Investigation of Orderliness of Breath Figures on Polydimethylsiloxane.

Breath figures, the self-assembled water droplet patterns formed on cold surfaces, are ideally hexagonal. A deviation from the ideal honeycomb pattern can occur due to variation of roughness of the substrate, change in vapor from water to other liquids, etc. The thermodynamics of breath figure formation is complex, and any deviation from ideality is even more difficult to understand. In the absence of a unified theory to understand such patterns and experimental difficulties in monitoring all aspects of formation of breath figures, the patterns formed are studied in terms of their orderliness by determining their Voronoi entropy. We report here the Voronoi entropy calculations of the breath figure fabricated over the smooth and constrained surfaces using polydimethylsiloxane (PDMS) of molecular weight 235 g/mol in two different environments: (a) water and (b) binary mixture of ethanol-propanol over the entire concentration range. Ordered honeycomb patterns are seen on the smooth surfaces, and disordered patterns are seen on constrained surfaces when imaged using confocal microscopy. The latter is attributed to the depinning of the triple-phase contact line, implying that the underlying constraints influence the pore morphology. Contact angle studies of water over the breath figure patterned surfaces indicate the hydrophobic nature of the patterned surfaces.