Electrically charged droplet: case study of a simple generator

Abstract

We studied the behavior of a simple apparatus that allows charging droplets by induction. We imposed the following constraints: (i) any liquid should be charged (more precisely whatever its surface tension and therefore whatever its wettability, capillary length, etc.); and (ii) the system is to remain as simple as possible. The design of the device results of a compromise regarding both constraints. The device developed is based on a planar capacitor in which the droplets are generated. The influence of the nature of the liquid (i.e., presence of ions in solution, polarity, surface tension, and conductivity) on the charge induced was measured and was found to be independent of the liquid properties (except for the silicone oil). We deduced that, in the considered configuration, fluids behave like a “perfect conductor” as soon as conductive relaxation time is smaller than the droplet ejection time. Under this condition, the charge has the time to move in the droplet to reach the electrostatic regime.