The forces acting on a bubble were analyzed to determine the criteria for bubble departure and lift-off from a heated wall. Mechanistic models for the bubble departure and lift-off diameters were developed from the analysis results to predict the evaporation heat flux under low heat flux and low flow velocity conditions. The developed model for the departure diameter depends strongly on the contact angle, but the model for the lift-off diameter was found to be a function of the lift-off number, which indicates the ratio between the shear lift force and the growth force. Wall boiling experiments were performed to measure bubble departure and lift-off diameters using digital image processing. From the experimental data, the developed models were validated, and the results showed good agreement between the experimental data and the predicted diameters. (C) 2011 Elsevier Ltd. All rights reserved.