Collision of two Hopfions
  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. 
M. Array\'as, J. L. Trueba
physics.class-ph math-ph math.MP