= Quasistatic spherical Stefan problem with capillarity
{title2=$\dot a=(\kappa/S)(a-a_c)/a^2$}
For a one-phase spherical <Stefan problem> with melting-temperature boundary value $T_s-\Gamma/a$ and supercooling $\Delta T=T_s-T_\infty>0$, use the latent-to-sensible heat ratio $S=\mathcal L/(c_p\Delta T)$. Large $S$ separates heat diffusion from slow interface motion. The exterior harmonic temperature is $T=T_\infty+(\Delta T-\Gamma/a)a/r$. The <Stefan condition> gives the displayed radius equation with $a_c=\Gamma/\Delta T$. This is the large inverse-<Stefan number> convention; the common sensible-to-latent Stefan number is $1/S$.
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