addition, numerical simulations of melt convection in a spherical electromagnetically levitated CuCo droplet exposed to a static magnetic field were performed assuming laminar flow. We observed the emergence of many intermittent bright spots due to recalescence on the entire surface of the levitated droplet, and the frequency of the bright spots decreased markedly as the static magnetic field increased, with no bright spots observed at fields larger than 1.5 T. Also, the Reynolds numbers were evaluated from the measured and calculated velocities in the droplet for various static magnetic fields and compared with the critical Reynolds number of approximately 600, at which the laminar-turbulent transition of a magnetohydrodynamic (MHD) flow in an electromagnetically levitated droplet occurs, as proposed by Hyers et al. The above results clearly revealed that the marked change in the phase separation structures in undercooled molten CuCo droplets at approximately 1.5 T is due to a convective transition from turbulent flow to laminar flow in the levitated droplets, as speculated in our previous work.

Liquid phase separation in undercooled Cu–Co alloys under the influence of static magnetic fields

“…The influence of the static magnetic fields on the metastable liquid phase separation in the glass-fluxed Cu-Co samples exhibits similarities and differences compared to those observed for electromagnetically levitated samples [40][41][42][43][44]. The similarities are seen in the sphere-like morphology and the monomodal size distribution of the Co-rich droplets frozen in at lower intensities or smaller gradients of the magnetic fields.…”

“…Recent studies by Zhang and colleagues [40][41][42] showed that imposition of static magnetic fields on electromagnetically levitated Cu-Co alloys damped forced convection and brought about a microgravity-like effect on the droplet size distribution. Such a magnetic field effect was observed also by Sugioka and colleagues [43,44]. Compared to parabolic flights or space experiments, ground-based experiments with the use of static magnetic fields are cheap and offer a new opportunity for studies of liquid phase separation in undercooled Cu-Co and other Cu-based alloys.…”

“…This difference explains why the droplets with the elongated morphology were not observed by Zhang and colleagues [40][41][42]. However, recent studies by Sugioka and colleagues [43,44] showed the formation of elongated Co-rich droplets in a Co 80 Co 20 sample electromagnetically levitated under a magnetic field of B = 4 T. Those elongated droplets may also be interpreted by assuming suppression of the transverse stretching of the coagulated droplets. Because of smaller thermal gradients, the Marangoni convection in the levitated bulk samples are supposed to be weaker than in the glass-fluxed samples.…”

Droplet Manipulation under a Magnetic Field: A Review

“…In microgravity, Bojarevics et al [163] performed a numerical simulation on the melting of silicon droplets in an AC+DC magnetic field and presented the typical temperature and velocity field. The stable droplet suspension helped in studying heat transfer in microgravity environments [164], ensuring demanding chemical reactions without bacterial and virus pollution, smelting high-purity materials [165], promoting the fusion of incompatible alloys [166] and optimizing the welding effect [167].…”