Free electron charging of microdroplets in a plasma at atmospheric pressure

TL;DR

This study investigates how microdroplets acquire charge in a plasma at atmospheric pressure, revealing high surface electric fields, charge levels, and chemical changes, advancing understanding of plasma-droplet interactions.

Contribution

It provides the first measurements of droplet charge in a fully collisional plasma and explores the mechanisms influencing charge and chemical modifications.

Findings

Droplets acquire up to 2.5×10^5 electrons depending on power.

Simulations suggest water cluster ion formation reduces charge by ~40%.

H2O2 concentrations up to 33 mM are produced in droplets.

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.