Melting kinetics in microgravity

Abstract

We analyzed the large data archive obtained from microgravity experiments called the Isothermal Dendritic Growth Experiment (IDGE). The final orbital mission of the IDGE allowed streaming of real-time video of crystal growth and melting in low-Earth orbit of high-purity pivalic acid anhydride (PVA). Melting kinetics was observed in detail for the first time under microgravity conditions. Normally, terrestrial gravity precludes observation and analysis of conduction-limited melting by stimulating buoyancy flows and inducing collapse of the solid-liquid structure. The initial dendritic mushy zone melts under steady heating into isolated crystallite clusters that remain stationary within the melt, and eventually shrink toward their complete extinction. Near the end of conduction-limited melting, a few needle-like crystallites remain, exhibiting various axial ratios, C/A, of their major ellipsoidal axis, C, to their minor axis, A. Analysis of their convection-free melting, based on quasi-static thermal conduction, yields rates in agreement with the microgravity video and IDGE thermal data. Theory and experiment compare well, when the melting Stefan numbers, based on thermal data telemetered from the space-borne thermostat are used. Isolated PVA fragments shrink in scale at an accelerating rate toward total extinction. The 'point effect' of thermal conduction operates during most of the melting process when C/A remains almost constant. Prior to extinction, when the radius of a typical crystallite's minor axis approaches circa 100 micrometers, capillary effects become important. Then the crystallite's C/A ratio abruptly falls toward unity, as the needle-like crystals contract into spheres by accelerated melting near their poles. These experiments, and the accompanying field-theoretic analysis, introduce some new considerations, and raise questions concerning the roles of capillarity, heat conduction, interface shape, and solid-liquid phase change kinetics.