Why are there are no dry sand castles? Two words: Friction and gravity. Gravity is the force that pulls things down toward the earth's surface, and is responsible for all things in nature seeking their lowest energy state. Friction is a function of the surface area and shape of the particles.
Dry BBs, mostly flat, only a few on top of others. BBs are spherical and have little surface area, thus they have little friction. Gravity pulls them into a single layer.	Dry sand forms a mound with regular angles.  Sand grains are irregularly shaped particles (spheroids, columnoids, etc.) with little cohesion, so they form an internal angle of about 30 degrees due to friction among particles. This angle is called the angle of repose. In the larger picture, the shape of the largest sand grains can be observed.	Dry silica flour has fine particles, so there is more surface area. More surface area creates more friction, and some cohesion, allowing particles to stack higher. The angle of repose is greater for these finer particles.
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Updated 06-23-2008. Copyright 2005. Clay Robinson, Ph.D., as to all resources: Materials may not be reproduced without Dr. Robinson's written consent. Students are prohibited from selling (or being paid for taking) notes or webpages during this course to or by any person or commercial firm without the express written permission of the developer of these pages.

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