Can self-propelled objects escape from compression stimulation?

Abstract

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 (Δγ) increased with a decrease in A_in. P is discussed in relation to Δγ 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.