Hydrophobicity’s Role in Electrostatic Charge Decay of Levitated Solids Open Access

McGrath, John (Spring 2021)

Permanent URL: https://etd.library.emory.edu/concern/etds/cz30pt931?locale=en


Electrically charged particles suspended in gas exist in numerous planetary environments. Salient examples include volcanic plumes, airborne Saharan sand, Martian dust devils, and the dune fields of Titan. Yet the microphysical processes that lead to electrification are not well understood. Moreover, the lifetime of charge on electrified particles is not well quantified and the events preceding electrostatic neutrality are poorly understood. Our laboratory measurements aim to answer these questions through acoustic levitation to ensure that charge is not lost through physical contact or through any other mechanism of charge transfer other than the natural processes of charge loss. Two hemispherical transducers produce an acoustic standing wave that lofts particles of millimetric scale. An ionizer then charges these particles, and the remaining charge is measured over days or weeks by moving the particle through a Faraday cup.

Previous measurements showed that the lifetime of charge is dependent on the relative humidity of its environment. These measurements, however, were limited to low-density, porous particles. Now we have constructed an acoustic levitation device capable of suspending dense materials such as copper. Additionally, in environments of varying humidity, we investigate the mechanisms leading to airborne particle charge loss, and its relation to the material’s hydrophobicity. We find that water dissociation is the dominant mechanism of charge transfer in these systems. 

Table of Contents

Introduction Motivations from Nature Lifetime of Charge Acoustic Levitation as a Novel Scientific Tool Acoustic Levitation Background Sound and the Acoustic Radiation Force The High-Powered Acoustic Levitation Device Charge Measurements Preliminary Charge Decay Results Humidity and Discharge Behavior Positive and Negative Charge Decay Water Film Formations QCM Background and Theory Hydrophobicity and Hydrophilicity The Sauerbrey Equation QCM Experimentation Experimental Design Preparation Prior to Experimentation QCM Results and Discussion Water Film Thickness Measurements Interpretation of QCM Results Conclusions and Future Directions

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