Nature of Static Charge at Interfaces

What is static charge? Static charge is conventionally considered a “point charge”, from which the theory of electrostatics is well established. In practice, however, static charge is very poorly understood. The identity of the species of static charge is not known. The mechanisms of generation and behaviors of static charge are also not well understood.

It is important to understand static charge. Static charge is the fundamental element of electrostatics and driving force for a vast range of applications, including manufacturing processes, energy, and sustainability. Static charge also causes many undesirable consequences in industry (e.g., explosions). Despite its importance, the severe lack of understanding of the static charge has greatly hindered the development of technologies.

This presentation discusses the nature of static charge in practice: its identity, the mechanisms of generation, and the behaviors at all interfaces, including solid, liquid, and gas. At the solid-solid interface, we performed molecular-scale analysis of contact electrification using well-defined surfaces functionalized with a self-assembled monolayer of alkylsilanes. Analyses show the elementary molecular steps of contact electrification: the exact location of heterolytic cleavage of covalent bonds (i.e., Si-C bond), exact charged species generated (i.e., alkyl carbocation), and transfer of molecular fragments. Static charge is thus an ionic molecular fragment (and an alkyl carbocation in this study). At the solid-liquid interface, we found that two opposite states can be switched flexibly: charge dissipation and charge generation. We found that static charge transfers (i.e., dissipates) readily to all types of liquids, charging nonpolar liquids unexpectedly very highly. By performing a simple flow transition, charge can be generated substantially instead of dissipating away. We thus developed a system to harvest energy of rain effectively for electricity generation. At the solid-gas interface, we found a dynamically reversible solid-gas charge transfer that gives rise to a fundamental relationship between shapes and charge states of flexible materials. Because charge generates and transfers actively at all interfaces of matter, these findings reveal the dynamic nature of static charge.