A device for generating sprays of charged droplets, and resulting nanoparticles, the device comprising a first needle connected to an electrical potential line to generate a first spray of charged particles from the first needle, and a second needle spaced apart from and facing the first needle, and connected to an electrical line configured to ground the second needle or to apply a voltage to the second needle that is the same polarity as the voltage applied to the first needle. The device also comprising an electric field modifier connected to the first needle, and configured to modify an electrical field to generate a second spray of charged particles from the second needle.