Five-Fold Symmetry in Liquids
Entry by Emily Redston, AP 226, Fall 2012
Five-fold symmetry in liquids by F. Spaepen. Nature 408, 781-82 (2000)
Even though liquids are essential ingredients in soft matter, researchers have only recently begun to understand their structure. Previous theories describing liquids as disordered crystals or as dense gases have fallen short, and it has accepted that the liquid structure is a well-defined phase in its own right. While there are still many unanswered questions, the current method is to explain the liquid structure as a dense packing of tetrahedral building blocks. Ideally one would like to have a simple structural method for describing the liquid structure as we already have with the periodicity of crystal structures. Unfortunately, since the atomic structure of a liquid changes over space and time, conventional scattering experiments using X-rays or electrons or neutrons can only provide directionally average information.
Reichert et al  were the first to report direct evidence for polytetrahedral structures in a monatomic liquid trapped at a solid interface; they saw the characteristic five-fold symmetry of the bonds.