Drop Electrification

When water rubs against a solid surface, it naturally acquires electric charge. We study the consequences of this spontaneous droplet electrification.

Using a specialized microscope that can “see” electric charge at the nanoscale, we watched this process unfold: each time a droplet evaporates, it leaves behind a patchwork of positive and negative regions — a unique electrical footprint that depends on the surface material. Remarkably, on some surfaces, the droplet can spontaneously self-propels as a result. This work reveals a subtle link between electrostatics and droplet evaporation, and could open new possibilities for controlling water droplets in applications such as cooling technology, water harvesting, and self-organizing “active” materials.