Mapping
the Functional Tortuosity and Spatiotemporal
Heterogeneity of Porous Polymer Membranes with Super-Resolution Nanoparticle
Tracking
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Abstract
As particles flow
through porous media, they follow complex pathways
and experience heterogeneous environments that are challenging to
characterize. Tortuosity is often used as a parameter to characterize
the complexity of pathways in porous materials and is useful in understanding
hindered mass transport in industrial filtration and mass separation
processes. However, conventional calculations of tortuosity provide
only average values under static conditions; they are insensitive
to the intrinsic heterogeneity of porous media and do not account
for potential effects of operating conditions. Here, we employ a high-throughput
nanoparticle tracking method which enables the observation of actual
particle trajectories in polymer membranes under relevant operating
conditions. Our results indicate that tortuosity is not simply a structural
material property but is instead a functional property that depends
on flow rate and particle size. We also resolved the spatiotemporal
heterogeneity of flowing particles in these porous media. The distributions
of tortuosity and of local residence/retention times were surprisingly
broad, exhibiting heavy tails representing a population of highly
tortuous trajectories and local regions with anomalously long residence
times. Interestingly, local tortuosity and residence times were directly
correlated, suggesting the presence of highly confining regions that
cause more meandering trajectories and longer retention times. The
comprehensive information about tortuosity and spatiotemporal heterogeneity
provided by these methods will advance the understanding of complex
mass transport and assist rational design and synthesis of porous
materials