Next we performed comparison experiments on conditions different from the major experiments (See section major results). In these experiments, a disk 140 mm in diameter and 1.01 kg in weight was used. All conditions of the experiments are shown in Table 1, and the condition of major results is listed as number I. Comparison experiments were performed under the following conditions:
II-1. Starch and silica sand were used as powder materials. The starch layer was prepared in the same way as the flour layer, and the silica-sand layer was made into a layer 2.5 - 3 mm thick with the surface smoothed by hand since the sieve machine could not handle the silica sand.
II-2. The powder layer was pressed to examine the influence of a void fraction of the powder layer on sliding. An iron pipe was used as a roller to press down the powder layer. However, this experiment was not performed with silica sand because it could not be made into a "light and fluffy" layer.
II-3. The disk having the basal surface which each powder stuck onto was used; this type of disk is called the S-type while the other type is called M-type. To make an S-type disk, we fixed the powder onto the base of the disk with spray paste. These comparison experiments were not performed with silica sand because it did not cling onto either base types.
II-4. The surface of the track board was also changed to check the effect of the track-board surface on sliding. The particles of the powder can cling onto a plywood board but not onto a galvanized iron sheet. These comparison experiments was performed for only flour.
Movie 4. State of the flour layer.