Newsletter 2021.1 Index
Theme : "Mechanical Engineering Congress, 2020 Japan (MECJ-20)”
Statistical Mechanical Analysis of Nanoparticle Behavior for Measurement of Flow Velocity Distribution in Nanochannels by Particle Tracking Velocimetry
Tokyo University of
Agriculture and Technology
Understanding fluid flows in nano-space (10-1000 nm) is important for development of nanofluidics. Commonly used method for measuring fluid flows is particle image velocimetry (PIV), which visualizes the flow by tracking the motion of tracer particles suspended in fluid. The conventional PIV developed for micro-spaces is micro PIV(1). Micro PIV measures the flow velocity at focal plane and scans across the channel by moving the focal plane to obtain the depth-wise flow profile. Since the spatial resolution of micro PIV is few micro-meters, which is determined by the depth of field of microscope, micro PIV cannot be applied to the measurement of fluid flows in nanochannels. Our research group therefore developed defocus nano particle image velocimetry(2), which exploits out of focus particle image to detect the depth-wise position of particles with the spatial resolution of 10 nm. However, in nanospaces, displacement of the particles due to Brownian motion compared to the size of the space becomes relatively large, causing significant error in detecting the particle position. Thus, it is necessary to find out time resolution in which case the influence of Brownian motion is small enough to be neglected. In the present study, we carried out simulation based on Langevin equation to evaluate the influence of Brownian motion and found that the time resolution of 360 μs or less is required to reduce the influence of Brownian motion. Based on this finding, we experimentally measured velocity distribution of pressure driven flow in a nanochannel of 400 nm depth and succeeded in reducing the influence of Brownian motion. The result suggested unique characteristics of nanochannel flow, which will further be studied in future work.
Particle tracking velocimetry, Defocus, Brownian motion, Langevin simulation, Nanochannel, Nanofluidics
Fig.1 Principle of micro PIV(left) and defocus nano PIV(right)