SURFACE-TENSION-ANISOTROPY MEASUREMENTS OF SUCCINONITRILE AND PIVALIC ACID - COMPARISON WITH MICROSCOPIC SOLVABILITY THEORY

New determinations of the surface-tension-anisotropy parameter epsilon-4 of succinonitrile (SCN), pivalic acid (PVA), and a PVA-1%-ethanol mixture axe reported. Effects of temperature gradients and crystal orientation are analyzed in detail. The experiments utilized numerical interpolation techniques previously employed by Dougherty and Gollub to enhance the digital image resolution. The values found for epsilon-4 in SCN, PVA, and PVA-1%-ethanol are 0.0055+/- 0.0015, 0.025+/-0.002, and 0.026+/-0.002, respectively. From these values, the selection parameter sigma(theor)* predicted by microscopic solvability theory for the three-dimensional axisymmetric case was computed and compared to the sigma(expt)* values determined directly from previous dendritic-growth experiments. We find that sigma(theor)*/sigma(expt)* is 0.56+/-0.20 for SCN, and 2.14+/-0.50 for PVA. Possible sources for these discrepancies are discussed.