SELF ASSEMBLED, ULTRA-HYDROPHOBIC MICRO/NANO-TEXTURED SURFACES

The formation of hierarchically ordered arrays of spherical cavities on polymer films is of interest due to potential applications in the preparation of photonic bandgap materials, environmental sen-sors, and patterned light emitting diodes (LED). While many methods are known for the prepara-tion of these porous materials, the breath figure approach has received significant scrutiny because of the simple and robust mechanism of pattern formation.1 Breath figures are patterned arrays of micrometer-sized defects in a polymer film, formed when water droplets condensed onto a polymer solution surface during film drying. By controlling variables such as relative humidity and solvent, the feature size and uniformity of the resultant pattern can be controlled. The breath figure approach is valuable because it provides the advantages of large area ordering in the nano and micrometer re-gime, is versatile, and inexpensive. The process is driven by the evaporation of an ap-propriate solvent under humid conditions, leading to a decrease in temperature at the air-liquid inter-face, resulting in water condensation.2 The drop-wise condensation proceeds according to several steps shown in Figure 1. The first stage involves nucleation of the water droplets on the polymer solution surface. During the second stage, the drop-lets become larger and coalesce to minimize interfa-cial energy differences, resulting in pattern forma-tion. With properly controlled conditions, a subsequent ordering of the droplets into a hexago-nal lattice is observed.3 The temperature difference between the surface and the ambient conditions is minimized once the surface of the film is covered with water droplets. At this stage, the water drop-lets sink into the solution (depending upon solution density). Upon complete evaporation of the sol-vent, the pattern of the droplets generated in the polymer matrix is preserved as a hexagonally or-dered array of pores with a honeycomb structure.4,