Volume percentage of the simulated domain where the dust density has at least increased to 2.5 times the initial dust density. All simulations have v₀ = 0.42 and are perturbed with wavelengths close to their most unstable value (see Fig. 5). The portion of the domain where the dust density is significantly enhanced is seen to increase linearly with time after the linear phase (t ~ 10 for δ = 0.01 and δ = 0.1 and t ~ 20 for δ ~ 1.0). The rate of increase decreases with dust-to-gas ratio δ.