The effect of the particle-fluid and particle-particle interactions of the flow within a hydrocyclone is investigated. These were studied by applying the time scale analysis. It is shown that the particle-particle interactions, due to the lubrication and collision mechanisms, only play an important role in the vicinity of a hydrocyclone wall, and near the air core. In the remaining region, particle-fluid interactions are dominating. These play a vital role on the separation efficiency as illustrated by the significance of the wakes generated behind larger particles on dragging finer particles. (c) 2005 Elsevier B.V. All rights reserved.