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.

The image above shows a sketch of the apparatus and a sampling of trajectories of individual particles as they are deflected into the x-direction by an applied horizontal electric field.

For the ∼300 μm diameter grains used in the experiments described 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 x 10^4 elementary charges per grain (corresponding to surface charge densities ∼1 elementary charge per μm^2).  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.

1. •Scott Waitukaitis and Heinrich M. Jaeger, “In situ granular charge measurement by free-fall      videography”, Review of Scientific Instruments 84, 025104 (2013). pdf file