The acoustic levitation of various materials can be realized by highly intensive ultrasound, which provides a free surface and containerless state for materials processing under space simulation conditions. The nonlinear effects such as acoustic radiation pressure, acoustic streaming, and ultrasonic cavitation open up special access to modulate the fluid dynamics and solidification mechanisms of liquid materials. Here, the physical characteristics of liquid flow, undercooling capability, phase separation, and crystal nucleation and growth within acoustically levitated droplets are explored comprehensively to reveal the extraordinary solidification kinetics of liquid alloys. The sectorial shape oscillations of the 2nd to 10th order modes accompanying internal potential flow are observed for water droplets with modulated ultrasound amplitudes, while the enhanced ultrasound intensity promotes ice nucleation and thus reduces water undercooling. The migration of Sn-rich globules during phase separation of immiscible Al-Cu-Sn alloy is dominated by the droplet deformation and rotation related to acoustic levitation. The high undercooling states of liquid Ag-Cu-Ge and Ni-Sn alloys during acoustic levitation result in the refinement of (Ag) dendrites and the formation of anomalous (Ni+Ni3 Sn) eutectics. The ultrasound-liquid interaction also induces surface waves during the containerless solidification of Ag-Cu and Ni-Sn eutectic alloys.
Keywords: acoustic levitation; dendritic and eutectic growth; drop dynamics; liquid undercooling; microgravity state; phase separation; space simulation.
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