Charged powders are very common in nature and industry and they pose many challenges in applications. In some cases, the charges are separated by contact electrification and the system as a whole is neutral. In other cases, the grains all carry the same sign of net charge. We have investigated the latter case, studying dry sand that is charged under a beam of ions from a corona discharge device taken from a laser printer. The yield stress of each pile is measured as a function of the voltage on the surface, which is related to the amount of charge in the bulk of the pile. Sand that is chemically modified to make it hydrophobic behaves very differently - surprisingly so, to us -- from untreated (hydrophilic) sand. I will show measurements of the yield stress as a function of the surface voltage as well as measurements of the transport of charge though these systems. I will end with a speculative discussion of how the spatial pattern of electric charge determines the macroscopic properties in these systems.
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