Cohesion and brittle fracture in negatively-charged piles of hydrophobic grains

Electrically charged powders are ubiquitous, from freshly ground coffee to regolith on the Moon or Mars. Whereas triboelectric charging can yield oppositely charged grains that aggregate, one might expect like-charged grains to repel and soften granular materials. Here we show that if like-charged grains are hydrophobic, ions are immobile and the material becomes surprisingly cohesive and brittle. We formed slab-shaped piles, deposited anions with a corona discharge device, and measured the surface voltage V_S. Using hydrophobically modified sand, we found that the material could be tilted to a maximum (failure) angle that increased beyond 90° and even 180° with increasing V_S, indicating enhanced shear rigidity. The yield mechanism changed from surface flow (V_S=0) to cracking and brittle failure (V_S < -1 kV). As a direct probe of yield shear stress, we measured the force to move an embedded mesh. Hydrophobic powders yielded at 55 Pa at V_S=0, and at up to 150 Pa at V_S=-2 kV. With ordinary (hydrophilic) sand, ions are mobile and the yield stress did not change with V_S. Our results may inform operations on the Moon or Mars and allow real-time control of granular rheology via applied charge.