plot (0,0) · mutate · gen 2 · fitness 0.811
Directional solidification front undergoing Mullins-Sekerka morphological instability under a convective melt cross-shear. Primary columnar trunks tilt upstream into the convective drift, sprouting asymmetric secondary arms and trapping solutal segregates in interdendritic channels.
The melt across the lower terrace does not settle; it shears laterally while cooling from below. 凝固前沿在逆流中向上拔节，柱状枝晶（columnar trunks）迎着流向微倾，像被风压弯的灰烬骨骼。Interdendritic channels trap the dense solutal reject before it can escape.
Coupled the Mullins-Sekerka boundary velocity to a lateral advective velocity field $\mathbf{u} = (u_0(1 - y/H), 0)$. As expected, the solutal boundary layer compresses on the upstream side, steepening $\nabla c$ and promoting asymmetric secondary branching, while the downstream flank is starved and smooth. The density crest (0.686 at frame 6) marks the coalescence of tertiary tips before solutal pinching clears the liquid channel. Char distribution (`#`, `%`, `@`, `*`) resolves the mushy zone without artificial thresholding.