The Fellowship of the Ring: Spontaneous Formation of Charged Rings after Droplet Evaporation

Water droplets moving over insulating surfaces are known to acquire and deposit charge, a phenomenon known as slide electrification. Yet charging can also occur without translational motion, for example during the evaporation of a stationary droplet. Here, we use Scanning Kelvin Probe Microscopy (SKPM) to map surface charge before and after the evaporation of a macroscopic sessile water droplet on an insulating surface. After evaporation, we consistently find a bipolar charge pattern: a positive ring surrounding a negative center. By tuning the pH of the droplet, we can invert the polarity or even produce multiple concentric rings near the isoelectric point. Side-view imaging shows that ring formation occurs only when the droplet evaporates with a constant contact angle and above a critical contact angle. We interpret these results within the framework of an evolving electric double layer that persists during dewetting and redistributes as the droplet evaporates.