Statistical Analysis of Droplet Charge Acquired during Contact with Electrodes in Strong Electric Fields

Elton, Eric S.; Tibrewala, Yash V.; Ristenpart, William D.

doi:10.1021/acs.langmuir.8b04254

Cited by 10 publications

(14 citation statements)

References 61 publications

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“…Indeed, we observe undercharging during the majority of our experiments, especially at high electric field intensities. Similar undercharging events involving metallic particles and water droplets have recently been attributed to localized melting of the electrode surface at high current density and electrohydrodynamic instabilities that impede charge transfer [58,59]. Other effects such as nonuniform charge accumulation might also affect the particle charge and the electrostatic force between the particle and the electrode [22].…”

Section: Theoretical Backgroundmentioning

confidence: 91%

Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

et al. 2020

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Processes based on electrostatic projection are used extensively in industry, e.g., for mineral separations, electrophotography, or manufacturing of coated abrasives, such as sandpaper. Despite decades of engineering practice, there are still unanswered questions. In this paper, we present a comprehensive experimental study of the projection process of more than 1500 individual spherical alumina particles with a nominal size of 500 μm, captured by high-speed video imaging and digital image analysis. Based on flight trajectories of approximately 1100 projected particles, we determine the acquired charge and dynamics as a function of the relative humidity (RH) and the electric field intensity and compare the results with classical theories. For RH levels of 50% and above, more than 85% of distributed particles are projected, even when the electric field intensity is at its minimum level. This suggests that, beyond a critical value of the electric field intensity, the RH plays a more critical role in the projection process. We also observe that the charging time is reduced dramatically for RH levels of 50% and above, possibly due to the buildup of thin water films around the distributed particles, which can facilitate charge transfer. In contrast, projected particles at the 30% RH level exhibit excessive amounts of electric charge, between 2 and 4 times than that of the saturation value, which might be attributed to triboelectric charging effects. Finally, the physics of electrostatic projection is compared and contrasted with those of induced-charge electrokinetic phenomena, which share similar field-square scaling, as the applied electric field acts on its own induced charge to cause particle motion.

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Section: Theoretical Backgroundmentioning

confidence: 91%

Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

et al. 2020

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“…In Figures 7 and S4, even if all the data is plotted, the velocity data is displayed along a single curve, confirming that there is no significantly lower charging event (see also Figures S5−S9, which show the velocity vs time graph of the data shown in Figure 5a.). Moreover, we were not able to observe the formation of various types of water bridges 18 but only a single Taylor cone bridge, as shown in Figure 6. (All photos near the electrode were examined to select the best picture in contact with the electrode for numerical simulation.…”

Section: ■ Results and Discussionmentioning

confidence: 83%

“…One possible explanation for these different results from two studies covering the same droplet CCEP system is to use different types of electrodes. In the recent report, gold coated glass was used as the electrode, 18 but copper electrodes were used in this study. The imperfection of the gold coating or static electricity on the glass might affect the droplet charging, which can make significantly lower charging event or induce various shapes of bridge formation.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…3,34 Later, the contact charge was analyzed by normalizing it to the theoretical maximum charge derived by the perfect conductor theory called Maxwell charge. 1,18,35,36 For accurate charge measurement, a rigorous study has been conducted on the drag of a moving droplet 37 and high-resolution electrometer was used to directly measure the droplet charge. 35,38 In many reports, the charge acquired by aqueous droplets was less than the Maxwell charge.…”

Section: ■ Introductionmentioning

confidence: 99%

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Effect of Deformation on Droplet Contact Charge Electrophoresis

Yang

2020

Langmuir

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The effect of deformation on the droplet contact charge electrophoresis (CCEP) was investigated for consistent droplet movement control. Through systematic experiments and numerical simulations, it has been found that overcharging by deformation is up to about 130% of the sphere and is mainly driven by the concentration of the electric field near the tip of the droplet rather than an increase in the surface area. Dimensional analysis revealed a consistent droplet CCEP motion with the electric capillary number range of 0.01–0.09. We also found that the dimensionless droplet charge follows a universal curve proportional to the electric capillary number, regardless of the droplet size, and the weak dependence on the droplet size shown in the experimental results is due to hydrodynamic effects, not electrostatic ones. Changes in droplet velocity distribution with droplet size and the electric capillary number were also investigated. Using the perfect conductor theory and Stokes law, we derived an analytical relationship between the droplet center velocity and the electric capillary number and analyzed the experimental results based on this relationship. This study implies that if proper hydrodynamic correction is applied, the droplet CCEP and its deformation effect can be explained by a perfect conductor theory.

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“…Indeed we observed under-charging during the majority of our experiments, especially at high electric field intensities. Similar under-charging events involving metallic particles and water droplets have been recently attributed to localized melting of electrode surface at high current density and electrohydrodynamic instabilities which impede charge transfer [58,59]. Other effects such as nonuniform charge accumulation might also affect the particle charge and the electrostatic force between the particle and the electrode [57].…”

Section: Theoretical Backgroundmentioning

confidence: 79%

Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

Sayyah,

Mirzadeh,

Jiang

et al. 2019

Preprint

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Processes based on electrostatic projection are used extensively in industry, e.g. for mineral separations, electrophotography or manufacturing of coated abrasives, such as sandpaper. Despite decades of engineering practice, there are still unanswered questions. In this paper, we present a comprehensive experimental study of projection process of more than 1500 individual spherical alumina particles with a nominal size of 500 µm, captured by high-speed video imaging and digital image analysis. Based on flight trajectories of approximately 1100 projected particles, we determined the acquired charge and dynamics as a function of relative humidity (RH) and electric field intensity and compared the results with classical theories. For RH levels of 50% and above, more than 85% of disposed particles were projected, even when the electric field intensity was at its minimum level. This suggests that, beyond a critical value of electric field intensity, relative humidity plays a more critical role in the projection process. We also observed that the charging time is reduced dramatically for RH levels of 50% and above, possibly due to the build-up of thin water films around the particles which can facilitate charge transfer. In contrast, projected particles at 30% RH level exhibited an excessive amount of electric charge, between two to four times than that of saturation value, which might be attributed to triboelectric charging effects. Finally, the physics of electrostatic projection is compared and contrasted with those of induced-charge electrokinetic phenomena, which share similar field-square scaling, as the applied field acts on its own induced charge to cause particle motion.

show abstract

Statistical Analysis of Droplet Charge Acquired during Contact with Electrodes in Strong Electric Fields

Cited by 10 publications

References 61 publications

Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

Effect of Deformation on Droplet Contact Charge Electrophoresis

Physics of Electrostatic Projection Revealed by High-Speed Video Imaging

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