Flow Measurements in Microspace Using Sub-Micron Fluorescent Particles - An Effect of Brownian Motion on Velocity Detection

For the purpose of measuring a flow field on the order of dozens or several hundred microns, sub-micron diameter particles must be utilized as a tracer particle not to disturb the flow field. Since the diameter of particles is close to the wavelength of light source, the scattered light from a particle is blurred by the diffraction limit and it is difficult to image particles. Fluorescent particles were selected to obtain clear images of particles by filtering the excitation light, as the wavelength of the emission of fluorescence is different from that of the incident light source. The sub-micron tracer particles flow with fluctuation even in a laminar flow of a microchannel, affected by Brownian motion, and it is significant for lower velocity measurement. The methods to eliminate such random components are averaging with integration of multiple data. The time-averaging methods are usually employed and show very smoothed vector fields, but the methods lose the information of temporal variation of flow fields. While the averaging of the velocity vectors within a certain observation area, spatial averaging, provides higher time resolution ever losing the spatial resolution. The present report examined the spatial averaging method to keep a spatial resolution as well. The method was successful by employing PIV as a clue to find local flow directions of particles and applied the PTV to the field. The method could be applied to a denser seeding flow field and the area for averaging becomes smaller. The method shall be called as Spatial Averaged Time-resolved PTV (SAT-PTV). The spatial averaging method provides an instantaneous vector of flow in a small local area and Brownian diffusion of particles individually. It would contribute further to such as vibrant flow, acceleration and deceleration flows in microchannels.