The Production of Preferred Orientation in a Concentrated Dispersion of Elongated and Flattened Grains

A mechanism is discussed in which a statistical alignment of long and short axes might be produced as a result of the encounters occurring between elastic particles during the shearing of a concentrated dispersion of them in a fluid of negligible viscosity and inertia. Predictions are made about the type of orientation to be expected. The flow of sand on the lee faces of dunes is like the motion considered theoretically, and the grain orientation was estimated, using a method based on the measurement of magnetic susceptibility, of material from two sets of dunes. The results agree with prediction. Finally, a suggestion is made about possible differences to be expected in the grain orientation when the inertia and viscosity of the interstitial fluid are not negligible.