Triboelectrification and razorbacks: geophysical patterns produced in dry grains.

Electrostatic interactions between particles can dramatically affect granular flows, creating industrial safety and handling problems [K. N. Palmer, (Chapman and Hall, London, 1973), pp. 388-389]. We present experimental data demonstrating that charging of grains can also cause spontaneous self-assembly that may generate lasting geological patterns under arid conditions. Paradoxically, we find that grains that tribocharge enough to produce small explosions, ejecting grains meters into the air, leave little net charge on grains. Rather, grains charge into strongly heterogeneous polar clusters. These assemble into stereotyped residual structures that resemble geological features, for example, razorbacks observed on Mars ["The Razorback Mystery," July 16, 2004, http://www.jpl.nasa.gov/missions/mer/images.cfm?id=701].