Being two-faced isn't always a bad thing. In the case of Janus microspheres — micrometre-sized particles whose two hemispheres have distinct properties — it can be extremely desirable, as Shin-Hyun Kim and colleagues show (S.-H. Kim et al. Angew. Chem. Int. Edn doi:10.1002/anie.201000108; 2010). They report Janus microspheres that form a flexible, superhydrophobic barrier on the surface of water.
The process of making the particles began by forming equally sized droplets of a light-reactive compound in water. The droplets also contained silica particles, which migrated to the droplets' surface. The authors irradiated the droplets with ultraviolet light, triggering a polymerization reaction, and then etched away the silica from the resulting polymeric microspheres, creating dimpled surfaces.
Kim et al. deposited the microspheres onto a film to prevent them from moving, then treated them with a compound that attached fluorine atoms to the upper, exposed surface of the spheres. Once detached from the film, the particles had two distinct faces — a hydrophilic (water-attracting), non-fluorinated side and a superhydrophobic (water-repelling), fluorinated side. The dimples on the surfaces of the spheres were essential for generating superhydrophobicity.
Kim et al. found that the Janus particles form hexagonal arrays on water surfaces, with their hydrophilic hemispheres facing the water. The resulting monolayers of particles are surprisingly robust: when the authors dropped a small amount of water onto a monolayer, it didn't pass through, but instead formed spherical droplets up to 2.4 millimetres in diameter. Larger droplets were heavy enough to force their way through the monolayer by widening the gaps between the particles.
If poked with a hydrophilic glass rod, the monolayer deformed to prevent the rod from coming into direct contact with the water. What's more, when water droplets were rolled on a bed of microspheres, the droplets became coated with the particles, creating 'liquid marbles'. The marbles could be picked up and squeezed by tweezers, without bursting (pictured).
The authors speculate that their microspheres could be used to form buoys for floating micromachines, or size-dependent, semipermeable membranes at the interfaces of immiscible fluids. They might even find their way into rain-resistant cosmetics.
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