Can self-propelled objects escape from compression stimulation?

We studied circular papers impregnated with camphor (CPs) and CPs with magnets (MCPs) as self-propelled objects floating on water under the compression of the water surface as an inanimate system for evacuation in an emergency. Two water chambers-C in and C out -were connected via a plastic gate, and eight CPs or eight MCPs were placed on C in . We monitored the movement of the CPs or MCPs from C in to C out when the gate was opened and the area of C in ( A in ) was decreased using a barrier. When A in was large, CPs moved stochastically from C in to C out while exhibiting random motion. The escape probability from C in to C out ( P ) at time t = 20 s increased with a decrease in A in , and the rate of increase in P increased depending on the width of the gate ( W g ). By contrast, clustering was observed for MCPs. Consequently, P of MCPs was lower than that of CPs. The difference in the surface tension between C in and C out (Δ γ ) increased with a decrease in A in . P is discussed in relation to Δ γ as the driving force for emergencies and the repulsive forces between CPs or attractive forces between MCPs. These results suggest that the repulsive force enhances the self-propulsion of objects towards the gate, that is, as a result, higher values of P are obtained.