Yu, H. K. ; Yi, G. - R. ; Kang, J. - H. ; Cho, Y. - S. ; Manoharan, V. N. ; Pine, D. J. ; Yang, S. - M. Surfactant-Assisted Synthesis of Uniform Titania Microspheres and Their Clusters. Chemistry of Materials 2008, 20, 2704-2710. Publisher's VersionAbstract

In this study, we demonstrate the surfactant-assisted synthesis of uniform-sized titania microspheres and their clusters for highly responsive optical materials. Monodisperse titanium glycolate microspheres were produced by controlled hydrolysis of glycolated titanium butoxide in acetone using surfactant as a stabilizer. The diameter of as-prepared particles ranged from 230 to 650 nm and was finely controlled by changing the concentrations of titanium butoxide and surfactant. In particular, the tunable range of the particle size was at a few tens of nanometers scale by the surfactant concentration, which was much finer than that by the concentration of titanium butoxide. Then, as-prepared titanium glycolate microspheres were transformed into monodisperse titania microspheres of anatase phase by heat treatment. Pure clusters of titania microspheres with an identical configuration were obtained by encapsulating a certain number of microspheres in emulsion droplets and subsequently evaporating the emulsion phase to induce self-assembly. Dynamic and static light scattering experiments show that the pure clusters with an identical configuration were well dispersed, and the refractive index of anatase phase titania microspheres was about 2.2. In addition, monodisperse Eu-doped titanium glycolate microspheres were prepared for phosphorescence sources by adding both a soluble precursor Eu(NO3)3-5H2O and glycolated titanium butoxide in acetone. Finally, photoluminescence properties of Eu-doped titania microspheres were examined after annealing.

Manoharan, V. N. Colloidal spheres confined by liquid droplets: Geometry, physics, and physical chemistry. Solid State Communications 2006, 139, 557-561. Publisher's VersionAbstract

I discuss how colloidal particles organize when they are confined by emulsion droplets. In these systems, the interplay between surface tension and interparticle repulsion drives the formation of complex, non-crystalline 3D arrangements. These can be classified into three groups: colloidosomes, or Pickering emulsions, structures that form when particles are bound to the interface of a spherical droplet; colloidal clusters, small polyhedral configurations of colloids formed by capillary forces generated in an evaporating emulsion droplet; and supraparticles, hall-shaped crystallites formed in the interior of emulsion droplets. I discuss the preparation, properties, and structure of each of these systems, using relevant results from geometry to describe how the particles organize.

Cho, Y. - S. ; Yi, G. - R. ; Lim, J. - M. ; Kim, S. - H. ; Manoharan, V. N. ; Pine, D. J. ; Yang, S. - M. Self-organization of bidisperse colloids in water droplets. Journal of the American Chemical Society 2005, 127, 15968-15975. Publisher's VersionAbstract

Most of the colloidal clusters have been produced from oil-in-water emulsions with identical microspheres dispersed in oil droplets. Here, we present new types of binary colloidal clusters from phase-inverted water-in-oil emulsions using various combinations of two different colloids with several size ratios: monodisperse silica or polystyrene microspheres for larger particles and silica or titania nanoparticles for smaller particles. Obviously, a better understanding of how finite groups of different colloids self-organize in a confined geometry may help us control the structure of matter at multiple length scales. In addition, since aqueous dispersions have much better phase stability, we could produce much more diverse colloidal materials from water-in-oil emulsions rather than from oil-in-water emulsions. Interestingly, the configurations of the large microspheres were not changed by the presence of the small particles. However, the arrangement of the smaller particles was strongly dependent on the nature of the interparticle interactions. The experimentally observed structural evolutions were consistent with the numerical simulations calculated using Surface Evolver. These clusters with nonisotropic structures can be used as building blocks for novel colloidal structures with unusual properties or by themselves as light scatterers, diffusers, and complex adaptive matter exhibiting emergent behavior.