Two-dimensional DEM-CFD simulations have been performed in order to examine the effect of surface energy on the transitional behaviour from fixed bed to bubbling bed for Geldart Type A particles. The results of the simulations presented in the paper show that any effect of surface energy on the magnitude of Urnf is not due to increasing bed resistance as a result of increasing the interparticle bond strength. It is demonstrated that U-mf corresponds to a deterministic (isostatic) state that is in effect the initiation of the transition from solid-like to fluid-like behaviour. It is also shown that the so-called 'homogeneous expansion' regime is not in fact homogeneous. This is because the system, when U>U-mf, consists of agglomerates. Consequently, the idea that bed expansion is due to the 'elasticity' of the bed is not tenable. In order to break up the agglomerates and create a fully fluidised bed that will allow bubbling to occur, higher superficial gas velocities are required for higher values of surface energy. Once the bed is fully fluidised and bubbling occurs the effect of surface energy becomes insignificant. (C) 2012 Elsevier Ltd. All rights reserved.