Roughness and hydrophobicity studies of nanofiltration membranes using different modes of AFM

Abstract

Determination of the surface roughness by AFM is crucial to the study of particle fouling in nanofiltration. It is, however, very difficult to compare the different roughness values reported in the literature because of a lack in uniformity in the methods applied to determine surface roughness. AFM is used in both noncontact mode and tapping mode; moreover, the size of the scan area is highly variable. This study compares, for six different nanofiltration membranes (UTC-20, N30F, Desal 51HL, Desal 5DL, NTR7450, NF-PES-10), noncontact mode AFM with tapping mode AFM for several sizes of the scan area. Although the absolute roughness values are different for noncontact AFM and tapping mode AFM, no difference is found between the two modes of AFM in ranking the nanofiltration membranes with respect to their surface roughness. NTR 7450 and NF-PES-10 are the smoothest membranes, while the roughest surface can be found with Desal 51HL and Desal 5DL. UTC-20 and N30F are characterized by an intermediate roughness value. An increase in roughness with increasing scan area is observed for both AFM modes. Larger differences between the roughnesses of the membranes are obtained with tapping mode AFM because of the tapping of the tip on the surface. Phase imaging is an extension of tapping mode AFM, measuring the phase shift between the cantilever oscillation and the oscillation of the piezo driver. This phase shift reflects the interaction between the cantilever and the membrane surface. A comparison with contact angle measurements proves that a small phase shift corresponds to a large contact angle, representing a hydrophobic membrane surface.

The role of cell-surface interactions in bacterial initial adhesion and consequent biofilm formation on nanofiltration/reverse osmosis membranes

“…When looking at AFM images of NF membranes the differences in roughness vary considerably depending on the scan size [84,85] and the area where the topography is measured. In some areas, the membrane is very smooth [86] but in others, membrane defects from the manufacturing process show deep ridges and valleys that could accommodate bacteria and protect them from cross-flow, as can be seen in Figure 5 for the NF 270 and the NF 90 membranes.…”

Atomic force microscopy visualises a hydrophobic meshwork in the central channel of the nuclear pore

“…The oxide sharpened Si 3 N 4 tips used in these experiments were hydrophilic [25], and therefore, interactions between the tip and the hydrophobic meshwork were expected to be rather repulsive. It has been shown in theory and in practice that repulsive tip-sample interactions cause a positive phase shift or phase advance, whereas attractive interactions result in a negative phase shift or phase lag [17,18,26]. The positive peak seen in the central channel in the phase image of Fig.…”

“…In addition to its amplitude, the probe motion in tapping mode can be characterised by detecting the phase difference between the exciting signal and the cantilever oscillation. The measured phase shift is very sensitive to several surface properties such as elasticity, adhesion, hydrophobicity/hydrophylicity and surface charges [17,18]. The phase difference or phase shift, which is recorded simultaneously with the changes in the amplitude, can be used to create an image in which the degree of phase shift is colour coded.…”

“…Support for the existence of peripheral channels in the NPC comes from later studies investigating the NPC both from structural and functional aspects using permeability investigating approaches and the nano-imaging technique atomic force microscopy (AFM), respectively [12][13][14][15]. AFM not only yields extraordinary high-resolution images [16] but can also be used as a nano-mechanical tool for studying numerous properties of the imaged sample such as hydrophobicity [17,18]. In this study, by using AFM under physiological conditions, we provide, for the first time, structural evidence for the presence of a hydrophobic meshwork in the NPC central channel.…”

Introduction: Opportunities and Challenges of Real Time Monitoring on Membrane Processes

“…AFM is mostly used for UF membranes to look at pore size, but according to Bowen (see Chapter 6) one can actually see nanopores using AFM. When using AFM it is also possible to make scans on pore size distribution and on the roughness of samples [8]. Both of these characteristics would be important for online measurements because both fouling and wear of membranes could be observed as a function of time, which is of great value in industrial applications.…”