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

A theoretical and experimental study on the feasibility of determining the size of a single submicron particle by observing its Brownian motion characteristics has been carried out. The method is based on measurement of the particle motion interferometrically using the light scattered from a pair of intersecting laser beams. The particle is assumed to be in thermal equilibrium with a fluid medium. due to the viscous damping of the fluid, the motion of the particle exhibits relaxation behavior. The relaxation time may be obtained from the interference signal. Knowing the temperature and the viscosity of the fluid, the mass, hence the size of the particle may be determined from the relaxation time. Monte Carlo simulation of the Brownian motion detector response has shown that the relaxation time may be obtained from the statistics of the time between extrema of the signal. A proof-of-concept experiment was conducted using latex spheres of known sizes as test particles. The signal to noise ratio of the experimental setup, however, was not adequate for determining the size of the particles.