The effect of inlet velocity, inlet particle size distribution, particle sphericity and particle concentration on the separation efficiency was studied with a mini-hydrocyclone of 5 mm diameter to study its performance when miniaturised. For this mini-hydrocyclone, the separation efficiency was found to increase with decreasing particle size resulting in a fishhook curve for particle diameters smaller than 15 mu m. The fishhook effect was found to increase with increasing inlet velocity. The presence of large particles in the feed was found to enhance the fishhook effect supporting the hypothesis that entrainment of small particles within the wake of a large particle is the major cause of the fishhook effect. As the inlet velocity is increased, the larger particles have larger slip velocities which give rise to larger wakes that are more effective in entraining small particles, thereby enhancing the fishhook effect. Spherical particles were found to give rise to a more pronounced fishhook effect compared to non-spherical particles. A simple empirical model that accounts for the fishhook effect shows that the relative particle size and inlet velocities play a major role in effecting the fishhook effect. (c) 2014 Elsevier Ltd. All rights reserved.