Triboelectrification and Razorbacks

Introduction

This website is supplemental information for the article "Triboelectrification and Razorbacks: patterns produced by charging effects in dry grains," by Troy Shinbrot, Keirnan LaMarche and Benjamin J. Glasser. (Phys. Rev. Lett., 96, 178002, 1-4, 2006)

Figure 1. Picture taken after flow has stopped. Grains continue to jump off the surface for several minutes after the bed has come to rest. Razorbacks can be seen on the ridges. Here the chute angle is 32.1° with no static eliminator present.

Recent explorations of the surface of Mars by the NASA Mars Exploration Rover Opportunity have revealed intriguing features called "razorbacks". A picture of the razorbacks discovered by Opportunity can be seen in figure 2. One possible mechanism for the creation of these features is electrostatic attraction. The very low water vapor content in the Martian atmosphere could allow very high charges to accumulate on Martian sand grains.

Figure 2. Martian razorbacks discovered by the Mars rover Opportunity on sol 160. Credit: NASA/JPL-Cornell. To learn more see NASA's website at http://www.jpl.nasa.gov/missions/mer/images.cfm?id=701.

Even on earth triboelectrification of powders can generate charges large enough to cause dangerous dust explosions in many industries. We have studied the possibility that static electricity could be responsible for the creation of razorbacks. We allowed colored art sand to flow down a plastic chute in dry conditions (relative humidity between 13 and 20%).

Examples of the resulting structures can be seen in figure 1 as well as figure 3. Sand particles were observed to often become airborne and sometime jump long distances (as high as 2 meters). These particles often land on the peaks of the razorbacks making them longer. Occasionally a large peace of a razorback will break off and fly down the chute, where they often reattach. Examples of this can be seen on the Razorback Growth page.

Figure 3. A close up of an experimental razorback from the side. Here the angle of the chute is 27.8° and there is no static eliminator present.

These experiments show the large role that electrostatics can play in the flow of granular materials both here on Earth and potentially on Mars.

During the flow sand grains gather together as they flow down hill. These cluster can become surprisingly large and occasionally fly off the chute while remaining intact. Examples of these clusters can be seen on the Particle Clusters page.

Other interesting features of the flow have been observed. Occasionally a cluster of grains will leave the surface and run down hill intact. Movies of these "runners" can be found by clicking on Running Clusters in the side bar or by clicking here.

Other interesting patterns can be seen in the indented circles and grooves page here. These include circular formations which appear spontaneously on the flowing surface of grains as well as groove shaped indentations which also form on the flowing surface.