Surface-tension-anisotropy measurements of succinonitrile and pivalic acid: Comparison with microscopic solvability theory

New determinations of the surface-tension-anisotropy parameter ε₄ of succinonitrile (SCN), pivalic acid (PVA), and a PVA--1%-ethanol mixture are 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 ε₄ 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 σₜₕₑₒᵣ* predicted by microscopic solvability theory for the three-dimensional axisymmetric case was computed and compared to the σₑₓₚₜ* values determined directly from previous dendritic-growth experiments. We find that σₜₕₑₒᵣ*/σₑₓₚₜ* is 0.56±{}0.20 for SCN, and 2.14±{}0.50 for PVA. Possible sources for these discrepancies are discussed.