Iron-precipitated cacti

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These segments of iron wire melted incompletely before sudden cooling.

cactus-like structure on partially melted iron wire, large scale

cactus-like structure on iron wire, medium scale

The cactus spines are most obvious when viewed in profile.

The wire on the right is covered with fine rows of "cactus spines".

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The cactus spines tend to bifurcate, just like the dendrites on cobalt.

occasionally bifurcating iron-precipitated cactus spines

Tentative explanation: A strong electric field first folds surface graphite, then draws dendrites from the creases. Creases in surface graphene will concentrate electrostatic fields and facilitate the formation of compound curves.

Alternative explanation: A solid surface carbide phase may be present underneath the graphene layers on all metals investigated so far (iron, nickel and cobalt). Surface carbides will not generally have the same coefficients of thermal expansion as graphene, so differential thermal contraction--and, thus, spontaneous film curvature--should be expected.