A method includes separately conveying a first liquid solution (36) containing the first material and a second liquid solution (46) containing a liquid polyelectrolyte. The method includes forming of a series of drops (78) at the outlet (50), each drop (78) including a central core (80) formed from a first solution (36) and a peripheral film (82) formed from a second solution. The method includes immersing each drop (78) in a gelling solution (70) containing a reagent capable of reacting with the polyelectrolyte of the film (82) so as to form the gelled casing. The second solution (40) contains at least one surfactant before the former contacts the first solution (36).