Single-nanoparticle tracking reveals mechanisms of membrane fouling

While membrane fouling has been studied for decades, it remains challenging to obtain direct information about the dominant mechanism of fouling in a specific scenario. Here, we employed a high-throughput particle-tracking approach, which enabled the visualization of particle transport in actual microfiltration membranes under flow conditions and provided direct evidence for distinct fouling mechanisms under different operating conditions. Our results suggest that the “stickiness” of particles can qualitatively change the dominant fouling mechanism. In particular, the evolutions of effective flux, particle velocity and pathway tortuosity were found to be systematically different under “sticking” vs “reduced-sticking” conditions in two different microfiltration membranes, composed of PVDF and PTFE, respectively. Under “sticking” conditions, fouling was rapid, and individual pathways were observed to disappear with the reduction of flux. However, the average particle velocity and the tortuosity of particle trajectories were unchanged throughout the fouling process, consistent with the complete blocking of random pathways. Conversely, under “reduced-sticking” conditions, the average particle velocity decreased and the tortuosity of particle pathways increased systematically with fouling, consistent with the gradual narrowing of pathways causing increased resistance. The comprehensive information about particle dynamics in membranes achieved with this approach will assist design and optimization of reduced-fouling separation processes as well as advance the understanding of complex mass transport.