Free electron charging of microdroplets in a plasma at atmospheric pressure

Abstract

Gas-phase microdroplets have recently demonstrated exceptional chemical properties via suspected mechanisms such as contact electrification and surface charge pinning, producing high surface electric fields. Here, microdroplets are injected into a low-temperature atmospheric-pressure plasma and exposed to an excess free-electron flux. We report the first measurements of droplet charge in a fully collisional plasma, with averages up to 2.5 $\times 10^5$ electrons for 15 $\mu$m droplets, dependent on absorbed power. Simulations indicate solid particles under similar conditions acquire $\sim$40\% less charge, likely due to low-mobility water cluster ion formation around evaporating droplets, and predict surface electric fields up to $10^7$ V m${}^{-1}$ for the smallest droplets (3 $\mu$m). Plasma exposure also produces H${}_2$O${}_2$ in the liquid, with concentrations up to 33 mM, corresponding to remarkable generation rates exceeding 275 M s${}^{-1}$. The system involves complex, not yet fully elucidated mechanisms. A controlled plasma environment enables defined charge levels and exposure times, allowing systematic study and enhancing droplet properties.

Main manuscript 7 pages. Supplementary information 14 pages. Submitted to Nature Communications