Controlled Microdroplet Transport {\&} Charging in an Atmospheric Pressure Microplasma

We have measured charge and evaporation rates of a stream of micron-scale H$_{2}$O droplets transported through a low-temperature helium-neon rf plasma. N$_{\mathrm{e}}$ and T$_{\mathrm{e}}$, estimated from plasma impedance, were $\sim$ 10$^{13}$/cm$^{3}$ and $\sim$ 5eV respectively; gas temperature, from N$_{2}$ spectroscopy, was \textless 400K. With a log-normal aerosol droplet size distribution, 15 micron CMD and droplet velocity distribution within a parabolic envelope of $\sim$ 75{\%} of the local gas speed, the plasma induced evaporation caused an average diameter reduction of \textless 2 microns. This is equivalent to an average evaporation rate $\sim$ 2 orders of magnitude higher than reported for similar droplets in a comparable gas flow without plasma. Using charged droplet collection and current amplification, we have measured sub-millisecond charge pulses of up to 10$^{7}$e from a droplet stream with $\sim$ 2.5x10$^{3}$ droplets/s demonstrating the transport of droplets beyond the plasma and recombination region with negative charge retained. Time averaged measurements using an alternative technique show the mean charge per droplet is $\sim$ 10$^{5}$e. Results from an enhanced resolution charge measurement apparatus, currently being tested and using individual and size selectable droplets will be reported. \\[4pt] [1] P. Maguire et al: Appl. Phys. Lett. 106 (2015) 224101