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Abstract:
The directional solidification of the transparent binary alloy succinonitrile-poly(ethylene oxide) was studied in an experiment in which solidification speeds of about 2 mm/sec could be reached without loss of the linear temperature gradient. The low diffusivity of the polymer solute allowed the study of the dynamics of rapid solidification using an optical microscope. For both normal and doublonic dendrites we observed a transition to large triangular "superdendrites" above a certain solidification speed and we report measurements of the primary and secondary spacing as a function of the pulling speed. Our measurements suggest that the observed triangular shape is due to a decoupling of primary and secondary growth at large undercooling.
