EXPERIMENTAL-STUDY OF DENDRITIC GROWTH WITH AN EXTERNAL FLOW

In this paper, we report new experimental results on dendritic growth in the presence of an imposed flow; we also summarise the theoretical approaches to the problem, and discuss previous results obtained on th e subject. In a first part, we describe the available theoretical desc riptions of the flow around a paraboloid of revolution, performed unde r various approximations: potential, uniform or Oseen flow. Each of th ese leads to an expression for the temperature or concentration field, and relates the supercooling or supersaturation to the Peclet number of the dendrite. Several predictions on the selected growth constant a re also presented. The experiments themselves are performed on the gro wth of ammonium bromide with an external flow; we measure the concentr ation field around the crystal during the growth process. We also char acterise the velocity field. We find supersaturations in good agreemen t with predictions based on potential or uniform flows. The growth con stant is also determined in this system. We finally suggest that the a vailable experimental results on dendritic growth in the presence of a flow can be interpreted within a unique theoretical framework.