Superhydrophic surfaces enable spherical opalescent arrays to be made with precise shapes
Crystalline arrays of colloidal particles — some pretty examples in nature are opals — have uses in photonics, coatings and sensing. They can be made by wet self-assembly, but these methods do not give robust control over the final particle shapes. Now, Orlin Velev, of North Carolina State University, and colleagues have developed a dry self-assembly technique1 that controls the shape of the supraparticles, and enables them to be removed from the substrate more easily.
Droplets of an aqueous suspension of latex particles were dispensed onto a silicon-nanowire-coated substrate so hydrophobic that they formed almost perfectly spherical shapes. As water evaporates from the suspension, spherical colloidal assemblies of latex particles are formed. The addition of gold nanoparticles to the suspension results in supraparticles with metallic domains on the surface, because the smaller particles migrate with the solvent in between the larger ones. A further benefit of the superhydrophobic substrate is that the assemblies are easily removed from it because they simply roll off.
Like opals, the assemblies sparkle colourfully. Velev and co-workers discovered that these reflection bands do not originate from Bragg scattering within the supraparticles, but from rows of crystals on the surface. The simple fabrication technique could be used in parallel manufacturing processes, making uniform supraparticles for a variety of optical — or even drug delivery — applications.
Rastogi, V., Melle, S., Calderón, O. G., García, A. A., Marquez, M. & Velev, O. D. Synthesis of light-diffracting assemblies from microspheres and nanoparticles in droplets on a superhydrophobic surface. Adv. Mater. 10.1002/adma.200703008 (2008).
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Withers, N. Diffracting droplets. Nature Chem (2008). https://doi.org/10.1038/nchem.50