VIDEO IMAGING OF THE MELTING AND SOLIDIFICATION PROCESSES OF THE PBBR2-PBCL2 SYSTEM UNDER MICROGRAVITY

Using a sounding rocket, real-time observation of the melting and soli dification of the PbBr2-PbCl2 system sealed in a quartz ampoule was su ccessfully performed under microgravity. The interface shape is convex toward the melt during melting and becomes almost flat on solidificat ion, which agrees well with the results of a computational thermal ana lysis. The liquid flow observed during melting had a velocity between 0.2 and 1.5 mm/s. This is much higher than expected and is most likely due to Marangoni convection. Although the PbBr2 melt wets the quartz ampoule, traces of microscopic free-surface areas during solidificatio n are observed on the crystal surface. The microscopic free surface is the origin of Marangoni convection. Results from this ''first-time'' observation of the Marangoni flow in a liquid sealed in an ampoule, an d capable of wetting the ampoule wall, will be useful in future microg ravity experiments and will lead to a better understanding of crystal growth from a melt.