Mesoscale periodic structures have been fabricated via directed assembly of colloidal inks. Concentrated colloidal gels with tailored viscoelastic properties were designed to form self-supporting features. The inks were deposited in a layer-by-layer sequence to directly write the desired 3-D pattern. Periodic structures with spanning features that vary between ∼100 µm and 1 mm were assembled. Shear rate profiles were calculated on the basis of the measured rheological properties of the inks under slip and no-slip boundary conditions during flow through a cylindrical deposition nozzle. Deflection measurements of spanning elements were used to probe the relationship between gel strength, deposition speed, and shear rate profiles in the nozzle. These observations revealed that the ink adopted a rigid (gel) core-fluid shell architecture during assembly, which simultaneously facilitated bonding and shape retention of the deposited elements.

The ability to pattern ceramic materials in three dimensions (3D) is critical for structural, functional, and biomedical applications. One facile approach is direct ink writing (DIW), in which 3D structures are built layer-by-layer through the deposition of colloidal-or polymer-based inks. This approach allows one to design and rapidly fabricate ceramic materials in complex 3D shapes without the need for expensive tooling, dies, or lithographic masks. In this feature article, we present both dropletand filament-based DIW techniques. We focus on the various ink designs and their corresponding rheological behavior, ink deposition mechanics, potential shapes and the toolpaths required, and representative examples of 3D ceramic structures assembled by each technique. The opportunities and challenges associated with DIW are also highlighted.

A new mechanism for regulating the stability of colloidal particles has been discovered. Negligibly charged colloidal microspheres, which flocculate when suspended alone in aqueous solution, undergo a remarkable stabilizing transition upon the addition of a critical volume fraction of highly charged nanoparticle species. Zeta potential analysis revealed that these microspheres exhibited an effective charge buildup in the presence of such species. Scanning angle reflectometry measurements indicated, however, that these nanoparticle species did not adsorb on the microspheres under the experimental conditions of interest. It is therefore proposed that highly charged nanoparticles segregate to regions near negligibly charged microspheres because of their repulsive Coulombic interactions in solution. This type of nanoparticle haloing provides a previously unreported method for tailoring the behavior of complex fluids. Colloidal suspensions enjoy widespread use in applications ranging from advanced materials to drug delivery. By tailoring interactions between colloidal particles, one can design stable fluids, gels, or colloidal crystals needed for ceramics processing (1), coating (2), direct write (3), photonic (4-9), and pharmaceutical (10, 11) applications. Long range, attractive van der Waals forces are ubiquitous and must be balanced by Coulombic, steric, or other repulsive interactions to engineer the desired degree of colloidal stability.The self-organization of highly charged nanoparticles and their influence on the behavior of complex fluids in which they dwell has received scant attention. The traditional view is that small particles or other species (e.g., polyelectrolyte, polymer, or micelles) in solution can promote flocculation of stable colloidal suspensions via an entropic depletion interaction (12-15). The term ''depletion'' describes the exclusion of these smaller species from the gap region between colloidal particles that arises when their separation distance becomes less than the characteristic depletant size. The resulting concentration gradient between the gap region and bulk solution gives rise to an attractive force, whose magnitude scales with the volume fraction of smaller species, their charge, and the size ratio of large to small species (12,15,16). However, emerging theoretical work (17-19) suggests that charged species in solution may affect system stability through other self-organizing pathways. For example, charged nanoparticles have been predicted to segregate to regions surrounding large uncharged colloids, especially in systems with high size asymmetry and many more small to large spheres (18). This segregation is driven solely by a Coulombic repulsion between smaller species in solution and occurs simply because the larger particles represent a big volume without charge. The key question we wish to explore is whether this type of haloing process can provide a mechanism for stabilizing colloidal species.Here, we study the effects of highly charged nanoparticles on the behavior of ne...