Title: A comparison of vertical scanning interferometry (VSI) and atomic force microscopy (AFM) for characterizing membrane surface topography

Journal: Journal of Membrane Science

Authors: Ismail Koyuncua, Jonathan Brantb, Andreas L?ttgec and Mark R. Wiesnerb, d, 

Corresponding author: Mark R. Wiesner

Institute: 
aDepartment of Environmental Engineering, Istanbul Technical University, Maslak, Istanbul 34469, Turkey
bDepartment of Civil and Environmental Engineering, Rice University, MS-317 Houston, TX 77005-1892, USA
cDepartment of Earth Sciences, Rice University, MS-126 Houston, TX 77005-1892, USA
dDepartment of Chemical Engineering, Rice University, P.O. Box 1892, MS-317 Houston, TX 77005-1892, USA

The original and creativity of paper: 

The roughness statistics of nanofiltration and reverse osmosis membranes generated by atomic force microscopy (AFM) and vertical scanning interferometry (VSI) was compared. 


Summary: 
?    When same membrane was scanned, VSI showed higher values of roughness values compared to those from AFM (Table 1).  This is because AFM is able to capture dramatic changes in surface height of several micron or more.

[Table 1] Mean membrane surface roughness statistics and associated standard deviations determined using AFM and interferometry


?    Surface roughness values obtained by VSI found to increase with increasing scan-size up to a scan-size of 250,000 μm2. Then the roughness values remained relatively constant. 
?    The limitation of AFM is small scan-size (maximum scan-size = 100 ?m2), however, VSI provides larger scan-sizes (maximum scan-size = 0.5 mm2) than AFM which provide a more comprehensive characterization of membrane surface roughness.  
?    At substantially larger scan-sizes, VSI provides more rapid and accurate characterization of membrane roughness than AFM. 
?    Fig. 1 shows (a) average roughness, (b) root mean square roughness as a function of scan-size obtained with the interferometric method. The result showed that each membrane had a similar inflection point at approximately 250,000 μm2, this indicated that this value is statistical and not physical in origin.

 

[Fig. 1] (a) Average (Ra) and (b) root mean square (Rq) roughness as a function of scan-size obtained with the interferometric method. 

?    Therefore, when VSI is used to examine membrane surface roughness, scan-sizes of at least 250,000 μm2 should be used. This is because this scan-size appears to represent the necessary sample size for which to obtained statistically sufficient results. 


Application & further study: For researchers who are using AFM for membrane characterization, VSI can be a useful tool for characterize membrane morphology.  Because of VSI provides the larger scan-size than AFM. Therefore, the roughness values obtained from VSI are more accurate compared to roughness values obtained from AFM.

By Monruedee Moonkhum
Email: moon@gist.ac.kr