The site‐specific engineering of colloidal surfaces has provided a powerful approach to pushing the boundaries of today's materials research. The resulting surface‐anisotropic and patchy particles have become the center of vital research areas, ranging from the need for large‐scale fabrication techniques to exploring new applications of these materials. This Review summarizes patchy particle fabrication techniques, including but not limited to particle and nanosphere lithography and glancing‐angle deposition. The variety of existing patchy particle fabrication techniques is revealed and the need for a scalable approach to high‐volume patchy particle production is identified. Ongoing modeling efforts describing patchy particle interactions and properties are reviewed as potential predictive tools. Research endeavors that deal with the directed assembly of patchy particles in electric and magnetic fields, as well as with supraparticular assembly through chemical interactions, are discussed. The Review is concluded with a note on the future application of patchy particles as phoretic motors.magnified image

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The application of glancing angle deposition (GLAD) as a means to produce patchy particles is reported. Shadow effects are caused by neighboring particles within the particle monolayer. The patch geometry is determined by the angle of incidence of the vapor rays and the monolayer orientation. A mathematical model is used to study the patch geometry and to calculate the area of the patch. The smallest patch produced with GLAD is 3.7% of the particle surface.

There are many new approaches to designing complex anisotropic colloids, often using droplets as templates. However, droplets themselves can be designed to form anisotropic shapes without any external templates. One approach is to arrest binary droplet coalescence at an intermediate stage before a spherical shape is formed. Further shape relaxation of such anisotropic, arrested structures is retarded by droplet elasticity, either interfacial or internal. In this article we study coalescence of structured droplets, containing a network of anisotropic colloids, whose internal elasticity provides a resistance to full shape relaxation and interfacial energy minimization during coalescence. Precise tuning of droplet elasticity arrests coalescence at different stages and leads to various anisotropic shapes, ranging from doublets to ellipsoids. A simple model balancing interfacial and elastic energy is used to explain experimentally observed coalescence arrest in viscoelastic droplets. During coalescence of structured droplets the interfacial energy is continuously reduced while the elastic energy is increased by compression of the internal structure and, when the two processes balance one another, coalescence is arrested. Experimentally we observe that if either interfacial energy or elasticity dominates, total coalescence or total stability of droplets results. The stabilization mechanism is directly analogous to that in a Pickering emulsion, though here the resistance to coalescence is provided via an internal volume-based, rather than surface, structure. This study provides guidelines for designing anisotropic droplets by arrested coalescence but also explains some observations of "partial" coalescence observed in commercial foods like ice cream and whipped cream.