Angew. Chem. Int. Ed. doi:10.1002/anie.201000108 (2010)

The natural world is often a source of inspiration for researchers looking to develop new synthetic materials. Substances that repel water, such as lotus leaves, are of particular interest and their structures have been mimicked to develop a range of practical superhydrophobic surfaces. However, these artificial surfaces have been predominately formed on flat solids. Shin-Hyun Kim, Su Yeon Lee and Seung-Man Yang at KAIST have now created microspheres that can act as a flexible superhydrophobic interface.

Inspired by small water-repelling objects such as the scales of butterflies, the researchers synthesized microspheres that have both a superhydrophobic and a hydrophilic face using Pickering emulsions, in which solid particles stabilize the interface between two immiscible liquids. Monodisperse emulsion droplets of a photocurable resin, which also contained silica particles and iron oxide nanoparticles, were first generated using a glass capillary device. The droplets were then photopolymerized, creating microspheres decorated with an array of silica particles. Next the silica particles were removed using wet-etching, leaving cavities on the surface of the microspheres. Half of each microsphere was then coated in a hydrophobic layer by reactive-ion etching with sulphur hexafluoride.

When placed at the interface between air and water, the microspheres provide a flexible monolayer in which the hydrophilic hemispheres face the liquid phase. This microsphere membrane can support a droplet of water on top and retain integrity when disturbed by a glass stick. Furthermore, the microspheres can coat a water droplet to form liquid marbles that can be manipulated with magnets or tweezers.