Measurement of particle behavior in micro/nano channels using defocusing nano-PIV*

With the recent progress of micro/nanotechnology, understanding particle transport in small spaces with dominant surface effects becomes important. For this issue, micro-particle image velocimetry (micro-PIV) has been developed, but the spatial resolution is insufficient for study on spaces smaller than micrometers. We have developed defocusing nano-PIV that determines particle positions with spatial resolution overcoming the optical diffraction limit using defocused particle images with diffraction rings due to spherical aberration. However, the measurement accuracy is still insufficient to obtain results that reflect actual phenomena.

The present study improved the spatial resolution of defocusing nano-PIV by determination of the positions of center peak and diffraction ring of the particle image. In our previous work, the spatial resolution in streamwise and depthwise directions were respectively 40.0 nm and 42.6 nm, but in this study, they were 7.4 nm and 9.9 nm. We archived the spatial resolution similar to the dispersion of particle diameter. Based on these improvements, we measured the flow velocity profile in a channel with a depth of 2500 nm for particles of 200 nm diameter, and the result was close to the theoretical flow velocity distribution. Thus, we succeeded in measurement of the particle behavior in micro channels that reflects actual phenomena. In the future, we will measure the particle behavior in nanochannels.