The emulsion directed self-assembly is based on the confinement of colloidal nanoparticles in the dispersed phase of an emulsion that results in nanoparticle clusters. As shown in Figure , depending upon the nanoparticle concentration in the dispersed phase and the size of the dispersed phase droplet, the size of the nanoparticle clusters is determined. Also, for nanoparticles dispersed in an aqueous medium (or another polar medium), a reverse emulsion strategy needs to be adopted in order to trap the aqueous colloidal droplets in a hydrophobic (e.g. oil) phase. Usually, emulsion directed self-assembly can be quite straightforward. For example, Schmitt et al. demonstrated the formation of 3D dimensional clusters of small Au nanoparticles (~2 nm) through oil-in-water emulsions in a straightforward way, by simply mixing a thiol stabilized nanoparticle colloid in a non-polar medium (toluene, cyclohexane, dodecane, or hexadecane) with an aqueous solution of nonionic copolymer surfactant P123 (EO20PO70EO20, Pluronic BASF), Figure (b). The role of the surfactant should be emphasized here, as Lacava et al. demonstrated that for Au nanoparticles and hexane-in-water systems, SDS and Triton X-100 were better than others in forming stable emulsions with the nanoparticles encapsulated in the dispersed phase. While Span 20 and Tween 85 resulted in reasonably stable emulsions and significant entrapment of the nanoparticles at the oilwater interface was observed. In other cases, the formation of so-called Pickering emulsions can also result in ring-like assemblies upon drying. Interestingly, the organization of colloidal particles in an emulsion droplet while forming a cluster is largely driven by thermodynamics, resulting in precisely defined shell structures for a certain number of particles. As demonstrated by Wang et al. using free energy calculations, colloidal clusters with specific "magic numbers" possess higher thermodynamic favourability than those without it, Figure ). The system finds it free energy minimum configuration, which is a icosahedral structure, through a complex pathway. A molecular dynamic simulation study for tens of thousands of nanoparticles shows that the icosahedral structure is unique to the packing at this length scale in contrast to long range FCC structures. Another possibility in the emulsion formation is when the nanoparticles themselves stabilize the oil (or water) droplets by their assembly at the interface. The formation of such Pickering emulsions naturally leads to the formation of hollow nanoparticle assemblies, Figure . Ligand capped nanoparticles can also get trapped in the interfacial barriers for the formation of the Pickering emulsions, Figure . Interestingly, with large particles, relatively small droplets of water can be permanently stabilized by Pickering emulsion of water in oil, Figure (f) by careful control of the quantity of water. For a clear comparison, Table summarizes emulsion directed self-assembly procedures from different significant literature reports and the resulting structures. ©2016, American chemical society. (c) Formation of icosahedral clusters by polystyrene nanoparticles ©2018 CC-BY 4.0. (d) Hollow assembly of 5.8 nm iron-oxide nanoparticles due to the formation of oil-water Pickering emulsion. Reprinted with permission ©2016 John Wiley & Sons. (e) Pickering emulsion stabilized PEGylated Au nanoparticles. Reprinted with permission ©2007, American chemical society. (f) Permanently assembled PMMA particles (radius 1.08 μm) in CHB-decalin at the oil-water interface in various configurations: free-floating spherical water droplets (green) surrounded by particles in hexagonal packing (left), and small water droplets (green) linking together PMMA particles (right). © 2007, National Academy of Science. Lin et al. (2005)