Microlightning: Avalanche-Driven Blue Emission and Reactive-Species Formation in Charged Microdroplets

Charged microdroplets have recently been observed in experiments performed without a strong externally imposed field to emit flashes of blue light and initiate chemical reactions, a new phenomenon referred to as "microlightning." This observation adds to a growing body of evidence that charged microdroplets can host redox chemistry not accessible in bulk solution, yet the mechanistic origin and chemical consequences of this emission remain poorly understood. A fundamental chemical question regarding spontaneous microdroplet reactivity thus naturally arises: can electrostatic energy stored on a microdroplet be converted into chemical activation, including molecular excitation, ionization, and reactive-species formation? Here, we establish a framework that links droplet charging, avalanche dynamics, and the resulting reactive-species and photon production. Under the local-field approximation, Townsend theory establishes analytical scaling for finite avalanches in isolated heavily charged droplets; particle-in-cell Monte Carlo simulations quantify multiplication up to ∼1000 and resolve electron fates via ionization, excitation, and attachment, generating reactive oxygen and hydrogen species (e.g., OH•, O2•-, and H•) or their precursors relevant to microdroplet-accelerated organic reactions and atmospheric chemistry. Further, pairs of oppositely charged droplets create intensified fields that enable detectable N2 second-positive-system emission and promote significant reactive-species formation. Finally, we identify the necessary conditions for microlightning: micrometer-scale droplets carrying substantial fractions of the Rayleigh limit, seed electrons, and ionization-enhancing gas environments, providing a predictive basis for understanding microdroplet reactivity in electrospray and other reaction platforms. These findings characterize microlightning as a form of finite-size interfacial plasma-chemistry, bridging microscopic discharges with macroscopic lightning.