 Much of the theory of classical non-equilibrium thermodynamics is concerned with the spatially continuous motion of fluids, but fluids can also move with spatial discontinuities.Helmholtz (1868) wrote about how in a flowing fluid, there can arise a zero fluid pressure, which sees the fluid broken asunder.This arises from the momentum of the fluid flow, showing a different kind of dynamical structure from that of the conduction of heat or electricity.