Holographic Deflection Imaging Measurement of Electric Charge on Aerosol Particles
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We propose a simple noninvasive technique called Holographic Deflection Imaging (HDI) to measure electric charge on individual aerosol particles. Many flows including atmospheric turbulence and pneumatic transport involve charged aerosols, where the electric forces between particles affects particle dynamics in a complex way. We devised a novel technique to measure bipolar charge distributions in polydispersed particle samples. The technique uses common optical hardware and a novel particle sizing algorithm for polydispersed size distributions. A charged particle sample is introduced into a vertical channel air flow and a horizontal electric field. For each particle, In-line digital holography is used to simultaneously track particle positions over successive holograms and decode size information from hologram interference fringes. This enables measurement of the electric-force-induced horizontal component of terminal velocity and the particle size. From velocity and size, particle charge is calculated using a force balance, assuming Stokes drag on spherical particles. Two experiments were conducted, one to validate HDI measurements of a bipolarly charged particle sample against an electrometer, and the other to demonstrate particle size effect on triboelectric charging of aerosols in a fan-driven isotropic turbulence chamber. The HDI setup was configured for each experiment, and measurement uncertainty was estimated. Experiment 1 found good agreement (<5%) in mean charge between measurements by HDI and the electrometer. Experiment 2 found nearly symmetric bipolar charge distributions, which broadened with increasing particle surface area, as expected for triboelectric charging by the symmetrically installed fans.