It is well known that three modes (the constant contact radius mode, the constant contact angle mode and the mixed mode) are existed during the droplet evaporation process. However, the mechanisms of these modes and their transitions are still in dispute. In this paper, pinning force is introduced and formulated to give a quantitative explanation to the evaporation modes and their transitions. The effect of surface morphology on the pinning force is discussed using our experiments of water droplets evaporating and contact line receding on the surfaces covered by PDMS (polydimethylsiloxane) spherical cap arrays with gradually increasing height. (C) 2011 Elsevier B.V. All rights reserved.