Experimental measurements of sidebranching in thermal dendrites under terrestrial-gravity and microgravity conditions

We perform sidebranch measurements on pure succinonitrile dendrites grown i n both microgravity and terrestrial-gravity conditions for a set of superco olings within the range 0.1-1.0 K. Two distinct types of sidebranch regions , uniform and coarsening, exist, and are characterized by the distance from the tip at which the region began, D-i, and the average spacing of sidebra nches within that region, lambda(i). There does not appear to be any signif icant dependence on either gravity level or supercooling when D-i or lambda (i) are normalized with respect to the radius of curvature of the tip, R. T he apparently constant normalized proportionality factor between D-i, lambd a(i), and R, regardless of the relative importance of diffusion and convect ive heat transport, demonstrates self-similarity between dendrites of diffe rent length scales propagating under various heat transfer conditions. Howe ver, when the form of the sidebranch envelope is defined by a power law rel ating the amplitude and relative positions of the sidebranches normalized t o the radius of the tip, the form is seen to have significant variations wi th supercooling between the terrestrial gravity and microgravity growth den drites. Furthermore, both the amplitude coefficient and exponent from the p ower-law regressions of the microgravity data are statistically different ( 95% confidence level) than their terrestrial counterparts. [S1063-651X(99)1 1612-X].