Alternate Route Selection of Self-Propelled Filter Papers Impregnated with Camphor for Two-Branched Water Channels

The route selection of self-propelled filter papers impregnated with camphor for two-branched water channels was investigated. The two-branched water channel was composed of a stem channel and two branch channels, and the branch channels were connected to the stem channel at a junction. When a single camphor paper reached the junction from the stem channel, it selected one of the two routes equivalently. Three or five camphor papers which were placed on a stem channel exhibited either alternate or random route selection depending on the characteristic length between the leading and following papers, L_c. That is, the alternate route selection of the camphor papers for the two-branched water channels was observed at L_c ≤ 25 mm. By contrast, the alternate route selection was broken at L_c > 25 mm. The physicochemical meaning of the threshold value, L_th ∼ 26 mm, between the alternate and random route selections was discussed based on the experimental results. In addition, the distribution length of camphor molecules developed from the leading camphor paper and the change in the spatial gradient of surface tension around the junction supports the value of L_th. These results suggest that autonomous phenomena using inanimate self-propelled objects are important to understand collective motion in living organisms.