Atomic force microscopy of nanofiltration membranes: Effect of imaging mode and environment

[Display omitted] ► Detailed study on nanofiltration membrane of different degree of hydrophobicity in liquid using AFM tapping and contact modes. ► AFM studies were complimented by the membrane transport method and contact angle measurements. ► A number of features were observed when imaging PES 20% PVP 20% membrane with AFM tapping mode in water. ► The image is backed up by the relatively hydrophobic nature of the membrane, the shape being similar to nanobubbles previously described in the literature. The atomic force microscope (AFM) has become a useful tool for studying the morphology of membrane surfaces as well as their fouling characteristics. One principle advantage of the AFM over other high resolution imaging techniques is the ability to make observations in both ambient air and liquid environments. Diverse imaging modes also exist, each with their own advantages and disadvantages. In this study two different imaging modes in both air and water are compared when examining two different nanofiltration membranes, to compare the strengths and weakness of different methods of obtaining surface topography when applied to nanofiltration membrane characterization. When imaging the more hydrophobic of the two membranes using tapping mode in a water environment features consistent with the existence of surface adhered nanobubbles were observed. Such features have implications for the fouling of membranes by hydrophobic materials, as well as effects on the ability to image hydrophobic membrane surfaces under such conditions.