(153a) Triboelectrification and Razorbacks: Patterns Produced by Charging Effects in Dry Grains
AIChE Spring Meeting and Global Congress on Process Safety
Tuesday, April 25, 2006 - 4:30pm to 4:50pm
Static electricity plays a huge role in many granular processes whether they be industrial, natural or even extraterrestrial. Several industries use static charging to transfer particles controllably. Industries use electrostatic spray coatings, separators and filters. Xerography has been using charged ink particles to make copies for over 60 years. Other industries spend huge amounts of money on preventing electric charging to avoid 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 dryer environments, like the ones found on Mars and the moon, electrostatics is likely to have an even larger effect. If humans are to travel to Mars they must be able to utilize the resources available on the surface of the planet. That will mean invariably dealing with triboelectric charging. Yet the role electrostatics plays in granular flow is still not well understood. Understanding this behavior could be a huge 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 on both experiments and a simple model, which 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, which could be a useful behavior for products that need to be agglomerated but are either soluble or sensitive to degradation in water. 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. In these experiments the size and number of the sand clusters was shown to have a strong relationship with the distance of a grounded static eliminator from the surface of the flow. In this work we show that electrostatics plays a complex but important role in the behavior of granular flows. Our results give insight into the forces at play on granular materials as they move and tribocharge and it is hoped that this understanding can eventually be used in order to control and take advantage of electrostatics for both our industrial processes as well our exploration of space.
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