  We report the controlled injection of near-isolated micron-sized liquid
droplets into a low temperature He-Ne steady-state rf plasma at atmospheric
pressure. The H2O droplet stream is constrained within a 2 mm diameter quartz
tube. Imaging at the tube exit indicates a log-normal droplet size distribution
with an initial count mean diameter of 15 micrometers falling to 13 micrometers
with plasma exposure. The radial velocity profile is approximately parabolic
indicating near laminar flow conditions with the majority of droplets
travelling at >75% of the local gas speed and having a plasma transit time of <
100 microseconds. The maximum gas temperature, determined from nitrogen
spectral lines, was below 400 K and the observed droplet size reduction implies
additional factors beyond standard evaporation, including charge and surface
chemistry effects. The successful demonstration of controlled microdroplet
streams opens up possibilities for gas-phase microreactors and remote delivery
of active species for plasma medicine.
