Bubble-particle encounter interaction is the first step of the particle collection by rising air bubbles in flotation and has been predicted based on the potential flow condition by Sutherland and others, leading to the approximate generalised Sutherland equation (GSE). In this paper, the bubble-particle encounter interaction with the potential flow condition has been analysed by solving the full motion equation for the particle employing a numerical computational approach. Together with other inertial forces, the gravitational forces were fully included in the motion equation. The numerical results were compared with the GSE models for the encounter efficiency. The effect of particle density on the encounter interaction was very significant and could counterbalance the "negative" effect of the inertial forces on the particle interaction with air bubbles with a mobile surface under the potential flow condition. For the typical particle size range used in flotation, significant deviation of the Sutherland approximate models from the numerical results due to the particle density effect and gravitational forces was observed. (C) 2008 Elsevier Ltd. All rights reserved.