Size and concentration of particles in syton using the fiber optic Doppler anemometer, FODA

The fiber optic Doppler anemometer, FODA, utilizes laser light directed into a communication-type optical fiber to obtain a heterodyne measurement of scattered light from an object or objects near the tip of the fiber. The FODA is applied in this paper to the measurement of the Brownian motion of a colloidal suspension of submicron particles, Syton W-30. By comparison of the power spectrum of scattered light with appropriate mathematical models, it is possible to determine the size distribution of the particles of the colloid. In addition, the concentration of particles in the colloid may be determined from the mean square photocurrent of the FODA. The accuracy of the FODA was assessed by verifying the assumption of heterodyne detection and by determining the noise and signal-to-noise ratio of the instrument. The accuracy of the particle size distribution determination with the FODA was tested by comparing it with that obtained from electron microscope photographs of the particles in Syton. It was found that the laser-scattering size distribution obtained with the FODA is strongly affected by the scattering amplitude of the spherical particles in Syton, since large particles scatter more light than do small ones. A method of obtaining the true particle size distribution, from the laser-scattering size distribution was shown to yield accurate results, within the statistical accuracy of the data.