  We discuss the reversible-irreversible transition in low-Reynolds
hydrodynamic systems driven by external cycling actuation. We introduce a set
of models with no auto-organisation, and show that a sharp crossover is
obtained between a Lyapunov regime in which any noise source, such as thermal
noise, is amplified exponentially, and a diffusive regime where this no longer
holds. In the latter regime, groups of particles are seen to move
cooperatively, yet no spatial organization occurs.
