Charge transfer of pre-charged dielectric grains impacting electrodes in strong electric fields

https://doi.org/10.1016/j.elstat.2022.103705Get rights and content

Highlights

  • Charge transfer in collisions of dielectric particles depends on the contact time.

  • In short-contact collisions charges of both polarity are transferred randomly.

  • If particles temporarily stick to a surface or to another particle, charges are transferred along an external electric field.

  • The different characteristics can be explained by charge carriers in a thin adsorbate layer of water.

Abstract

The charging of dielectric grains colliding with metal is poorly understood. We study collisionally pre-charged submm-sized glass beads interacting with electrodes in a strong electric field. The charge transfer for rebounding collisions (millisecond) or temporarily sticking particles (second) differs. For short contacts, charges of both polarities are randomly exchanged. However, if grains stick to an electrode before being ejected again, they tend to recharge in line with the electric field. The different timescales and polarity dependencies can be modeled with low mobility charge carriers in a thin adsorbate layer, i.e. water ions. The grains lose charge carriers in both cases.

Introduction

If two solid surfaces get in contact, charge is regularly transferred. There are some systematic trends in the polarity of the transferred charge related to material or grain size differences [[1], [2], [3], [4]]. However, there are many unknowns left and there is no unified model for charge transfer - if such a model exists at all [5]. Especially for dielectric grains, it is still debated, what the underlying charge carriers are. If the material is not soft enough to transfer material, the two most likely options are electrons on one side and ions of volatiles sitting on the surfaces of the grains on the other side. There is evidence for or against both. E.g., on one side, works by Refs. [6,7] clearly point to water ions on glass particles as charge carriers while [8,9] showed that water can enhance contact electrification but its presence is not mandatory. Also, [10] found that the loss of net charge during collisions can only be explained if charge carriers of both signs are present, which strongly hints to ions. On the other side, charge transfer is still efficient on KCl and ZnS at 500 K temperature [11]. Water molecules on the surfaces might hardly be present then, though a monolayer might still exist [12]. Additionally, size dependent charging can rather naturally be explained by electron transfer of trapped electrons [13,14] even though thermoluminescence measurements show that the density of trapped electrons might not always be high enough [2].

Therefore, if it cannot be decided which charges are transferred, maybe different charging mechanisms are at work at the same time and it might only be a matter of the specific setting whether one dominates over the other.

In the context of the work presented here, also strong external electric fields are present at a contact. So it is important to note that [6] find that charge transfer adapts to external electric fields. In their experiments the polarity of transferred charge in many collisions of a single particle systematically follows the electric field. I.e. particles impacting a negative electrode charge negative. Charge transfer might also be influenced by local fields, e.g. generated by dipoles [15,16] but the experiments reported here will not answer to this. Finally, we note that [17] discussed the importance on collisional timescales for charging.

What we report here are charge transfers in collisions of insulating grains with a metal surface. This has been studied before. For metal and rubber particles charge transfer has e.g. been shown to depend on the contact area [18,19]. We make use of this idea below. Otherwise, in contrast to earlier work, we focus on collisions which differ in the duration of the contact times. These are either in the millisecond range in a rebounding collision or on the second scale with a phase in between where grains stick to the electrode. Otherwise all experimental parameters are identical for all collisions studied.

Section snippets

Microgravity experiments

The data presented here are from microgravity experiments carried out at the drop tower in Bremen. The basic setup can be found in Fig. 1.

The setup is placed in a vacuum chamber, which is evacuated and flushed with CO2 in order to keep the atmosphere constant and dry. The experiments are carried out in a CO2 atmosphere at a pressure of 1000 mbar. A sample of identical glass beads of 434 ± 17 μm in size is vibrated for 15 min in a sample confinement with the same glass beads glued to its inside.

Fast rebounding collisions

In rebounding collisions, particles are in contact with the electrodes for only a short time on the order of 10−4 s [22]. The data of rebounding collisions can be seen in detail in Fig. 3. For all collisions analyzed it shows the charge transfer between grains and electrodes, plotted versus the initial grain charge before the collision.

With 6.25–83.3 kV/m, the electric field applied is strong and grains with net pre-charges of both signs collide with the respective electrode they are drawn to.

Grain-grain collisions

In addition to collisions with the electrodes, we also observed collisions and charge transfer among grains. This gives further data to set the amount of charge transferred at the electrodes in context as outlined below. Fig. 8 shows the charge transfer for each single grain-grain collision as it depends on the initial charge before the collision. As already before, the data for the charge transfer per contact area is shown in Fig. 9, with the inset showing how the contact area scales with the

Transfer model

In the following, visualized by Fig. 10 for one electrode, we give a model that would be in agreement to the two charge transfer mechanisms. That does not imply that other models might not work, but this one has the least number of assumptions and no extreme assumptions. Our model solely builds on water ions as charge carriers. We discuss alternatives afterwards.

Fig. 10 a) shows the model situation before a collision. The electrode (copper) as well as the particles (glass) are covered by a

Conclusion

Drop tower experiments allowed us to study the charge transfer in collisions of precharged grains with capacitor electrodes on different timescales, during fast rebounds and during temporary sticking. In the first case large amounts of charges are transferred with arbitrary polarity on each electrode. In the second case charge transfer follows the field lines, increases with time, but, in the end, also reaches the same absolute amount of charge transfer as in the first case.

A model that can

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project is supported by DLR Space Administration with funds provided by the Federal Ministry for Economic Affairs and Climate Action (BMWKK) under grant numbers DLR 50 WM 1762 and DLR 50 WM 2142. We also thank the anonymous referee for a fast and constructive review.

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