Onset of sidebranching in directional solidification

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Abstract
We study the onset of sidebranching of growth cells in directional solidification of impure succinonitrile. Care is taken to obtain uniform cell spacing over large distances in order to use this variable as a true control parameter. Two sidebranching modes referring to different crystalline orientations are observed and their physical equivalence is shown. The onset of sidebranching is identified according to an order parameter and scanned with respect to pulling velocity, thermal gradient, and cell spacing. Its evolution with the control parameters surprisingly reveals that increasing thermal gradient at otherwise fixed velocity and cell spacing enhances sidebranching. These results show the need for improving the experimental characterization and the theoretical description of sidebranching in directional solidification.