Anisotropic particle synthesis inside droplet templates on superhydrophobic surfaces.

We demonstrate how droplet templates dispensed on superhydrophobic substrates can be used to fabricate both shape-anisotropic ("doughnut") and composition-anisotropic ("patchy magnetic") supraparticles. The macroscopic shape of the closely-packed particle assemblies is guided by the droplet meniscus. Aqueous droplets of monodisperse microsphere suspensions dispensed on the substrates initially acquire near-spherical shape due to a high contact angle. During the solvent evaporation, however, silica suspension droplets undergo shape transitions (concaving) guiding the structure of the final assemblies into doughnut supraparticles. Composition anisotropy is achieved by drying a droplet containing a mixed suspension of latex and magnetic nanoparticles, while exposing it to magnetic field gradients. Depending on the pattern of the magnetic fields, the magnetic nanoparticles segregate into single, bilateral, or trilateral, patched spherical supraparticles. The physical effects leading to the development of anisotropy are discussed. Unlike the conventional wet self-assembly (WSA) methods where the final structures need to be extracted from the liquid environment, this efficient one-step procedure produces ready to use "dry" supraparticles.