Forward-scattering particle image velocimetry (FSPIV): application of Mie and imaging theory to measure 3D velocities in microscopic flows using partially coherent illumination and high-aperture opt

摘要

Abstract: We have combined Mie scattering theory and image theoryto predict the forward scattering of light fromspherical particles in a seeded fluid using highnumerical aperture collection optics. Using thismethod, it is possible to determine all threecomponents of a fluid's velocity by measuring thescattering from homogeneous spherical particles withoutmoving the optics. The transverse velocity component isdetermined by following the centroid of the scatteringpattern (with respect to time), while the componentalong the optical axis is determined by comparing theexperimental data with numerical computations. We haveverified our theoretical model and computer code bymeasuring the scattering from polystyrene particlesilluminated with partially coherent, Koehlerillumination in a transmitted light microscope. Thethree-dimensional scattering data is in quite goodagreement with our model. To further verify ourapproach, we have measured the three- dimensional(parabolic) profile of a parallel flow of a lowviscosity, seeded fluid in a straight channel (6 mm by48 mm by 0.315 mm). The channel was placed on the stageof a conventional microscope equipped with a longworking distance microscope objective, with the narrowdimension along the optical axis (OA). Instead ofdirectly imaging the seed particles, the forwardscattered light is recorded from the spherical,polystyrene seed particles (7 micrometer diameter). !15