Abstract
The paper discusses the applicability and advantages of using electrically charged soap bubbles as a route to produce fine sprays composed of highly charged particles, commonly named as electroaerosols. It is indicated that such low in energy demand process may produce very fine droplets or particulates charged to a level higher than that obtained using classical spray charging techniques, involving no bubbles. A process of a soap bubble electrical charging is thus initially studied on a simple analytical basis pointing out to a possibility of producing air bubbles with charge-to-mass (Q/m) ratio up to 60 mC/kg (constrained by the Rayleigh limit) while just 2 mC/kg is commonly considered as a threshold value for an effective particle charging process. Finite element 3D electrostatic simulation method (3D FEM) is then applied to assess a single bubble charging level achievable in a laboratory setup using a DC high-voltage biased bubble machine producing soap bubbles 23 mm in diameter on average. The 3D FEM simulation results postulate a single bubble maximum charging level approaching 25 nC at 41.5 kV charging voltage (constrained by the Rayleigh limit), corresponding to (Q/m) = 17.6 mC/kg. Finally, a stream of bubbles characterized by (Q/m) = 10.4 mC/kg was produced experimentally in the laboratory setup using a contact charging method at 40 kV DC supply. A discrepancy between 3D FEM-simulated results and experimental data was discussed on a shielding effect basis.Graphic abstract
Highlights
Electroaerosols specified as sprays composed of charged particles have been recently extensively studied as well as broadly utilized in many industrial processes including crop spraying, pollination, spray painting, electrospray air cleaning, and pharmaceutical inhalation, just to name a few
Visual inspection of the picture showing a sequence of soap bubbles emerging from the bubble machine nozzle ascertains that the bubble making process employed in the discussed experiments does not differ from that described in the literature (Salkin et al 2016, Davidson et al 2017)
Electrostatic finite element method (FEM) simulation is sufficient to correctly predict the charge magnitude in case of a droplet induction or conductive charging performed in a real experimental arrangement (Pelesz et al 2020) as well as it provides the numerical link between the bubble charge and the magnitude of the voltage correlated to the electric field
Summary
Electroaerosols specified as sprays composed of charged particles have been recently extensively studied as well as broadly utilized in many industrial processes including crop spraying, pollination, spray painting, electrospray air cleaning, and pharmaceutical inhalation, just to name a few. The main advantage of the charged aerosol formation involving the bubble step is related to much higher possible (Q/m) ratios than achieved in case of a direct droplet formation and charging. It is expected that highly charged bubbles should burst spontaneously (due to field-induced bubble instabilities and Rayleigh limit surpassing leading to local partial discharges emanating from the bubble film). When ruptured they ought to produce finer secondary droplets (than in case of uncharged bubbles) due to strong and repulsive Coulomb force interactions in the fragmenting liquid film and between the resulting homo-charged liquid drops. There is a need to experimentally study the basic bubble charging process as well as a behavior of already-charged bubbles
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