Controlled Microdroplet Transport in an Atmospheric Pressure Microplasma

TL;DR

This study demonstrates the controlled injection and transport of micron-sized water droplets in an atmospheric pressure plasma, revealing size reduction mechanisms and potential applications in microreactors and plasma medicine.

Contribution

It introduces a method for controlled microdroplet injection into atmospheric plasma and analyzes droplet behavior and size reduction under plasma exposure.

Findings

Droplet size decreases from 15 to 13 micrometers during plasma exposure.

Droplets travel at >75% of local gas speed with laminar flow characteristics.

Maximum gas temperature remains below 400 K during experiments.

Abstract

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.