Effect of solvent choice on breath-figure-templated assembly of "holey" polymer films

Dew or breath figures is a disordered pattern formed by polydisperse water drops that form when a cold surface comes in contact with breath or moist air. Unexpectedly, self-assembled arrays of non-coalescent monodisperse water drops form when dilute solutions of polymers in volatile organic solvents are exposed to moist air. After solvent evaporation is complete, these condensation figures dry out as well, leaving "holey" polymer films, containing hexagonally ordered arrays of pores (0.2 to 10 μm). While the macroporous films are produced easily for a wide variety of solvents and polymers, there is no existing theory that describes how the pore size depends upon tunable parameters like humidity, air temperature and velocity as well as on the choice of solvent and polymer. In this study, we propose a transport model to elucidate the role of the solvent and airflow in determining the rate and extent of evaporative cooling, and contrast our model results with the corresponding experimental measurements for polystyrene/carbon disulfide solutions. We describe how modeling evaporative cooling is essential for the quantitative understanding of dominant processes that lead to growth, non-coalescence and self-assembly of water drops, and the subsequent formation of ordered arrays of pores.