Microstructuring of polystyrene surfaces with nonsolvent sessile droplets.

Herein, we study the microstructuring of toluene-vapor-softened polystyrene surfaces with nonsolvent sessile droplets. Arrays of microvessels are obtained by depositing non-evaporating droplets of ethylene glycol/water on the original polystyrene surfaces and subsequently exposing them to saturated toluene vapor. The droplets act as a mask on the polymer, thereby impeding the toluene vapor to diffuse and soften the polystyrene surface below them. Alternatively, the formation of microcraters at random positions--with an average depth-to-width aspect ratio of 0.5 and a diameter as small as 1.5 microm--is achieved by condensing water droplets on a softened polystyrene surface. The cross-sections of the microvessels and the contact angle of the sessile water droplets suggest that the structures are formed by the combined action of the Laplace pressure at the bottom of the droplet and the surface tension acting at the three-phase contact line of the droplets. As a support, the rim height and the depth of the microvessels are fitted with an elastic theory to provide Young's modulus of the softened polystyrene surface.