Controlling breath figure patterns on PDMS by concentration variation of ethanol-methanol binary vapors.

In this paper, the self-assembly of condensed droplets on smooth and constrained surfaces under saturated vapor atmosphere of ethanol and methanol binary system is reported. Hexagonally ordered array of pores are obtained on smooth surfaces with saturated vapors of binary liquids without the assistance of any additives. The results show that the addition of a small amount of ethanol to methanol plays a role very similar to that of surface active agents in inducing the formation of a regular droplet array. The effect of constraints on a self-assembled droplet pattern such as the movement of the contact line and the depinning of the contact line is also investigated. It is observed that the pore size, pore shape, pore depth and ring diameter are influenced by the atmosphere of binary vapors in addition to the commonly held attribution to the surface tension of the solvent. Contact angle studies of the patterned substrates show hydrophobicity with high adhesiveness and transitions between the Wenzel and Cassie impregnating state over the entire concentration region.