(385g) Clustering of Sand Grains Due to Triboelectrification

Static electricity plays an important role in many granular processes, whether they are industrial, natural or even extraterrestrial. Industries use static charging to transfer particles controllably. Applications include electrostatic spray coating, separation and filtration. Xerography has charged ink particles to make copies for over 60 years. Other industries spend significant resources to prevent electric charging, which causes troublesome difficulties in processing or dangerous electric discharges. Even for geophysical processes, electrostatics is involved in the transport of dry grains, from volcanic plumes to dune movements. And in drier environments, like the ones found on Mars and the Moon, electrostatics is expected to have an even larger effect. If humans are to realize current governmental plans to travel to Mars, they must be able to utilize the resources available on the surface of the planet. This will unavoidably mean dealing with triboelectric charging. Yet the role electrostatics plays in granular flow is still not well understood. Understanding this behavior could be of great benefit to industry and a critical step for a manned mission to Mars.

We must also understand the history of Mars in order to predict what resources are available today. Many of the recently discovered features on Mars appear to have analogs on Earth that formed in the presence of liquid water. However, some of these Martian features appear to lack erosional damage, which suggests a contemporary mechanism for their creation. Here we report experiments that offer an alternative explanation, based on electrostatic effects, for the creation of some of these features. When sand grains are poured down an inclined acrylic sheet at low relative humidity, the grains cluster together into sharp pointed spikes, which resemble the features on Mars referred to as ?razorbacks?.

These features also demonstrate the ability of electrostatic charging to agglomerate a normally free flowing material, dry sand, simply by allowing the material to flow down an insulating chute. This work also supplies an exceedingly simple test for the effectiveness of anti-static devices such as static eliminators, anti-static sprays or even anti-static nanoparticle coatings.