The bimodal growth of PTCDI on NaCl is an interesting case in that one may observe in detail the dewetting transition over the course of days at room temperature. Other multimodal systems generally either dewet on much faster or slower time scales.
The dewetting process in general is akin to other coarsening scenarios such as Ostwald ripening in that the growth process is competitive. Stable islands grow at the expense of metastable islands through mass transport driven by the minimization of the global free energy.
Needle islands grow in both height and area, and tend to elongate rather than widen. This anisotropy is expected, given the hydrogen bonding that typically causes PTCDI molecules to form rows.
Due to the predominant nucleation of needle islands at step edges, the dewetting process is dependent upon the initial state of the sample, and is highly sensitive to substrate structure. Substrate areas with high step density tend to contain many small needle islands and dewet quickly, while areas with low step density tend to slowly form fewer, larger islands as dewetting is limited by diffusion and nucleation site density.
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