= Brine-ice sandwich with disparate diffusion scales
{title2=$\epsilon\lambda_a\sim\tfrac14[\sqrt\pi-\sqrt{\pi+8/S}]$}
For an <ice layer between freshwater and cold brine>, set $\epsilon=\sqrt{D/\kappa}$ and use solutal similarity positions $a=2\lambda_a\sqrt{Dt}$, $b=2\lambda_b\sqrt{Dt}$. A linear ice temperature, two <Stefan conditions> and interfacial salt conservation determine both negative interface constants. As $\epsilon\to0$ with fixed <latent-to-sensible heat ratio> $S$, $\lambda_b=O(1)$ but $\lambda_a=O(\epsilon^{-1})$. Freezing above occurs over the thermal diffusion scale, while dilution-driven melting below has the much shorter solutal scale.
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