In recent years CFD has proven to be a valuable and powerful tool for advancing our understanding of complex multiphase flow systems arising in industrial applications. However, the predictive capabilities of this tool are determined by many factors of physical and numerical origin but in particular by the quality of the closures adopted for the description of the interface forces. The objective of this study is to use direct numerical simulations to validate and improve these closures using an improved front tracking model. We have studied the drag force on single air bubbles rising in viscous liquids over a wide range of viscosities. Dedicated experiments were conducted to validate the model and to highlight the effect of contaminants. The results show an excellent agreement between the numerical simulations and available analytical theory, whereas existing drag force correlations and the in-house experiments (using liquids with the same physical properties) gave a much higher drag force. This demonstrates the important effect of contaminants on the drag force, which is an important subject for future research. © 2009 Elsevier Ltd. All rights reserved.