Free electron charging of microdroplets in a plasma at atmospheric pressure

Gas-phase microdroplets have recently demonstrated exceptional chemical properties via suspected mechanisms such as contact electrification and surface charge pinning, leading, in turn, to high electric fields at the surface. In this work, microdroplets are injected into a low temperature flowing gas plasma at atmospheric pressure, and during flight, they are subject to an excess free electron flux. We report the first measurements of particle charge acquired in a fully collisional plasma with average values up to 4.2 × 105 electrons, dependent on absorbed power, for droplets with an average diameter of < 20 μm. Simulations indicate that for similar plasma conditions, the acquired charge on solid particles would be ∼65% lower, at 1.5 × 105 electrons, with the difference likely due in part to low mobility water cluster ion formation around the evaporating droplet. Simulation also indicates surface electric fields up to 107 V m−1 for the smallest droplets (3 μm radius). A quasi-1D model of droplet transport through afterglow and effluent regions provided accurate estimates of charge loss in flight due to recombination with positive ions, mainly water clusters. The microdroplet system involves a complex interplay of various potential mechanisms, yet to be elucidated. The inclusion of a controlled plasma environment offers the capability for creating known charge levels and defined exposure times to allow more systematic study as well as enhancing their already exceptional characteristics.