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It is in the lexicon of crystal growth that the shape of a growing crystal reflects the underlying microscopic architecture. Although it is known that in weakly nonequilibrium conditions the slowest growing orientations ultimately dominate the asymptotic shape, is the same true for melting? Here we observe and show theoretically that while the two-dimensional steady melt shapes of ice are bounded by six planes, these planes are not proper facets but instead are rotated 30 degrees from the prism planes of ice. Finally, the transient melting state exposes 12 apparent crystallographic planes thereby differing substantially from the transient growth state.

We investigate the microscopic mechanisms underlying the dynamical faceting of crystals. Partially faceted crystal shapes of CCl4 are formed from a melt contained in a Bridgman apparatus and pressure is used to control growth which is observed using optical microscopy. In contrast to predictions of models in which the local interfacial motion is greatest where the step density is the highest, the loss of rough orientations is observed to occur via a local decrease in curvature which results in the formation of discontinuities-shocks-in the surface of the growth forms, a feature predicted by a recent theory of kinetic faceting.

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