Hardening by softening in a flow of chainlike self-driven objects

A two-dimensional square lattice system, on which flexible, chainlike, self-driven objects move randomly but are drifted to a same direction, causing a unidirectional net flow, is investigated by numerical simulations. It is shown that the objects exhibit a freezing transition from a smoothly flowing state to a completely jammed state, in which the objects become immobile and cannot move anymore. Comparison with the flow of rigid objects shows that this complete jamming (hardening) results from the flexibility (softening) of each self-driven object. This is the first report of the freezing transition of a free transport system (without obstacles) where the net flow is not multidirectional (as in the case of opposing flows or crossing flows) but unidirectional.