Energy harvesting using air bubbles on hydrophobic surfaces containing embedded charges

Technology to harvest electrical power from waste micro-mechanical energy is increasingly in demand. A promising approach lies in manipulating the electrical double layer on hydrophobic surfaces; however, the underlying mechanism is still unclear. Here, we demonstrate that ascending air bubbles in water can produce electrical power in a mode similar to other systems that use descending water droplets. Although the two systems, which are analogous to electrons and holes in semiconductors, are similar in fundamental principle, their detailed electrification mechanisms are significantly different. In the air bubble system, only the pre-existing charges on the surface are involved. However, electrification in the water droplet system is dominated by triboelectric charges accumulated on the surface over time. An air bubble can produce a maximum of nine times more energy than a water droplet due to its advantages in terms of its geometry, hydrodynamics, and electro-circuitry. We also suggest an innovative approach to improve energy-harvesting efficiency using artificially embedded charges.