Millimeter Wave Scattering from Neutral and Charged Water Droplets

TL;DR

This study examines how millimeter wave scattering differs between neutral and charged water droplets, developing a model that explains increased scattering from charged droplets, with implications for remote sensing of radioactive gases.

Contribution

The paper introduces a classical electrodynamics model for scattering from charged dielectric spheres, aligning with experimental data on millimeter wave scattering from charged water mist.

Findings

Charged mist shows increased MMW scattering compared to uncharged mist.

The model predicts increased scattering for small charged droplets, consistent with experiments.

Results suggest potential for remote sensing of radioactive gases through mist charging effects.

Abstract

We investigated 94GHz millimeter wave (MMW) scattering from neutral and charged water mist produced in the laboratory with an ultrasonic atomizer. Diffusion charging of the mist was accomplished with a negative ion generator (NIG). We observed increased forward and backscattering of MMW from charged mist, as compared to MMW scattering from an uncharged mist. In order to interpret the experimental results, we developed a model based on classical electrodynamics theory of scattering from a dielectric sphere with diffusion-deposited mobile surface charge. In this approach, scattering and extinction cross-sections are calculated for a charged Rayleigh particle with effective dielectric constant consisting of the volume dielectric function of the neutral sphere and surface dielectric function due to the oscillation of the surface charge in the presence of applied electric field. For small droplets with (radius smaller than 100nm), this model predicts increased MMW scattering from charged mist, which is qualitatively consistent with the experimental observations. The objective of this work is to develop indirect remote sensing of radioactive gases via their charging action on atmospheric humid air.