THE EFFECT OF CONVECTION ON DENDRITIC GROWTH UNDER MICROGRAVITY CONDITIONS

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Abstract
The Isothermal Dendritic Growth Experiment (IDGE) is an orbital space flight experiment, launched 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 obtained at supercoolings in the range 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 received 3 months after the flight. The IDGE flight data has now been analyzed, permitting a comprehensive comparison between dendritic growth under terrestrial and microgravity conditions. The measured growth kinetics, in the form of velocity versus supercooling, is markedly different from those observed in terrestrial experiments. Above 0.4 K supercooling in microgravity, the process of dendritic growth is diffusion controlled, i.e., thermal conduction is the rate limiting process. Under terrestrial conditions, dendritic growth of SCN remains dominated by convective transport of heat until a supercooling of ca. 1.7 K is exceeded. Beyond a supercooling of 1.7 K, there is excellent agreement between terrestrial dendritic growth measurements, and a theory with one adjustable parameter determined form the microgravity measurements. Surprisingly, however, even under microgravity conditions, dendritic growth of SCN becomes dominated by convective transport at supercoolings of ca. 0.4 K and below. The observations confirm that convection, which depends as a sublinear power of the supercooling, will always dominate at low supercoolings, whereas diffusion, which depends on the superlinear power of the supercooling, will always dominate at high supercoolings.