Summary and Conclusions
We measured dendritic growth velocities and tip radii of curvature of succinonitrile in
microgravity using the IDGE instrument flown in low-earth orbit. The on-orbit data, when
compared to terrestrial dendritic growth data obtained using the same apparatus and techniques,
demonstrate that: 1) convective effects under terrestrial conditions cause growth speed increases
to a factor of 2 at the lower supercoolings ( DeltaT < 0.5 K), and convection effects remain discernible
under terrestrial conditions up to supercoolings as high as 1.7 K. 2) In the supercooling range
above 0.47 K, microgravity data remain virtually free of convective or chamber effects, and may be
used reliably for examining diffusion-limited dendritic growth theories. 3) A diffusion solution to
the dendrite problem, combined with a unique scaling constant, sigma*, fails to yield accuratepredictions
of the growth velocity and dendritic tip radii. 4) Growth Péclet numbers calculated
from Ivantsov s solution deviate systematically from the IDGE data observed under
diffusion-limited conditions. 5) The scaling parameter sigma* does not appear to be a constant independent
ofsupercooling. Finally, 6), the sigma* measurements from the terrestrial and microgravity data are ingood
agreement with each other, despite a difference of over six orders of magnitude in the quasi-static
acceleration environment of low-earth orbit and terrestrial conditions.
The most important of these findings is that Ivantsov s description of the conduction of latent heat
from adendrite needs to be modified in order to describe the growth of SCN dendrites accurately. We offer here
one key observation: the diffusion field of the Ivantsov formulation is based on a steady-state dendrite which
is a paraboloid of revolution. But we know from our own measurements and others, that neither assumption
is true [Huang, 1981][Koss, 1995]. The microgravity data reported here in the diffusion limited regime should help to constrains
the possibilities for theoretical modeling. Diffusion-limited growth data can serve as a benchmark for testing
and developing theories and provide a better basis for phenomenological models of dendritic growth kinetics.
The publication of our dendritic growth data in tabular form is meant to facilitate such studies.
The data from our recent microgravity experiment will be added to this as soon as it is available.