Size-Dependent Oscillation in Optical Spectra from Fly Ash Cenospheres: Particle Sizing Using Darkfield Hyperspectral Interferometry

Abstract

Fly ash by-products are emerging biocompatible fillers for a number of construction materials. The value of fly ash as a filler is higher if the content of hollow cenospheres is increased. Here we describe a new method for detection and sizing of fly ash spheres based on darkfield microscopy with hyperspectral image capture to perform white light interferometry. Our method is cost-effective and can provide rapid means for evaluating cenosphere content during the enrichment process. We show that fly ash cenospheres produce a strong oscillation over wavelength in optical recordings. The phenomenon is easiest to observe using microscope imaging techniques that preserve both spatial and spectral information. Frequency is observed to increase in direct proportion to the sphere diameter. The oscillation appears in light recorded from any focal plane on the sphere which indicates that the entire sphere is involved in sustaining the signal, making the detection of cenospheres of different size and displacement within a recording volume productive. There is no oscillation from nonspherical particles of fly ash or other material, so this detection method is highly selective for the cenospheres.