Accurate measurement of particle velocity distribution in micro/nano spaces by defocusing nanoparticle tracking velocimetry

Micro/nanofluidics has led to a growing emphasis on the study of particle behavior in confined spaces, where surface effects dominate. According to the previous studies, in laminar flow (Re < 1) within 100 µm channels, the velocity of particles slows relative to the flow velocity. This deceleration is attributed to the hydrodynamic wall interactions when the size of the particles is comparable to the channel depth. Recently, we have developed a defocusing nanoparticle tracking velocimetry with a 10 nm spatial resolution for the measurement of particle behavior in micro/nano spaces. The particle depth-wise position is determined from the size of defocused particle images with interference rings caused by spherical aberration. However, the calibration method to determine the relationship between particle image size and depth-wise position had not been established.

The present study developed a novel calibration method employing nanochannels of varying depths. A correction method for errors of particle depth-wise position caused by field curvature and channel tilt was also proposed. The velocity distribution of particles 60-1000 nm diameter was measured in 2500 and 400 nm depth micro/nanochannels. The measurement results well agreed with the theoretical particle velocities, which were derived from the hydrodynamic wall interactions.