Studying the droplet sliding velocity and charge transfer at a liquid–solid interface

The contact electrification between liquid and solid has attracted widespread attention in the fields of energy, physics and chemistry, but its basic mechanism still remains unclear, especially for charge transfer at a liquid–solid interface. Here, the droplet triboelectric nanogenerator (droplet-TENG) was used as a probe to measure the charge transfer at different sliding speeds of a moving droplet. By varying the sliding speed and concentration of the moving droplet, the transferred charges generated at a liquid–solid interface are found to be highly dependant on the sliding velocity of the droplet on the hydrophobic surface: the faster sliding speed leads to more transferred charges. However, such effect is not obvious when the droplet has higher concentration, which provides direct evidence of a “two-step” model for the formation of the electric double-layer (EDL): where electron transfer occurs when the liquid droplet contacts a hydrophobic surface for the very first time, and then ion adsorption follows. The process of surface charge transfer is probed by droplet-TENG. This work is significant for better understanding of the charge transfer at a liquid–solid interface, and devises a method to maximize tribocharging in droplet-TENG, describing a concept potentially suitable for velocity sensing applications based on the self-powered droplet-TENG.