ONSET OF SIDEBRANCHING IN DIRECTIONAL SOLIDIFICATION

We study the onset of sidebranching of growth cells in directional sol idification of impure succinonitrile. Care is taken to obtain uniform cell spacing over large distances in order to use this variable as a t rue control parameter. Two sidebranching modes referring to different crystalline orientations are observed and their physical equivalence i s shown. The onset of sidebranching is identified according to an orde r parameter and scanned with respect to pulling velocity, thermal grad ient, and cell spacing. Its evolution with the control parameters surp risingly reveals that increasing thermal gradient at otherwise fixed v elocity and cell spacing enhances sidebranching. These results show th e need for improving the experimental characterization and the theoret ical description of sidebranching in directional solidification.