A very usefull photography technique to illustrate the collective dynamics of active elastic solids ! It consists at taking pictures with long exposure times in a very dark background, so that luminous objects will leave a trace highlighting their displacements. To apply this method to our experimental system, we just need to apply some fluorescent paint on the nose of each active units constituting the active solid, and to illuminate with some « black light« . Thus one can visualize on a single picture the collective dynamics of the self-propelled objects.
