Growth of multiparticle aggregates in sedimenting suspensions

The process of multiparticle aggregation in a dilute sedimenting suspension is rigorously simulated, with precise hydrodynamical interactions. The primary particles are monodisperse non-Brownian spheres at zero Reynolds number, with short-range molecular attractions. The rigid aggregates grow, as they settle downwards, by sequential particle addition - a valid assumption for dilute suspensions during the initial stages. The growth starts from doublet-particle interaction, but the indeterminate initial doublet concentration does not affect the results for cluster geometry and settling velocity. A new particle is generated far below a cluster with uniform probability density, and many trial particle-cluster relative trajectories are computed with high accuracy until a collision is found. The new cluster is then assumed to be rigid and allowed to reach a steady sedimentation regime (which is a spiral motion around the axis of steady rotation, ASR) before another particle is added, and so on. The ASR is typically far away from the cluster centre of mass. The Stokes flow solution algorithm for particle-cluster interaction works very efficiently with high-order multipoles (to order 100) and is extended to arbitrarily small particle-cluster separations by a geometry perturbation adapted from the conductivity simulations of Zinchenko (Phil. Trans. R. Soc. Lond. A, 1998, vol. 356, pp. 2953-2998). Clusters are generated to N = 100 spheres, with extensive averaging over many growth realizations. The fractal scaling similar to N-0.48 for the cluster settling speed is quickly attained once N >= 25, and the exponent 0.48 is practically independent of the strength of molecular forces. The cluster fractal dimension is predicted to be d(f) = 1.91 +/- 0.02 (in contrast to the existing views that sequential addition can only produce high-d(f) clusters). Several average characteristics of the cluster size are also computed. The theoretical settling speed has no adjustable parameters and agrees reasonably well with prior experiments for a moderately polydisperse system in a broad range of cluster sizes.