  We study the collision of two hopfions or Hopf-Ra\~nada electromagnetic
fields. The superposition of two of such fields, travelling in opposite
directions, yields different topology for the electric and magnetic field
lines. Controlling the angular momentum of such fields, we can control the
topology of the flow associated to the field lines, and the energy
distribution. The concept of electromagnetic helicity and the exchange between
its magnetic and electric components are used to explain the different
behaviours observed when the angular momentum is reversed.
