The microfluidic Kelvin water dropper.

The so-called "Kelvin water dropper" is a simple experiment demonstrating the spontaneous appearance of induced free charge in droplets emitted through a tube. As Lord Kelvin explained, water droplets spontaneously acquire a net charge during detachment from a faucet due to the presence of electrical fields in their surroundings created by any metallic object. In his experiment, two streams of droplets are allowed to drip from separate nozzles into separate buckets, which are, at the same time, interconnected through the dripping needles. In this paper, we build a microfluidic water dropper and demonstrate that the droplets get charged and break up due to electrohydrodynamic instabilities. A comparison with recent simulations shows the dependence of the acquired charge in the droplets on different parameters of the system. The phenomenon opens a door to cheap and accessible transformation of pneumatic pressure into electrical energy and to an enhanced control in microfluidic and biophysical manipulation of capsules, cells and droplets via self-induced charging of the elements.