Bipolar surface charging by evaporating water droplet

Charging due to the evaporation of a water droplet on a solid surface is ubiquitous in both natural and industrial systems, and the charges present at the surface have a significant influence on interfacial processes. However, how an evaporating droplet deposits charges on solid surfaces remains poorly understood. In this study, we investigated charge deposition by evaporating micron-sized water droplets by first monitoring their evaporation dynamics and then using Kelvin Probe Force Microscopy to spatially map the surface charge patterns with micron-scale resolution on various substrates. Unlike sliding water droplets, which leave unipolar charges, evaporating water droplets deposit both positive and negative charges, generating a bipolar charge pattern on the substrate. To explain this phenomenon, we developed an analytical model that quantitatively describes the origin of the bipolar charging pattern.