Study of Submicron Particle Size Distributions by Laser Doppler Measurement of Brownian Motion.

Few nonintrusive techniques are available for particle measurement in the submicron size range o.1 micron diameter, yet measurement of these particles is basic to an understanding of important processes in combustion, such as soot formation and oxidation. The objective of the present research is the development and application of a technique for measurement of individual submicron particles in a gas stream. The approach is to measure the inertial relaxation time of individual particles of Brownian motion, by statistical analysis of the time resolved 100 MHz heterodyne signal obtained in an interferometer system resembling a more conventional laser Doppler velocimeter. Progress to date has included the development and refinement of an optical system for conducting the experimental studies. This optical system and experimental method are described in detail in conjunction with preliminary measurement of H20 particles which confirm the laser Doppler velocimeter mode of operation of the apparatus. A theory describing the statistical behavior of the signal from such a measurement signal has been developed. From theory, the probability distribution for the time intervals between the zeros of the Brownian motion velocity may be obtained. This program will later focus on measurements of particles of known size suspended in liquids in order to maximize the signal to noise of the optical measuring system, to refine the data acquisition methods, and to verify the theoretical understandin of the acquired signal. Followign the demonstration of the experimental approach, the technique will be applied to measurements of submicron particles suspended in gas flows.