Spin-up instability of a levitated molten drop in magnetohydrodynamic-flow transition to turbulence

We describe the spinning behavior of a suspended molten droplet subjected to electromagnetic heating. Our observations are derived from video images of droplets of palladium-silicon alloy in experiments on the MSL-1 (First Microgravity Science Laboratory) mission of the Space Shuttle (STS-83 and STS-94, April and July 1997). We inferred the resultant magnetohydrodynamic (MHD) flow inside the drop from motion of impurities on the surface. Digital particle tracking of the impurities is used to quantify the axial rotation of the levitated droplet. The analysis suggests that the levitated drop attains a constant rotational speed during the melting phase and formation of the co-rotating axisymmetric laminar toroidal structures. With continued electromagnetic heating, the sample's viscosity drops and the MHD flow accelerates, giving rise to instabilities of the internal flow. The rate of axial rotation increases significantly during this flow transition. The new data suggests a surprising interaction between the flow inside the levitated molten drop and the driving coils in the experiments. We explore the mechanisms that may be responsible for this spinning behavior.