Measurement of Friction Coefficient
on a Powder Layer and Fluid Dynamic Study

Isao Iizawa (Graduate School of Human and Environmental Studies, Kyoto Univ.)
Satoshi Sakai (School of Earth Sciences, IHS, Kyoto Univ.)
Eriko Suda (School of Earth Sciences, IHS, Kyoto Univ.)
Takeshi Saito (Graduate School of Human and Environmental Studies, Kyoto Univ.)
Hiroki Kamata (School of Earth Sciences, IHS, Kyoto Univ.)

abstract

We measured the friction coefficient for a disk sliding on a powder layer to observe whether such a phenomenon could explain the small apparent friction coefficient exhibited by debris avalanches. 

In the experiments, flour was mainly used as the powder material and metal disks as the rigid body that slides on the powder layer. The friction coefficient was calculated from reading the position of a sliding disk recorded on a digital video.

Small friction coefficient was observed when the speed of the sliding disk became higher than a certain (critical) value. 

The obtained friction coefficient was smaller than the friction coefficient between normal surfaces of rigid bodies. The traces left on the powder layer indicated that the disk floated down on the powder layer when the sliding speed was faster than the critical value.

A "dynamic pressure model" is proposed as the mechanism providing  the force that floats the disk. The critical speed derived from this model is consistent with experimental results. These results show that the dynamic pressure in a powder layer is the major source for the driving force that floats the disk.