Late-Stage Capillarity Effects During Crystallite Melting in Microgravity

The Isothermal Dendritic Growth Experiment (IDGE) flew in late 1997 as a primary experiment on the United States Microgravity Payload Mission (USMP-4). IDGE video data show that PVA dendrites melt into small fragments, or crystallites, without being subject to any detectable relative motion with respect to the surrounding melt phase. Although a mass density di! erence of about 4% exists between crystals and the melt, the lack of significant body forces on orbit precludes buoyancy e! ects, such as sedimentation and convection. Prior work reported shows that the observed dendritic melting and fragmentation occur by heat conduction through the melt. Specifically, in microgravity, melting occurs such that needle-like fragments shrink with an increasing C/A ratio, where C is the semi-major axis length, and A is the semi-minor axis length. We report new observations of late-stage melting kinetics a! ected by capillarity, where the previously rising C/A ratio of individual crystallites suddenly drops from ! 20 to ! 5 when the typical crystallite length, 2C, falls below about 5 mm. The sudden drop observed in the C/A ratios of ellipsoidal fragments undergoing late-stage melting is ascribed to capillary e! ects that accompany the melting of slender needle-like crystallites. Capillary-induced heat fluxes, received at the crystal-melt interface—both externally from the hotter melt, as well as internally—arise via the Gibbs-Thomson e! ect from the steep interfacial curvature gradients near the poles prior to total extinction. The nature of these unusual heat fluxes during late-stage melting was modeled using finite di! erence methods, the results of which will be discussed as elucidating an important phenomena during melting as observed in reduced gravity during space flight.