Tribocharge: a multi-scale approach to an enduring problem in physics
Tribocharging, i.e. charge transfer during contact, is a fundamental effect of widespread importance, yet we do not understand it. For insulators, even the most basic questions are open: What are the charge carriers? How do they bind to a surface? What drives them between surfaces? 
The aim of this proposal is to establish the mechanism of insulator tribocharging. Building on my work to develop new experiments and test existing models, we will use a multi-scale approach to find the origin of this effect. We will address the apparent homogeneous macrophysics, through the stochastic signatures at the mesoscale, and down to individual charges at the microscale. Throughout, we will perform crucial tests for the promising ‘water island’ hypothesis.
The project consists of 3 experimental work packages:
WP1: At the macroscale, we will use atomic layer deposition on polymers to create clean contacts with oxides. We will compare charge transfer with measurements of water surface coverage to test the water island model.  
WP2: At the mesoscale, we will use acoustic levitation to measure the charge of particles colliding with a substrate. With unprecedented precision, we will analyze statistics to tease out water’s role as a charge driver vs. discharge driver.
WP3: At the microscale, we will use optical tweezers to watch individual charges jump on and off a microparticle. With sub-electron resolution, we will determine the binding energy to identify the carrier.
Our experiments will be interwoven with a theoretical thread to unify the physics between scales.
Facing a problem that has eluded description for centuries is inherently risky. However, our multi-scale approach has not been attempted, and gives comprehensive solutions to the challenges that have existed. Beyond fundamental relevance, our work will be useful in new technologies, e.g. triboelectric nanogenerators. Understanding tribocharging, though ambitious, promises exceptional scientific and technological rewards.