This work was motivated by the unexpected values of adhesion forces measured between all atomic force microscopy tip and the hydrophobic surface of ultra-high-molecular-weight polyethylene. Two types of samples with different roughness but similar wettability were tested. Adhesion forces of similar magnitude were obtained in air and in polar liquids (water and Hank's Balanced Salt Solution, a saline solution) with the rougher sample. In contrast, the adhesion forces measured oil the smoother sample in air were much higher than those measured in water or in the aqueous solution. Those experimental results Suggested the presence of nanobubbles at the interface between the tough sample and the polar liquids. The existence of the nanobubbles was further confirmed by the images of the interface obtained in n tapping mode. The adhesion forces measured in a nonpolar liquid (hexadecane) were small and of the same order of magnitude for both samples and their values were in good agreement with the predictions of the London-Hamaker approach for the van der Waals interactions. Finally, we correlate the appearance of nanobubbles with surface topography. The conclusion of this work is that adhesion forces measured in aqueous media may be strongly affected by the presence of nanobubbles if the surface presents topographical accidents. (C) 2008 Elsevier Inc. All rights reserved.