(569c) Revealing Particle Transport Mechanisms in Microfiltration Processes Using Single-Particle Tracking

Filtration membranes are used extensively in processes including water treatment, pharmaceutical sterilization, food/beverage processing, and heterogeneous catalysis. Therefore, it is important to fundamentally understand particle transport mechanisms during filtration processes. While conventional filtration experiments (e.g., flux or concentration measurements as a function of volumetric throughput) provide useful information about mass transport at the ensemble level, they provide limited insight into the microscopic mechanisms that give rise to complex phenomena, including particle remobilization upon flow interruption and membrane fouling. To address this issue, we have developed refractive index matching imaging systems, combined with single-particle tracking methods, allowing the direct visualization of single-particle motion within a variety of porous materials.

Using single-particle tracking methods, we directly visualized particle transport in microfiltration membranes under flow conditions that included stoppage intervals and therefore provided direct evidence for particle remobilization during flow stoppage. Our results suggest that during flow interruption, a sub-population of trapped/retained particles detach from their retention sites and may escape to less trapped areas through Brownian motion within the pore space. This mechanistic information about particle transport remobilization will permit the design of filtration flow profiles that meet customized requirements for particle retention during complex filtration cycles.

In addition, we also visualized the whole membrane fouling processes using single-particle tracking. We found that internal membrane fouling consisted of three stages, fouling site formation, fouling site growth, and fouling site coalescence. Larger particles had a greater chance to be retained initially, nucleating the fouling sites. These sites tended to capture other particles passing through the membrane, causing the fouling sites to grow in size. Eventually, isolated fouling sites grew large enough to coalescence with neighboring sites, blocking a large region of membrane pores, leading to significant fouling. This mechanistic information about internal membrane fouling will assist in the design and optimization of filtration processes to reduce membrane fouling and expand the fundamental understanding of complex mass transport of polydisperse particle distributions.