Effect of finite spatial and temporal resolutions on super-resolution particle tracking velocimetry for pressure-driven flow in a nanochannel

With developments of nanofluidics, understanding the behavior of fluids confined in nanospaces becomes important. Particle tracking is an efficient approach, but in nanospaces, it often suffers from the finite temporal resolution, which causes the Brownian displacement of nanoparticles, and the finite spatial resolution due to the decreased signal-to-noise ratio of nanoparticle images, both of which are factors that can cause artifacts. Therefore, in the present study, we simulated nanoparticle tracking velocimetry based on the particle dynamics given by the Langevin equation to evaluate the artifacts. The results revealed that for measurement of the velocity distribution of pressure-driven flow in a 400 nm nanochannel utilizing 60 nm tracer nanoparticles, high-speed (temporal resolution: Δt ≤ 360 µs) and super-resolution (spatial resolution: Δz ≤ 25 nm) measurement is required for errors less than 10%, while insufficient resolution causes an artifact that results in a flattened velocity distribution compared with the original flow profile. The proposed resolutions were experimentally verified by defocusing nanoparticle tracking velocimetry developed by our group. As the simulation predicted, at longer temporal resolution and larger spatial resolution, the measured nanoparticle velocity distribution in the nanochannel indicated a parabolic flow profile but became flattened because of the artifacts. In contrast, at measurement resolutions within the proposed range, the velocity distribution close to the profile given by the Hagen-Poiseuille equation, which was considered to be the actual flow profile, was successfully obtained. This work provides a guideline for nanoscale flow measurements and will accelerate the understanding of specific transport phenomena in nanospaces.