Friction coefficient was measured by sliding a rigid disk on a powder layer in the following method.
The powder materials used in this experiment are wheat flour, potato starch and silica sand. Of these materials, wheat flour is mainly used because it is inexpensive and easy to handle.
In most experiments, the powder was sieved and scattered directly on a plywood track board 10 m long and 0.5 m wide. In some cases, a galvanized iron sheet was placed over the track board in order to change the surface condition for the powder layer.
We lightly scattered powders over the track board using a sieve machine, making a fluffy, 2- mm thick layer. For comparison, the powder layer was pressed into a compact layer in several cases.
The sliding bodies are metal disks with different mass and/or different radius and/or different bottom surface. In most cases, powder was adhered to the lower surface of the disk, while in some cases, the disks with smooth surfaces were used for comparison.
Fig. 3. The experimental apparatus. Click on the name for details.
Movie 1. Video of the experiment.
These disks were accelerated by a catapult and slid on the powder layer. They were subsequently decelerated by sliding on powder layer. To decelerate the disk, we recorded the position of the sliding disk on a digital video with readings at 1/60-second intervals on a computer. The friction coefficient m is given as
m = -a/g,
where a is the acceleration of a disk and g is the gravity acceleration.
In addition, we observed tracks left in the powder layer to understand how a disks slides. The shear in the powder layer was visualized with a ditch filled with colored powder. The shear stress in the powder layer was visualized with colored powder filled in a ditch. The 2-10 mm wide ditch was prepared beforehand across the track board. The sliding attitude of the disk was also recorded using a laser displacement sensor set at the middle of the track board.