Uncovering the role of adsorbed water in the triboelectric effect with acoustic trapping*

Collisions between identical grains can cause large buildups of electric charge. However, the underlying causes of this charge separation process are still an open question. Acoustic trapping allows isolating a single grain, performing individual collisions on a substrate, measuring charge with high precision, and discharging grains using ionizing radiation to reset the system. Using this setup, we examine the fundamental processes behind contact electrification. In particular, we study the role played by adsorbed water on the contacting surfaces. We find that the direction of charge exchange correlates with how much water is on the surface, which itself is a function of the history of the sample. We investigate this by performing humidity ramps and selectively baking surfaces.