Filtration is ubiquitous in our daily lives and in many technological processes, from drinking water and microplastics to kidney dialysis. A wide range of filtration membrane materials are available and new chemistries are constantly being developed. AFM again shines as a powerful technology to rapidly analyze membrane manufacturing (pore size, distribution and defects), membrane failure analysis (clogging), and filtration performance (filtrate imaging).

Measuring pore distribution in polyether sulfone (PES) and polycarbonate (PCTE) filtration membranes.

The most simple and accessible measurement is pore size, pore distribution, and defect analysis. AFM allows rapid imaging without any modification of the filter membrane, even in liquid and including stiffness measurements if the material swells. See the examples below on how the membrane material impacts these parameters.

Topography image of a polycarbonate membrane with specified 0.8 um pore size. Note the defects where individual pores merge, but also the highly reproducible and accurate pore size.

Compare this to a polycarbonate filtration membrane with 1.5 um pore size and a polyether sulfone filtration membrane with 0.22 um pore size. Note the substantial differences in pore distribution and pore size.

Higher magnification scan of the polyether sulfone filtration membrane illustrates the variability in the pore size.

Protein dialysis membranes require nanopores and are commonly cellulose-based. A 10 kDa protein has a minimal radius of >1.42 nm. Note the orientation of the cellulose fibers and the fold in this dry membrane.

We tested the use of the polyether sulfone (PES) membrane above to filter solutes from cow’s milk, with the goal to image the components by AFM. Filtering milk rapidly clogged the membrane but retained enough solutes to identify casein micelles.

Home water filtration and the contamination of water with microplastics is a very active topic. Unfortunately, home water filters also release micro- and nanoparticles into the filtered water. Here we use the AFM to compare tap water before filtration, after filtration with a home water filter, and distilled water, by drying water samples onto mica. Filters leaching contaminations into water is also common with laboratory filtration systems.

Tap water before and after filtration with a home water filter, showing new particulates leaching from the filter.

The side by side of topography and phase image suggests that a new type of contaminant is now present, probably leached from the home water filter. Distilled water in comparison is almost perfectly free of particulates, except for a few nanoparticles.