Nanoscale Friction Forces between Hydrophobic Surfaces
The interaction between hydrophobic surfaces in aqueous solutions is particularly important because it is encountered in many industrial processes. Even though advances in surface science have been tremendous, the nature of the hydrophobic interaction remains one of the greatest challenges, from both experimental and theoretical perspectives. In this work, friction forces between hydrophobised silica surfaces were measured by atomic force microscopy (AFM). In addition to the friction force measurements the surfaces were tested in order to quantify the strength of the van der Waals, electrical double layer and non-DLVO forces. An Asylum MFP3D atomic force microscope was used to measure the normal and lateral interactions between a silica bead and an atomically smooth silica substrate hydrophobised by esterification with octanol. Prior to the experiments, the samples were initially rinsed with de-ionised water and ethanol. The friction force versus loading force curves were measured and recorded by the AFM. The environment was changed by gradually increasing and then decreasing the ethanol concentration in the AFM closed fluid cell. The presence of the nanobubbles was observed using the tapping mode imaging technique. The aim of our study was to understand the role of the air nano-bubbles on the overall friction between the investigated surfaces. It was found that the pre-treatment of the samples, specifically the rinsing order with water and ethanol had a significant impact on the friction coefficients and also on the general behaviour of the friction versus load curves. In addition to the friction versus load, the friction dependence on the scan rate was also recorded in order to study the dynamic behaviour of the nano-bubble layer of the dissolved gases. A mechanism was proposed to explain the observed interaction phenomena based on the presence of the bubble layer.