Characterization of cell tip curvature in directional solidification

We experimentally characterize the geometry of cell tips in directional solidification by determining their curvature radius. Observations are made on a dilute alloy of a plastic crystal (SCN) confined in a thin sample. They go from the cellular regime to the weakly dendritic regime. Attention is drawn on providing an objective measure, insensitive to the irrelevant features of the measurement procedure and capable of characterizing the overall cell tip region. This is achieved by fitting tip shapes to suitably selected parabolas. We obtain this way accurate and coherent measures that give sense to the evolution of tip curvature radius with control parameters. A scan over large ranges of velocity V, cell spacing A and thermal gradient G reveals no significant change at the cell to dendrite transition but some scaling laws which differ from those expected at low or large Peclet numbers. They thus provide definite information for understanding cell tip geometry over the cell to dendrite transition regime. (C) 2004 Elsevier B.V. All rights reserved.