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Le Lapin Électrostatique : Static Electricity is an Unpredictable Little Bunny
"Static electricity" defies our best attempts to make sense of it. More scientifically referred to as contact electrification (CE), the effect seems straightforward—touch two neutral materials together, separate them, and they will have exchanged some electrical charge. Simple as this sounds, the effect is plagued by unpredictability, and we don’t even know what is transferred (e.g., ions vs. electrons), let alone why. In my group, we focus on CE between "identical" materials, which, though counterintuitive, has long been known to occur. I will discuss two sets of experiments on this same-material CE, which are helping us tame the unpredictability and get closer to the mechanism. In the first, we use acoustic levitation to study the charging of an SiO2 sphere as it bounces on an SiO2 plate. We test a leading hypothesis for same-material CE, i.e., that it is due to local variability in surface properties, and show that this is not the case. Going further, we reveal that there is a critical dependence on sample history, especially RH exposure, temperature, and surface treatment, which points to a key role played by surface adsorbates. In the second experiment, we study the charge exchange between soft polymers—identically prepared samples of PDMS. Measuring the charge exchange for all pair combinations of an ensemble, we find they begin charging randomly, but over time evolve into a triboelectric series—i.e., a transitive ordering based on the polarity of the charge acquired. We find that this is caused by the act of contact itself ; samples that have experienced more contacts in their history charge negatively to ones that have experienced less. Based on this observation, we develop a model that explains why our materials evolve into a series, and going further leverage it to control the charging behavior. Ultimately, we discover that this memory effect is due to nanoscale changes in surface morphology, pointing to a mechanism intimately coupled with tribology and contact mechanics.