Periodic Reporting for period 4 - Tribocharge (Tribocharge: a multi-scale approach to an enduring problem in physics)
Período documentado: 2025-07-01 hasta 2025-12-31
Tribocharging, the transfer of electrical charge between materials during contact, is one of the oldest topics of scientific inquiry, and is important in daily life, new technologies, and fundamental science. In thunderclouds, colliding ice crystals exchange charge so vigorously that they induce electrical breakdown in the air around them—a contributing factor to lightning. Triboelectric generators utilize the effect to convert mechanical contacts into electrical energy, and are being incorporated into wearable tech. Even the origin of the Earth may be connected with tribocharging, as attractions between protoplanetary dust may allow it to aggregate quickly enough before gas-induced drag causes it to spiral into its star.
Despite this universal relevance, we know remarkably little about how tribocharging occurs. The one area we do understand is charge exchange between metals, which is related to the work function and photoelectric effect. With insulators, however, even the most basic questions remain unanswered: What are the charge carriers (ions vs. electrons)? How are they bound to the surface? And what drives them from one surface to another?
The objective of this project is to determine the mechanism of tribocharging for insulators. To do this, we are building experiments that address different aspects of the phenomenon ranging from the scale of the everyday effect to the scale of the atoms/molecules that must be involved. Our work is based on testing the hypothesis that ions in adsorbed water contribute to the effect.
Despite this universal relevance, we know remarkably little about how tribocharging occurs. The one area we do understand is charge exchange between metals, which is related to the work function and photoelectric effect. With insulators, however, even the most basic questions remain unanswered: What are the charge carriers (ions vs. electrons)? How are they bound to the surface? And what drives them from one surface to another?
The objective of this project is to determine the mechanism of tribocharging for insulators. To do this, we are building experiments that address different aspects of the phenomenon ranging from the scale of the everyday effect to the scale of the atoms/molecules that must be involved. Our work is based on testing the hypothesis that ions in adsorbed water contribute to the effect.
In this last reporting period, the primary objective has been to use our experimental platforms to uncover the mechanisms behind several diverse and surprising aspects of contact electrification. This work has already led to multiple results and publications.
In our first experiment, which studies the “everyday effect” with large samples of PDMS, we found that nominally identical materials can spontaneously self-order into a triboelectric series through repeated contacts. In other words, charge transfer becomes transitive only after sufficient contact history. This is important because it shows that “identical” samples can become systematically different due to the act of contact itself, pointing to mechanical history as a key control parameter rather than environmental water.
In our second project, we study the charge of levitated aerosol particles in real time with sub-electron resolution using optical tweezers. We demonstrated that the trapping laser itself can controllably charge a single silica microparticle in air, and we can simultaneously measure the resulting charge dynamics with high precision. We are now using this platform to understand what sets charging rates and fluctuations under controlled environmental conditions.
In our last experiment, we use acoustic levitation to study charge exchange in repeated collisions between identical glass samples without physically touching them. Our statistics show that the symmetry-breaking parameter is global and strongly dependent on sample and environmental history. In this system, we have found that adsorbed carbon, not adsorbed water, is the long sought symmetry breaker in the effect. Together, these results have enabled quantitative tests of competing mechanisms and provide a foundation for a mechanistic understanding of tribocharging.
In our first experiment, which studies the “everyday effect” with large samples of PDMS, we found that nominally identical materials can spontaneously self-order into a triboelectric series through repeated contacts. In other words, charge transfer becomes transitive only after sufficient contact history. This is important because it shows that “identical” samples can become systematically different due to the act of contact itself, pointing to mechanical history as a key control parameter rather than environmental water.
In our second project, we study the charge of levitated aerosol particles in real time with sub-electron resolution using optical tweezers. We demonstrated that the trapping laser itself can controllably charge a single silica microparticle in air, and we can simultaneously measure the resulting charge dynamics with high precision. We are now using this platform to understand what sets charging rates and fluctuations under controlled environmental conditions.
In our last experiment, we use acoustic levitation to study charge exchange in repeated collisions between identical glass samples without physically touching them. Our statistics show that the symmetry-breaking parameter is global and strongly dependent on sample and environmental history. In this system, we have found that adsorbed carbon, not adsorbed water, is the long sought symmetry breaker in the effect. Together, these results have enabled quantitative tests of competing mechanisms and provide a foundation for a mechanistic understanding of tribocharging.
All three of the described experiments go beyond the state of the art. For the first experiment, it had not been considered that identical insulating samples could spontaneously form a triboelectric series, nor that such ordering could emerge progressively through contact history. More generally, while many studies search for surface-property changes as signatures of contact electrification, few treat these changes as a mechanism that can systematically bias the outcomes of subsequent contacts. For the second, optical tweezers have mainly been used to measure the charge of particles in vacuum for precision force sensing and quantum experiments, whereas we demonstrated a platform in air where a single silica microparticle can be trapped, its charge measured in real time, and its charge controllably modified by the trapping laser itself. Finally, for the last experiment, we are to our knowledge the only group currently capable of performing acoustically levitated, automated collision-charging statistics on single insulating spheres, and this approach has revealed a global symmetry-breaking parameter that is connected to carbon adsorbates, not water adsorbates as many (including us) had presumed.