In flotation processes, bubble diameter (d(b)), bubble velocity (v(b)), and turbulence are the key factors involved in particle-bubble interactions. The collision efficiency (E-C) is used as an indicator to assess the extent of these interactions. In this work, the bubble surface is assumed mobile with potential flow conditions dominating the particle-bubble collision efficiency. The collision probability has been determined by Schulze and Generalized Sutherland Equation (GSE) models in the particle size range of 1-100 mu m. Bubble diameters of 0.08, 0.12, and 0.15 cm and bubble velocities of 10, 20 and 30 cm/s were selected to study the flotation of chalcopyrite. The results reveal that the collision efficiency of ultra-fine particles (1-10 mu m) is generally improved with bubbles of finer sizes, e.g. d(b) = 0.08 cm compared to those of larger sizes, i.e. d(b) = 0.12 and d(b) = 0.15 cm. Also, in the same particle size range, E-C decreases with increasing the bubble velocity. The best agreement between Schulze and GSE models for ultra-fine particles at all bubble sizes is achieved at the bubble velocity of 30 cm/s. The maximum E-C of chalcopyrite (0.12) using the GSE model is found to occur for coarser particles of 70-100 m in size at bubble conditions of v(b) = 30 cm/s and d(b) = 0.12 cm. Results reveal that for a given bubble diameter increasing the bubble velocity from 10 to 30 cm/s makes the inertial force more effective on finer particles. A detailed interpretation of the effect of bubble diameter and its velocity on particle-bubble interaction of chalcopyrite is discussed from a theoretical point of view. (C) 2016 Elsevier B.V. All rights reserved.