Colloidal clusters
Colloids consist of small particles suspended in a liquid by thermal agitation. That is, colloidal particles are sufficiently small that collisions with the molecules of the liquid in which they are immersed are sufficient to keep them from sinking or floating under the influence of gravity. This means that colloidal particles are generally smaller than a few micrometers, since larger particles usually sink or float. There is no lower limit to the size of colloids, other than to note that because they are made up of many atoms or molecules, they are generally larger than a nanometer or so. Many everyday products are colloids, including milk, paint, ink, bacteria, viruses, as well as globular proteins. Opals are essentially dried out colloids.
A suspension of identical colloidal particles can spontaneously self assemble to form a regularly-spaced lattice. Figure 1 on the right shows such a lattice, a "colloidal crystal", that has been photographed using an electron microscope. The particles in the photograph are made from polystyrene, a common commercial plastic. The water in which the particles were suspended was evaporated to facilitate taking the photograph, but the same ordered structure of particles apparent in the figure was present before the water was removed.
The lattice shown in Fig. 1 is a face-centered cubic (FCC) lattice, an arrangement favored by grocers stacking oranges. Other types of lattices can be made by mixing two or more particles of different sizes in just the right ratio. However, the number of different kinds of lattices that can be made with simple spheres is limited.
Colloids can be any shape in principle. Proteins often fold up into complex oddly-shaped globules. Many viruses are shaped like rods. Artificial colloids, however, such as those synthesized for use in paint, are typically spherical, so the repertoire of particles we can actually make is rather limited.
In order to expand the kinds of structures that can be made with colloids, we have developed methods for assembling colloids in to small aggregates with well-defined structures. Figure 2 shows a series of colloidal clusters consisting of 3 to 10 particles. The emulsion encapsulation method we developed for making these clusters results in a unique configuration of particles for all clusters of a given number: for example, all 7-mers look exactly the same – there are no isomers. The absence of isomers has been explained by Eric Lauga and Michael Brenner in Physical Review Letters 93, 238301 (2004) . You can read more about the clusters in Fig. 2 (and a few more), including how they are fabricated in the following publications: Science 301, 483-487 (2003) [& Supplementary materials ], Advanced Materials 16, 1204-1208 (2004) , and MRS Bulletin 29, 91-95 (2004) .
Working with our collaborators at the Korea Advanced Institute for Science and Technology (KAIST) in Daejeon, Korea, we have generalized our emulsion encapsulation method in order to form complex clusters made from colloidal particles of two different sizes. Figure 3 shows some of the results. From left to right, the clusters consist of 1, 2, or 3 large particles (2.3 μm diameter, shown in pink) and many small particles (0.23 μm diameter, shown in white). In this photograph, all the spheres are silica, but we made other clusters where the small and large particles were made out of different materials. You can read about this work in JACS 127, 15968-15975 (2005) .
The first cluster experiments were done by Vinny Manoharan as part of his Ph.D. thesis, with assistance from Mark Elsesser. Subsequently, several people have made important contributions to the development and understanding of these clusters including Dr. Gi-Ra Yi and Prof. Seung-Man Yang at the Korea Advanced Institute for Science and Technology (KAIST) in Daejeon, Korea. The bi-disperse clusters were developed as part of the Ph.D. thesis of Young-Sang Cho working at KAIST.
