We present an experimental study of the directional-solidification patterns of a nematic–smectic-$B$ front. The chosen system is ${\mathrm{C}}_4{\mathrm{H}}_9-({\mathrm{C}}_6{\mathrm{H}}_{10}{)}_2\mathrm{CN}$ (in short, CCH4) in $12\hspace{0.5em}\mu \mathrm{m}$-thick samples, and in the planar configuration (director parallel to the plane of the sample). The nematic–smectic-$B$ interface presents a facet in one direction—the direction parallel to the smectic layers—and is otherwise rough and devoid of forbidden directions. We measure the Mullins-Sekerka instability threshold and establish the morphology diagram of the system as a function of the solidification rate V and the angle ${\theta }_0$ between the facet and the isotherms. We focus on the phenomena occurring immediately above the instability threshold when ${\theta }_0$ is neither very small nor close to 90°. Under these conditions, we observe drifting shallow cells and a type of solitary wave, called “faceton,” which consists essentially of an isolated macroscopic facet traveling laterally at such a velocity that its growth rate with respect to the liquid is small. Facetons may propagate either in a stationary or an oscillatory way. The detailed study of their dynamics casts light on the microscopic growth mechanisms of the facets in this system.

• Received 5 June 2001

DOI:https://doi.org/10.1103/PhysRevE.65.011702