Building Materials by Packing Spheres

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Original entry: A.J. Kumar , APPHY 225, Fall 2009

Reference

V.N. Manoharan and D.J. Pine, MRS Bulletin 29 (2): 91–95 (2004)

Keywords

colloids, microspheres, optical materials, packing, structure.

Abstract from the original paper

"An effective way to build ordered materials with micrometer- or submicrometer-sized features is to pack together monodisperse (equal-sized) colloidal particles. But most monodisperse particles in this size range are spheres, and thus one problem in building new micrometer-scale ordered materials is controlling how spheres pack. In this article, we discuss how this problem can be approached by constructing and studying packings in the few-sphere limit. Confinement of particles within containers such as micropatterned holes or spherical droplets can lead to some unexpected and diverse types of polyhedra that may become building blocks for more complex materials. The packing processes that form these polyhedra may also be a source of disorder in dense bulk suspensions."

Summary

Figure 1: Scanning electron micrographs of clusters formed by particles on the surface of an evaporating droplet. Schematic illustrations show the sphere packings and polyhedra that minimize the second moment of the mass distribution.

In this paper, Manoharan and Pine conjecture that we can learn about the structure of dense suspensions by studying smaller structures of finite packed spheres. After discussing the geometry of solid spheres, the authors describe various methods to create clusters of finite packed spheres in two dimensional and three dimensional structures. In 2D, the particles can be patterned onto a functionalized patch or holes on a micropatterned surface. The size and shape of these holes or patches provides constraints on the types of geometry that can be realized in the finite sphere clusters. To produce additional 2D and 3D arrangements, the authors absorb the particles into the interface between water and oil in a droplet. The structures that result from these processes are predicted geometrically but are often unique from the structures allowed in a dense infinite packing (fcc). The structures can contain vacancies or symmetries that would not be found in an infinite packing. Figure 1 shows examples of some of the structures that were created using the droplet method to form structures.

The authors point out that these structures can be frozen and redispersed in a dense suspension of the particles, introducing designed disorder or vacancies and allowing for a novel approach to engineering new structures on a larger scale. This allows for the doping of bulk colloidal crystals to produce specific features or properties, such as permeability, density, and disorder.

Connection to Soft Matter

Colloids and colloidal clusters are a major topic of study within soft matter. This paper gives us insight into how colloidal structures form and even how we could control the formation of certain structures, allowing us to potentially engineer colloids for specific uses. One of the reasons we study soft matter is because that soft matter materials sometimes behave in a way that we would not expect based on our understanding of solids or fluids. The fact that finite spheres arrange in ways different from a bulk crystalline structure hints at such interesting behavior. Furthermore, this information gives us insight into how to improve use of colloids in making materials. The authors suggest that doping bulk colloidal suspensions with these finite packed sphere arrangement could help control nucleation of colloidal crystals and hence allow more control over the quality of crystals for optical materials.