Measuring the yield stress of charged granular media: how a net charge leads to cohesive powders

Charged powders are common in nature and industry yet still exhibit many surprising behaviors that are poorly understood. Here we report that piles of grains that all carry the same sign of net charge can, surprisingly, behave like a brittle, cohesive solid. Our experiments probe slab-shaped piles placed on an insulating substrate atop a grounded plate. Sample were irradiated with ions from a corona discharge device and the final surface voltage, V_S, was measured. For ordinary sand, removing the pile from the ground plate caused the charged grains to fly off. By contrast, sand grains with hydrophobic coating formed piles that were stable, cohesive and rigid. We measured the rigidity of these piles in two ways: the minimum tilt angle θ that led to failure, and the yield shear stress, σ_Y. We found that both quantities increased as V_S². θ can exceed 90^○ and σ_Y can reach three times its zero-charge value. We also measured the decay of V_S over time and found that a stretched exponential function fit the data over timescales of many days. These results may lead to new ways to manipulate charged granular materials and provide new insights into the properties of lunar or Martian soils.