  Nano- and microscale particles, such as colloids, commonly interact over
ranges much shorter than their diameters, so it is natural to treat them as
"sticky," interacting only when they touch exactly. The lowest-energy states,
free energies, and dynamics of a collection of $n$ particles can be calculated
in the sticky limit of a deep, narrow interaction potential. This article
surveys the theory of the sticky limit, explains the correspondence between
theory and experiments on colloidal clusters, and outlines areas where the
sticky limit may bring new insight.
