Packing in the Spheres
Original entry: William Bonificio, AP 225, Fall 2009
Packing in the Spheres David A. Weitz, Science , vol 303 968 (2004)
close, packing, spheres, oblate, ellipsoid
The paper discusses how nonspeherical objects pack compared to spherical ones. Surprisingly, ellipsoidal and oblate objects take less energy to reach a close packed structure. Some reasons for this unintuitive result are discussed.
Soft Matter Example
Colloidal physics often deals with the problem of packing. For the most part, scientists will approximate all object that are being packed as spheres and then calculate the packing efficiency with that basis. One such type of packing that can occur is when the spheres still have a completely random orientation, but are packed as tightly as they can be, this is called random close packing, or RCP. For spheres, the volume fraction for RCP is 0.64.
If we allow the spheres to be ordered, and pack them as tightly as possible they take a configuration called hexagonal close packed, or HCP. In this configuration the spheres form hexagonal layers, and each layer above will fall into the crevices formed by the layer below. The volume fraction for spheres in HCP is 0.74.