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, Lc. That is, the alternate route selection of the camphor
papers for the two-branched water channels was observed at Lc ≤ 25 mm. By contrast, the alternate
route selection was broken at Lc >
25
mm. The physicochemical meaning of the threshold value, Lth ∼ 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 Lth. These results suggest that autonomous phenomena using inanimate
self-propelled objects are important to understand collective motion
in living organisms.