Dynamics of a faceted nematic-smectic-B front in thin-sample directional solidification

We present an experimental study of the directional-solidification patterns of a nematic-smectic-B front. The chosen system is C4H9-(C6H10)2CN (in short, CCH4) in 12 microm-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 straight theta(0) between the facet and the isotherms. We focus on the phenomena occurring immediately above the instability threshold when straight theta(0) is neither very small nor close to 90 degrees. 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.