Airborne granular material in the size range from about 1 micrometer up to 1 millimeter can acquire comparatively large electrostatic charges because of multiple contacts between particles and particle collisions with different surfaces. Highly charged particulate matter of this size can be attracted to surfaces. Dust control in clean environments can become difficult because of these charged particles. On the other hand, charged granular material has many beneficial applications. Devices such as electrostatic copiers, inkjet printers, powder-coating machines, injection moldings, and electrostatic precipitators depend on controlled charging of these small particles. The Electromagnetic Physics Laboratory at KSC is engaged in studies leading to a better understanding of the electrostatics of granular material.

Surfaces can become charged as a result of contact and separation between materials of different Fermi electronic energy levels. In a vacuum, these excess charges can remain on surfaces indefinitely. In a gaseous atmosphere, excess charge can leak away because of the presence of free charges in the gas, such as ions, electrons, or other heavily charged particles. At low atmospheric pressures, the number of free space charges is reduced, affecting the discharging characteristics of surfaces in specific ways.

Figure 1. Computer Simulation of Particle Flow Around the Cylindrical Multisensor Electrometer

To study charge exchange phenomenon in granular material, a KSC-designed low-pressure dust impeller is used together with an aerodynamic multisensor electrometer. Small particles ranging in size from about 5 to 17 micrometers are launched toward several cylindrical polymers in a dry carbon dioxide atmosphere at 9 millibars. Electrostatic sensors embedded in the aerodynamic multisensor measure the electrostatic charge generated on these polymers in real time. Figure 1 shows a simulation of particle flow around the aerodynamic multisensor. In an initial attempt at characterizing this interaction, a triboelectric series was generated. In this series, the materials are ordered according to the relative positions of their electronic energy levels when brought into contact.

Figures 2, 3, and 4 show the electrometer responses to the charge exchanged between fiberglass-epoxy G-10, Lucite, and Teflon when in contact with silicon dioxide (SiO2), aluminum oxide (Al2O3), and iron oxide (Fe2O3) particles. The table lists the ordering of the minerals and the polymers in a triboelectric series based on the data shown in figures 2, 3, and 4. The average electrostatic responses of the polymers to the granular minerals may allow us to observe consistent differences in behavior, which could lead to a possible identification of such minerals.

Contact: Dr. C.I. Calle (Carlos.Calle-1@ksc.nasa.gov), YA-C2-T, (321) 867-3274
Participating Organizations: Florida Institute of Technology (Dr. J. Mantovani), YA-C2-T (E.E. Groop and M.D. Hogue), Swales Aerospace (Dr. C. Buhler), and Dynacs Inc. (A.W. Nowicki)