The rate of aggregation is in general determined by the frequency of collisions and the probability of cohesion during collision. If the collisions are caused by hydrodynamic motions (e.g convection or sedimentation) this is then referred to as orthokinetic aggregation.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 610 (https://doi.org/10.1351/pac197231040577)