The Velocity Profile Measurement In Sub-micron Near Wall Region

Lab on a chip is one of the world’s most cutting-edge research fields. To achieve varieties of functions of lab on a chip needs a good understanding of inherent laws in micro/nano-fluidics. And the velocimetry is the most effective tool to study the micro/nano-fluidics. To study micro/nano flows, velocimetry technology with higher spatial resolution is needed. Introducing TIRV(total internal reflected velocimetry) to Micro-PIV/PTV makes this possible. But there are some problems in the technology nowadays. We make some improvements of the TIRV and measure the near-wall velocity distribution in sub-micron scale in a rectangular cross-section microchannel.First, we measured the velocity profile of a few micrometers to the wall in a rectangular cross-section microchannel. And the results show good agreement with the theoretical profile from the N-S equation, which ensure the stability and reliability of relevant equipment and laid the foundation for subsequent experiments.Second, a numerical solution of the nanotracer intensity probability density function is given to predict the intensity distribution according to the exponential decay of the evanescent wave, the Lorentzian distribution of the nanotracer intensity, the tracer concentration distribution and the tracer size variation on the problem of determining the base intensity in TIRV. We measured the velocity profile of a rectangular cross-section microchannel in 700 nanometers to the wall. The results from 300nm- 700 nm to the wall show a good agreement with the theoretical value and the error is within 10%, while there is a deviation between the experimental and theoretical ones in the region below 300 nm, which mainly due to the size deviation of the nanotracers. The results useing the new method to determine the I0 could significantly improve the velocity measurement precision comparing with the traditional method.Finally, we measure velocity with various nanotracers and various concentration of electrolyte solution, and analyze the effect of size and the ion concentration. The result shows that the velocity closer to the wall could be measured if more tracers appear in the near wall region; and the concentration of tracers increase with the increase of the ion concentration in the electrolyte solution, which results in a more reliable measurement in near wall region. Furthermore, we analyze the error source in our experiment. The size deviation of the nanotracers may bring significant uncertainty in the vertical position of a single nanotracer. While for velocimetry, that error could be eliminated by averaging of a large number of tracers in every layer.