Spreading dynamics of terraced droplets

The liquid crystal (7) over bar S5 spreads as a two-terraced droplet on an oxide covered (100) Si wafer. The thickness of the upper and lower terraces are respectively similar to 200 and similar to 40 Angstrom. This is the si mplest system for which the de Gennes and Cazabat (dGC) terraced spreading model [C. R. Acad. Sci. II 310, 1601 (1990)] is applicable. We find that so on after the upper terrace acquires a flat top a hole develops in the cente r of this terrace. The hole propagates down to the depth of the first terra ce. In this contribution we demonstrate that the dGC model is unstable to t he formation of a hole in the center of the upper terrace for a two-terrace d droplet. Our extended dGC model, which includes a hole in the upper terra ce, provides a reasonable description of the average spreading dynamics for this system. However, this model has difficulties quantitatively accountin g for all of the features exhibited by the dynamics, perhaps because experi mentally the inner and outer borders of the upper terrace become irregular with time. These irregularities in the borders have not been included withi n the model.