Breath-figure self-assembly

Breath-figure self-assembly is the self-assembly process of formation of honeycomb micro-scaled polymer patterns by the condensation of water droplets. "Breath-figure" refers to the fog that forms when water vapor contacts a cold surface.[1][2][3] In the modern era systematic study of the process of breath-figures water condensation was carried out by Aitken[4][5] and Rayleigh,[6][7] among others. Half a century later the interest to the breath-figure formation was revived in a view of study of atmospheric processes, and in particular the extended study of a dew formation which turned out to be a complicated physical process. The experimental and theoretical study of dew formation has been carried out by Beysens.[8][9][10] Thermodynamic and kinetic aspects of dew formation, which are crucial for understanding of formation of breath-figures inspired polymer patterns will be addressed further in detail.

Schematic (bottom) and electron micrographs (top) of the growth of a honeycomb polystyrene film by breath-figure self-assembly.

SEM images of varied patterns created through an adapted breath figure approach.

A water filter membrane prepared by breath-figure self-assembly, viewed at different synthesis steps and magnifications. The membrane material is a mixture of poly(phenylene oxide) and silica nanoparticles.

Breakthrough in the application of the breath-figures patterns was achieved in 1994–1995 when Widawski, François and Pitois reported manufacturing of polymer films with a self‐organized, micro-scaled, honeycomb morphology using the breath-figures condensation process.[11][12] The reported process was based on the rapidly evaporated polymer solutions exerted to humidity.[13][14][15] The introduction into experimental techniques involved in manufacturing of micropatterned surfaces is supplied in reference 1; image representing typical breath-figures-inspired honeycomb pattern is shown in Figure 1.

The main physical processes involved in the process are: 1) evaporation of the polymer solution; 2) nucleation of water droplets; 3) condensation of water droplets; 4) growth of droplets; 5) evaporation of water; 6) solidification of polymer giving rise to the eventual micro-porous pattern.[16] This experimental technique allows obtaining well-ordered, hierarchical, honeycomb surface patterns.[13][16] A variety of experimental techniques were successfully exploited for the formation of breath-figures self-assembly induced patterns including drop-casting, dip-coating and spin-coating.[2][15] Adapted techniques to achieve varied pattern morphologies and hierarchical designs have also been developed.[17] The characteristic dimension of pores is usually close to 1 µm, whereas the characteristic lateral dimension of the large-scale patterns is ca. 10–50 µm.[2]