Some Problems in Size Measurement of Submicron Particles

Abstract

Some problems that the author has encountered in size measurement of submicron particles in air and in water have been outlined. When particle size is smaller than around 0.5μm, the mean velocity caused by Brownian motion of a particle, which cannot be repressed by any usual means, becomes comparable with the gravitational settling velocity in air and exceeds the velocity in water. When the number concentration of particles is higher than about 10⁸ particles/cm³ , Brownian coagulation, which cannot be repressed in air and in water unless the use of appropriate dispersion agents becomes significant. Size measurement by sedimentation methods in one or both of these cases gives an erroneous result. Another difficulty arises in size measurement of aggregate particles composed of submicron primary particles. When one may want to know the size distribution of aggregates as they are, sedimentation in air is effective. For measurement of primary particle size composing aggregates, on the other hand, sedimentation in water is effective since deaggregation in water is much easier than in air. As to submicron liquid droplets there is a problem of stability. Even if they have low vapor pressure such as oil, they can easily evaporate to decrease in size by the Kelvin effect which becomes significant as the droplet size decreases.