This project is focused on studying triboelectric charging in insulating granular systems of chemically identical materials. The proposed research will carry out experiments closely aligned with theory to resolve the nature of the charging that occurs in these systems and relate the charging to such effects as particle size distribution.
Intellectual merit. The electrostatic charging that occurs when two surfaces rub ("triboelectric charging") is one of the most well known phenomena in physics. It is thus very surprising that there is no clear scientific explanation for this phenomenon. The triboelectric charging of granular systems of chemically identical particles is particularly perplexing. While in mixtures with different types of particles, net charge transfer from one type of particle to the other can be attributed to a difference in the material properties, there is no such difference in material properties when all particles have the same composition, and yet these systems are known to undergo substantial charging when the particles are insulators. Furthermore, when all particles are composed of the same material, experiments show that smaller particles charge negatively and larger particles charge positively. Why there is a particle size dependence of the polarity of particle charge, and why charging occurs at all when all particles are composed of the same material is poorly understood.
The proposed research will elucidate the charging behavior and mechanism in these systems through two thrusts: 1) we will carry out experiments to quantitatively characterize the effect of particle size on the triboelectric charging of chemically identical granular materials, 2) we will carry out experiments specifically designed to test the hypothesis for the mechanism of triboelectric charging including non equilibrium dynamics in which collision-induced electron transfer generates electron accumulation. Our experimental methodology is uniquely capable of studying triboelectric charging in a controlled environment and solely from particle particle interactions, removing the effect of humidity and other contaminants (i.e. container or reactor wall). The results of the proposed work will aid in a fundamental understanding of the driving force and mechanism for charge transfer in chemically identical granular systems.
Broader impacts. A particular emphasis of this project is a strong effort to mentor undergraduate student researchers, including women and members of underrepresented minority groups. The PIs have excellent records in this regard, with a large number of undergraduate students in their groups publishing papers in refereed journals and proceeding to Ph.D. programs in chemical engineering.
The project can also benefit society by enhancing the understanding of triboelectric charging. Triboelectric charging occurs in a wide range of contexts, and can have implications on a range of issues including safety (e.g., dust explosions can by cause by triboelectrical charging).
The charging of materials when their surfaces are contacted, known as triboelectric charging, has been recognized for over 2000 years. Today, triboelectric charging has important consequences in technologies such as polymer powder processing and xerography, and in naturally occurring systems such as dust storms and volcanic eruptions. Triboelectric charging is well known to everyone, for example the classic demonstration of rubbing a balloon on hair which leaves the hair standing up due to static charge. Surprisingly, a basic understanding of key questions, including the driving force for charge transfer, what charge carriers are involved, and what material properties control charge, remain completely unknown. In this project, we studied the role of material strain on triboelectric charging, and show that it can govern the triboelectric charging process. To systematically relate material strain to triboelectric charging, macroscopic sheets of latex rubber were controllably strained (beyond any native strain) and then contacted with polytetrafluoroethylene (PTFE) surfaces. We found that material strain has a dramatic effect on the charge that results, and can even reverse the direction of charge transfer between the materials (see Fig. 1). The intellectual merit of these findings is that: 1) the same material can charge differently depending on the degree of strain, such that universal guidelines for triboelectric charging such as the triboelectric series may not be valid; we note that "undetectable" strains may be present on the micro- or nano-scale, due to the material’s thermal and mechanical history, which may influence the charging behavior. 2) These micro- or nano-scale strains are inhomogeneously distributed over the surface, which may give rise to charge inhomogeneities observed by other investigators. 3) The triboelectric charging that occurs when identical materials are contacted can be partly explained by the differences in the localized strain across their surfaces. In addition to the scientific contributions, broader impacts of the project include the implementation of international programs with the University of Botswana. We have established a NSF International Research Experience for Student (IRES) and an engineering study abroad course at the University of Botswana; these programs have brought over 80 US students to Botswana over the past 3 years. We have also hosted in our lab a sabbatical visitor, Dr. Rufus Akande, from the Department of Physics at the University of Botswana.
