Light scattering study of highly absorptive, non-fractal, hematite aggregates. (May 2020)

• Record Type: Journal Article
• Authors: Gautam, Prakash; Maughan, Justin B.; Ilavsky, Jan; Sorensen, Christopher M.
• Is Part Of: Journal of quantitative spectroscopy & radiative transfer. Volume 246(2020)
• Publication Date: 2020-05
• Subjects: Light scattering -- Aggregate particles -- Enhanced backscattering -- Multiple-scattering -- Hematite; Spectrum analysis -- Periodicals; Radiation -- Periodicals; Analyse spectrale -- Périodiques; Rayonnement -- Périodiques; Radiation; Spectrum analysis; Periodicals; 543.0858
• DOI: 10.1016/j.jqsrt.2020.106919
• Languages: English
• ISSNs: 0022-4073

Abstract

Highlights: The scattered light was enhanced in the backscattering regime despite large imaginary refractive index. Enhancement backscattering for the aggregate is due to internal multiple scattering between the grains within the aggregate. Bimodal size distribution causes an extended Guinier regime. Highly refractive particles displays two-dimensional diffraction properties. Abstract: We present measurements of the scattered light intensity by aerosolized hematite aggregate particles. The measurements were made at a wavelength of 532 nm in the scattering angle range from 0.32° to 157°. Hematite has high values of the real and imaginary parts of the refractive index m = n + iκ = 3 + i0.5 at the studied wavelength. Scanning electron micrographs (SEM) indicated that the particles were aggregates whereas the optical microscope pictures showed that the aerosol had a bimodal distribution with effective mean diameters of roughly 1 and 10 µm. This is consistent with the light scattering results which displayed two Guinier regimes. The aggregates were composed of smaller grains with an approximate size of 200 nm. Ultra Small-Angle X-ray Scattering (USAXS) indicate that the aggregates were uniform and non-fractal. Mie calculations for a sphere equivalent to the aggregate size were compared to the experimentally observed results. The observed results showed an enhanced backscattering, whereas the Mie calculations did not due to the large imaginary part of the refractive index. Hematite aggregates were simulated by assuming they were composed of spherical monomers inside a spherical volume. Then the light scattering was calculated using the T-matrix method for these simulated aggregates. The calculated results show an enhanced backscattering. We present a dimensional analysis to estimate the extent of multiple scattering within the aggregate and find a correlation between the average number of scattering events within the aggregate and the enhancement in the backscattering.