Structure evolution of growing aggregates in the sequential collision simulations for various impact velocities v _imp and critical rolling displacements ξ _crit. Panel (a) shows the radius of gyration and panel (b) shows the density. The solid lines show the resultant aggregates in our simulations, and the dashed lines indicate BCCA clusters. Filled circles indicate the critical number of particles N _crit to start compression, as estimated from Equation (6) with β = 0.5.