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Measurements of video data on melting dendritic crystal fragments in reduced gravity show that a fragment's ellipsoidal axial ratio, C/A, rises initially until it melts down to a pole-to-pole length of C ≈ 5 mm. At that point we observe a sudden fall in the C/A ratio with time, as the polar regions melt toward each other more rapidly than C/A times the melting speed, dA/dt, of the equatorial region. This accelerated melting allows the C/A ratio to fall from values around 10-20 (needle-like) towards values approaching unity (spheres) just before total extinction occurs. Analytical and numerical modeling will be presented that suggest that the cause of these sudden changes in kinetics and morphology during melting at small length scales is due to a crystallite's extreme shape anisotropy. Shape anisotropy leads to steep gradients in the mean curvature of the solid-melt interface near the ellipsoid's poles. These curvature gradients act through the Gibbs-Thomson effect to induce unusual thermo-capillary heat fluxes within the crystallite that account for the observed enhanced polar melting rates. Numerical evaluation of the thermo-capillary heat fluxes shows that they increase rapidly with the C/A ratio, and with decreasing length scale, as melting progresses toward total extinction. [ABSTRACT FROM AUTHOR]

The Isothermal Dendritic Growth Experiment (IDGE) is an orbital space flight experiment, lauched by NASA, in March, 1994, as part of the United States Microgravity Payload (USMP-2). The IDGE provided accurately measured dendritic growth rates, tip radii of curvature, and morphological observations of ultra-pure succinontrile in the supercoolings range between 0.05-2.0 K. Data were received in the form of pairs of digitized binary images telemetered to the ground from orbit in near-real-time, and as 35mm photographic film. The data taken above ca. 0.4 K supercooling provide the first comprehensive assessment of diffusion-limited dendritic growth, and those taken below this value yield new insights into the influence of convection. The IDGE also demonstrated that prior terrestrial dendritic growth data were generally corrupted by melt convection, making their comparison with diffusion-limited theories questionable. A key element in the design and operation of this experiment, which was in development for over ten years at a cost of approximately $12 million U.S., was its reliance on telescience for accomplishing in situ near-real-time monitoring and control of the experiment. [ABSTRACT FROM AUTHOR]