The microfluidic Kelvin water dropper

TL;DR

This paper presents a microfluidic implementation of Kelvin's water dropper, demonstrating spontaneous droplet charging and breakup due to electrohydrodynamic instabilities, with potential applications in energy conversion and microfluidic control.

Contribution

The authors develop a microfluidic water dropper that shows droplet charging and breakup, providing experimental validation and comparison with simulations, advancing microfluidic electrohydrodynamics.

Findings

Droplets acquire charge and break up due to electrohydrodynamic instabilities.

Charge depends on system parameters as shown by simulations.

Potential for converting pneumatic pressure into electrical energy.

Abstract

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 surrounding created by any metallic object. In his experiment, two streams of droplets are allowed to drip from separated nozzles into separated 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.