A pulse of current of several hundreds of volts is established between two electrodes immersed in a resistive liquid. By concentration of the current at the end of one electrode, which is bonded onto an insulating support, a volume of liquid in contact with the end of this electrode is vaporised, causing an abrupt drop in current. Because of the voltage of the pulse, which is several hundred volts, a greater current re-establishes itself immediately across the volume of vaporised liquid, as a result of a sort of ionization of the vapor, causing superheating and energy sufficient to expel a droplet of liquid through an opening provided in a membrane. In order to limit the energy of the superheating phase and control the size of the droplets, the current of the energizing pulse is limited.