Periodic Reporting for period 3 - Tribocharge (Tribocharge: a multi-scale approach to an enduring problem in physics)
Okres sprawozdawczy: 2024-01-01 do 2025-06-30
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 second reporting period, the primary objective has been to use the functional experiments to learn about many diverse and surprising aspects of contact electrification. This has already led to several results and publications.
In our first experiment, which involves studying the 'everyday effect’ with large samples of PDMS, we have found that these spontaneously self-order into a triboelectric series as a result of contacts. This means that the sign of charge transfer for several samples is transitive, but only after a sufficient number of contacts has been performed. This self-charging of identical materials has been observed before, but it was never known that they ordered into a series. We are excited about this because if we can identify what is different about our ‘identical’ samples, we may be able to pin-point the parameter that drives charge exchange. Importantly, we found that water does not play a significant role in this system, but rather mechanical history does.
In our second project, we study the charge on levitated aerosol particles in real time and with sub-electron resolution. We do this by holding the particles in the focus of an intense laser beam and applying an external electric field. The charge of these spontaneously changes on a timescale of seconds due to the desorption and adsorption of ions from the atmosphere. Currently, we are trying to understand what sets the statistics and rate of this process, using environmental conditions (humidity, air ion concentration) as the control knob.
In the last experiment, we use acoustic levitation to study the charge exchange between identical glass samples. With this setup, we can perform thousands of collisions and charge experiments with a single glass sphere and plate without ever physically touching either. Our charge transfer statistics show that the driving parameter of the exchange is not due to local differences in material properties, but rather global differences. In this system, we do conclude that water plays an important role; changes in humidity alone are enough to completely switch the sign of charge transfer. This leads us to suspect that it’s the surface hydroxylation state that controls charging. These results have led to three publications and more on the way.
In our first experiment, which involves studying the 'everyday effect’ with large samples of PDMS, we have found that these spontaneously self-order into a triboelectric series as a result of contacts. This means that the sign of charge transfer for several samples is transitive, but only after a sufficient number of contacts has been performed. This self-charging of identical materials has been observed before, but it was never known that they ordered into a series. We are excited about this because if we can identify what is different about our ‘identical’ samples, we may be able to pin-point the parameter that drives charge exchange. Importantly, we found that water does not play a significant role in this system, but rather mechanical history does.
In our second project, we study the charge on levitated aerosol particles in real time and with sub-electron resolution. We do this by holding the particles in the focus of an intense laser beam and applying an external electric field. The charge of these spontaneously changes on a timescale of seconds due to the desorption and adsorption of ions from the atmosphere. Currently, we are trying to understand what sets the statistics and rate of this process, using environmental conditions (humidity, air ion concentration) as the control knob.
In the last experiment, we use acoustic levitation to study the charge exchange between identical glass samples. With this setup, we can perform thousands of collisions and charge experiments with a single glass sphere and plate without ever physically touching either. Our charge transfer statistics show that the driving parameter of the exchange is not due to local differences in material properties, but rather global differences. In this system, we do conclude that water plays an important role; changes in humidity alone are enough to completely switch the sign of charge transfer. This leads us to suspect that it’s the surface hydroxylation state that controls charging. These results have led to three publications and more on the way.
All three of the described experiments go beyond the state of the art. For the first experiment, it has never be considered that identical materials can form into a triboelectric series, and certainly not that they would evolve towards a series as a result of contact. Additionally, while many experiments have looked for changes in surface properties as signatures of contact electrification, we are not aware of any that further consider these as capable of altering the outcome of subsequent contacts. For the second, there are several groups that use optical tweezers to measure charge of aerosol particles in vacuum for macro-quantum investigations, but our focus to use this in air in relation to tribocharging and the role of humidity is new. If we can validate our hypothesis that humidity changes alone can charge aerosol particles, this would be a big step toward understanding atmospheric electricity. Finally, for the last experiment, we are to our knowledge currently the only group in the world capable of doing acoustically-levitated collision charging statistics of single spheres. That this has revealed a global (vs. local) charging mechanism that is completely contrary to what most in the field have expected.