Laser Doppler velocimetry for submicrometer particle size determination

Abstract

In laser Doppler velocimetry, two laser beams are crossed within a sample. Interference between the two beams generates a set of light and dark fringes within the intersection region of the beams. A particle passing through the fringe region will generate a modulated light scatter signal. Conventionally, the modulation frequency of the light scatter signal is used to estimate the velocity of the particles. However, the modulated light scatter signal generated by laser Doppler velocimetry also provides excellent noise immunity for particle size determination. To utilize the light scatter signal for precise particle size determination, it is necessary to align particles to pass through the center of the fringe region. We utilize the sheath flow cuvette to produce a thin stream of particulate suspension at the center of the interference fringe region. For polystyrene particles in water, the relative standard deviation of the light scatter distributions typically falls in the 1-2% range and is dominated by the intrinsic size distribution of the particle standards. Particles as small as 40-nm radius may be studied using a low-power He-Cd laser.