Size measurement of very small spherical particles by Mie Scattering Imaging (MSI)

The Mie Scattering Imaging method (MSI) gathers out-of-focus images of dispersed spherical particles present in a laser light sheet and extracts the individual particle diameter from these images. The general idea of the method has been around for more than a decade and a number of papers has dealt with it over recent years. Our work focuses on small particle sizes from 20 mum down to 2 mum, a range which has not been tackled so far although it is of great importance in particle systems. We present an optical set-up with a special arrangement of camera lenses that allows to work in this range. An evaluation algorithm based on correlation of the experimental optical information with theoretical Mie scattering was found to give the most accurate results for particle sizing. Besides accuracy measurements on solid spheres the versatility of the method is demonstrated by an example of transient droplet growth between 2-7 mum.