In situ granular charge measurement by free-fall videography

Abstract

We present the design and performance characterization of a new experimental technique for measuring individual particle charges in large ensembles of macroscopic grains. The measurement principle is qualitatively similar to that used in determining the elementary charge by Millikan in that it follows individual particle trajectories. However, by taking advantage of new technology we are able to work with macroscopic grains and achieve several orders of magnitude better resolution in charge to mass ratios. By observing freely falling grains accelerated in a horizontal electric field with a co-falling, high-speed video camera, we dramatically increase particle tracking time and measurement precision. Keeping the granular medium under vacuum, we eliminate air drag, leaving the electrostatic force as the primary source of particle accelerations in the co-moving frame. Because the technique is based on direct imaging, we can distinguish between different particle types during the experiment, opening up the possibility of studying charge transfer processes between different particle species. For the ∼300 μm diameter grains reported here, we achieve an average acceleration resolution of ∼0.008 m/s(2), a force resolution of ∼500 pN, and a median charge resolution ∼6× 10(4) elementary charges per grain (corresponding to surface charge densities ∼1 elementary charges per μm(2)). The primary source of error is indeterminacy in the grain mass, but with higher resolution cameras and better optics this can be further improved. The high degree of resolution and the ability to visually identify particles of different species or sizes with direct imaging make this a powerful new tool to characterize charging processes in granular media.

Small-Scale Particle Interactions Are Having Significant Effects on Global Fluidized Bed Behavior

“…Thus, the particle tracking during freefall was essentially a diagnostic of the triboelectric or contact charging that had occurred during earlier particle-particle contacts, most likely inside the hopper during the outflow process. Waitukaitis et al (2013;2014) were able to show that such contact charging among particles made from the same material involves the transfer of negative charge and that the charging magnitude can be linked directly to the dispersion in particle size, as proposed earlier by Lacks et al (2007;2008). Particles smaller than the mean size tend to become charged negatively, while larger particles tend to acquire positive charge.…”

“…Electrostatic charging can, of course, become important with other types of insulating particle material. Waitukaitis et al (2013) performed experiments with freely falling streams of 200-300 micron zirconium dioxide silicate particles, in which they incorporated a Faraday cup at the bottom and had parallel electrodes along the particle free fall. By applying a horizontal electric field, the charge on individual particles could be extracted.…”

“…How- ever, the work of Royer et al (2009) and Waitukaitis et al (2013) clearly shows that material type and surface morphology have a significant role in the level of particle clustering. Though, the BFE might be able to trend with a cluster level, that trend is most likely to be different for each material with respect to size, surface chemistry, morphology, etc.…”

Effect of humidity on contact electrification due to collision between spherical particles

“…However, for most existing CE experimental results, the amount of net charge carried on a particle was determined by measuring the charge-to-mass ratio and size distribution of bulk samples of grains in a grain flow. 25 It is difficult to distinguish the role of moisture on the CE. In this paper, we carried out an experiment to observe the CE due to a single collision between two spherical particles in identical materials under different RH, which can identify the effect of RH on CE, and a model is proposed to reveal how and why the contact electrification depends on RH.…”

Size-Dependent Same-Material Tribocharging in Insulating Grains

“…For a full discussion, see Ref. [20].] For the granular material, we use fused zirconium dioxide -silicate (ZrO 2 :SiO 2 , Glenn Mills Inc.) because it exhibits strong charging behavior and because it is known to the thermoluminescence community for its capacity to store trapped electrons [22][23][24][25].…”