Journal of Membrane Science 278 (2006) 410–417 A comparison of vertical scanning interferometry (VSI) and atomic force microscopy (AFM) for characterizing membrane surface topography Ismail Koyuncu a , Jonathan Brant b , Andreas L ¨ uttge c , Mark R. Wiesner b,d,∗ a Department of Environmental Engineering, Istanbul Technical University, Maslak, Istanbul 34469, Turkey b Department of Civil and Environmental Engineering, Rice University, MS-317 Houston, TX 77005-1892, USA c Department of Earth Sciences, Rice University, MS-126 Houston, TX 77005-1892, USA d Department of Chemical Engineering, Rice University, P.O. Box 1892, MS-317 Houston, TX 77005-1892, USA Received 27 September 2005; received in revised form 16 November 2005; accepted 17 November 2005 Abstract In this paper, vertical scanning interferometry (VSI) and atomic force microscopy (AFM) were used to characterize the topography of several nanofiltration and reverse osmosis membrane surfaces. Comparing roughness results from the two different characterization techniques revealed unique results for the various membrane surfaces. Roughness values tended to be higher from the interferometry measurements compared to those from AFM measurements for the same membranes. This was attributed to the inability of the AFM to capture dramatic changes in surface height of several microns or more. Based on interferometric measurements surface roughness was also found to increase with increasing scan-size up to a scan-size of 250,000 m 2 after which it remained relatively constant. Because such large scan-sizes are too large to be captured through AFM measurements interferometry appears to provide a more comprehensive characterization of membrane surface roughness. © 2006 Elsevier B.V. All rights reserved. Keywords: Interferometric method; AFM; Roughness; Scan area; Membrane surface topography 1. Introduction There currently exists a variety of analytical tools for characterizing the morphology of polymeric and ceramic membrane surfaces [1–4]. Some of the more popular surface characterization techniques for membranes include scanning electron microscopy (SEM) and atomic force microscopy (AFM). Each of these tools is capable of providing atomic level quantitative analyses of the morphological characteristics of membrane surfaces. For instance, using these tools it is possible to characterize membrane properties like surface roughness [5–8], porosity and pore size distribution [9–11], and deposit layer thickness [12]. Such characterization is important given findings demonstrating the critical role of these characteristics in determining membrane performance [6,7,13]. Of the surface characterization techniques that are available to membrane scientists, AFM has been used due to the ease of sample preparation and its ability to characterize mem- ∗ Corresponding author. Tel.: +1 713 348 5129; fax: +1 713 348 5203. E-mail address: wiesner@rice.edu (M.R. Wiesner). brane surfaces in both wet and dry environments [2,5,7,9–11, 14–16]. An atomic force microscope operates using a combination of principles from the scanning tunneling microscope and the stylus profilometer [17,18]. Here, a sharp tip, with a radius of around 50–100 nm, is scanned over a surface with feedback mechanisms that enable piezo-electric scanners to maintain the tip at a con- stant force (to obtain height information), or height (to obtain force information) above the sample surface. Tips are typically made from silicon nitride and extend down from the end of a flex- ible cantilever. Surface morphology and/or surface interactions are measured based on the vertical deflection of the cantilever. The resolution of the AFM is determined by the sharpness of the tip and typically approaches the atomic scale. Thus, AFM may be used to provide high-resolution information regarding membrane surface morphology in addition to other character- istics. Nevertheless, despite the many advantages of AFM and its success in characterizing membrane surfaces a number of limitations do exist. One of the principle drawbacks of the AFM is the relatively small area that can be scanned at any given time. For instance, the maximum scan area for most AFMs is approximately 100 m 2 . 0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.11.039