  Using concepts from fluid dynamics, a partial differential equation for the
shape evolution of a metallic nanowire is derived from a semiclassical energy
functional that includes electron-shell effects. A rich dynamics, involving
movement and interaction of kinks connecting locally stable radii, leads to the
formation of a wire whose equilibrium shape is universal and consists of a
cylindrical part connected to unduloid-like leads. The universality of the
equilibrium shape may provide an explanation for the formation of cylindrical
nanowires observed in recent experiments.
